Ohio Agronomy Guide, 14th Edition

Bulletin 472-05

Chapter 4: Corn Production

By Dr. Peter Thomison, Dr. Pat Lipps, Dr. Ron Hammond, Dr. Robert Mullen, and Bruce Eisley

Successful corn production requires an understanding of the various management practices and environmental conditions affecting crop performance. Planting date, seeding rates, hybrid selection, tillage, fertilization, and pest control all influence corn yield. A crop’s response to a given cultural practice is often influenced by one or more other practices. The keys to developing a successful production system are to recognize and understand the types of interactions that occur among production factors, as well as various yield limiting factors, and to develop management systems that maximize the beneficial aspect of each interaction. Knowledge of corn growth and development is also essential to use cultural practices more efficiently to obtain higher yields and profits.

How Climate Affects Corn Production

Temperature

Corn can survive brief exposures to adverse temperatures—low-end adverse temperatures being around 32°F and high-end temperatures being around 112°F. Growth decreases once temperatures dip to 41°F or exceed 95°F. Optimal temperatures for growth vary between day and night, as well as over the entire growing season. For example, optimal daytime temperatures range between 77°F and 91°F, and optimal nighttime temperatures range between 62°F and 74°F. The optimal average temperatures for the entire crop growing season, however, range between 68°F and 73°F.

Even though corn seeds germinate and grow slowly at about 50°F, the first spring planting dates usually begin when the average air temperatures reach 55°F, and soil temperature at seed depth is more favorable for seedling growth. Poor germination resulting from below-normal temperatures is the greatest hazard of planting too early. The growing point of germinating seedlings remains below or near the soil surface and usually is not vulnerable to freeze damage until plants reach the five- to six-leaf collar stage. By this time, corn is about 10-inches tall, and the probability of freezing temperatures greatly decreases. The loss of leaves from frost generally does not seriously injure small plants, although such loss may delay plant development.

Temperatures less than 40°F reduce photosynthesis, even if the only symptom is a slight loss of leaf color. Frost injury symptoms may appear on leaves even when nighttime temperatures do not fall below the mid 30s; radiational cooling can lower leaf temperatures to several degrees below air temperatures on a clear calm night. If frost kills leaves but not stalks before physiological maturity (black layer formation) in the fall, sugars usually continue to move from the stalk into the ear. However, yields are generally lower, and harvest moisture may be high because of high grain moisture at the time of frost and slow drying rates following premature death.

High temperature stress during ear formation, reproduction, and grainfill can reduce yield, but temperatures less than 100°F usually do not cause much injury if soil moisture is adequate. Under rain-fed conditions, corn usually begins to stress when air temperatures exceed 90°F during the tasseling-silking (pollination) and grainfill stages. Corn yield may be reduced 1.5 bushels per acre for each day the temperature reaches 95°F or higher during pollination and grainfill. Extended periods of hot, dry winds may cause tassel blasting and loss of pollen. Pollen shed usually occurs during cooler morning hours, however, and conditions severe enough to cause this problem are unusual in Ohio.

Precipitation

A corn crop in Ohio typically uses 20 to 22 inches of water during the growing season. Water requirements of corn vary according to the stage of development, as shown in Table 4-1. Corn reaches its peak water use during pollination, when plants are silking.

Excessive rainfall, resulting in flooding and ponding of soils, may cause serious injury to a corn crop depending on its stage of development. The major stress caused by flooding and ponding is a lack of oxygen needed for the proper function of the root system. When plants are very small (prior to the six-leaf collar stage), they generally are killed after about five or six days of submersion. Death occurs more quickly (within two to four days) if the weather is hot, because warm temperatures speed up the biochemical processes that use oxygen, and warm water has less dissolved oxygen. Cool weather, on the other hand, may allow plants to live for more than a week under flooded conditions.

As soon as plants reach the six- to eight-leaf collar stage and the plant’s growing point is above the soil surface, plants can tolerate a week or more of standing water—not necessarily without harm. In older plants, total submersion may increase disease incidence, and plants will suffer from reduced root growth and function for some days after the water recedes. Tolerance of flooding generally increases with plant age, but reduced root function resulting from a lack of oxygen is probably more detrimental to yield before and during pollination than during rapid vegetative growth or grainfill.

Nutrient uptake is also reduced in soils saturated by excessive rainfall. Not only does poor aeration inhibit effective root development and function, but the anaerobic conditions associated with saturated soils promote denitrification. Frequent rainfall can also cause nitrate leaching.

For crop moisture to be adequate, available soil moisture must be more than sufficient to meet the atmospheric evaporative demand. On windy, hot, sunny days with low humidity, evaporation demand on a crop is high, and a high amount of available soil moisture must be present if the crop is to avoid stress. Under cloudy skies, high humidity, and cooler temperatures, atmospheric evaporative demand is low, and plants can get by with lower amounts of available soil moisture.

The soil must provide a corn crop with enough water to offset the amounts lost through transpiration. If these needs are not met, the plant will wilt. Table 4-2 shows the effect of drought on corn grain yield from four consecutive days of visible wilting. Through the late vegetative stage (the end of June in normal years), corn is fairly tolerant of dry soils. Mild drought during June may even be beneficial because roots generally grow downward strongly as surface soils dry, and the crop benefits from the greater amount of sunlight that accompanies dry weather. From the two weeks before through the two weeks following pollination, corn is very sensitive to drought, however, and dry soils during this period may cause serious yield losses. Most of these losses result from pollination failure, and the most common cause is the failure of silks to emerge from the end of the ear. When this happens, the silks do not receive pollen; thus, the kernels are not fertilized and will not develop. Drought later in grainfill has a less serious effect on yield, though root function may decrease and kernels may abort or not fill completely.

Source: Claassen, M. M., and R. H. Shaw. 1970. Water deficit effects on corn. II. Grain components. Agron. J. 62:652-655.

Drought stress often leads to plant nutrient stress. The shallow depths where fertilizer is placed are dry under drought situations, which may limit nutrient uptake.

Corn Growth and Development

Corn growers who understand how the corn plant responds to various cultural practices and environmental conditions at different stages of development are able to use management practices more efficiently and, thus, obtain higher yields and profits. Knowledge of growth and development may also help in troubleshooting problems related to abnormal growth caused by pest problems or inappropriate cultural practices.

Table 4-3 describes two of the most widely used staging systems for corn development. Extension agronomists use the Leaf Collar Method throughout the United States; crop insurance adjusters, however, use the Horizontal Leaf Method to assess hail and other weather-related plant damage. Table 4-4 shows a timeline relating corn growth and development to normal heat unit accumulation and calendar dates during the growing season.

Table 4-5 lists estimated yield loss resulting from varying amounts of leaf area destruction for several stages of development. Although this table was developed to determine yield losses resulting from hail damage, it can also be used to help assess losses resulting from other defoliation injuries (such as wind, insect feeding, herbicide damage, and foliar N “burn”). The most common damage from hail is loss of leaf area, although stalk breakage and bruising of the stalk and ear may also be severe. Note that the largest yield losses result from defoliation damage that occurs during the late vegetative stages and the reproductive stages (silking and tasseling). Defoliation at early growth stages does not affect yield the same way as it does at later growth stages because much of the plant’s total leaf area is not yet exposed. Extensive defoliation of plants in the 10-leaf growth stage (or V8, eight leaf collar stage) does not result in a large yield loss because only 25% of the leaf area is exposed, and the plant can easily recover from early damage. On the other hand, severe damage to plants during tasseling results in a large yield loss because, by that time, 100% of the leaf area has been exposed and cannot be replaced.

Early killing frost in the fall may damage immature corn and reduce yield. The effect of frost damage to corn depends on the severity of defoliation, stalk damage, and stage of growth (see Chapter 1, Figure 1-3, for average fall frost dates). Tables 4-6 and 4-7 provide yield loss and kernel moisture estimates resulting from premature plant death (defoliation) during grainfill.

Hybrid Selection

Selecting a group of hybrids for planting is a key step in designing a successful corn production system. To stay competitive, growers must introduce new hybrids to their acreage on a regular basis. During the past 40 years, the genetics of corn hybrids has improved steadily, which has contributed to steady increases in grain yield potential ranging from 0.7% to 2.6% per year.

Growers should choose hybrids best suited to their farm operations. Corn acreage, soil type, tillage practices, desired harvest moisture, and pest problems determine the need for such traits as drydown rate, disease resistance, early plant vigor, plant height, and more. End uses of corn should also be considered. Will the corn be used for grain or silage? Will it be sold directly to the elevator as shelled grain or used on the farm? Capacity to harvest, dry, and store grain should also be considered. The most important factors for hybrid selection in Ohio are maturity, yield potential and stability, stalk quality, and disease resistance.

Maturity

Growers should choose hybrids with maturity ranges appropriate for their geographic area or circumstances. Corn for grain should reach physiological maturity or “black layer” (maximum kernel dry weight) one to two weeks before the first killing frost in the fall. Use days-to-maturity and growing-degree-day (GDD) ratings along with grain moisture data from performance trials to determine differences in hybrid maturity. Although yields of full-season hybrids usually exceed those of short-season hybrids, early- to mid-maturing hybrids have been developed in recent years with yield potential approaching those of full-season types. Late- to full-season hybrids do not always mature or dry down adequately before frost, which results in wet grain. When confronted with delayed planting or replanting decisions, growers may need to switch to early- to medium-maturity hybrids adapted to their area, but they should avoid short-season hybrids that are earlier than those normally used in their area. For more information on selecting hybrids for late planting, see the section on Date of Planting.

Days-to-Maturity Rating System

The most common maturity rating system is the days-to-maturity system. This system does not reflect actual calendar time between planting and maturity—106-day hybrid, for example, does not actually mature 106 days after planting. A days-to-maturity rating is based on relative differences within a group of hybrids for grain moisture at harvest. A one “day” maturity difference between two hybrids is typically equal to a one-half to three-fourths percentage point difference in grain moisture. For example, a 106-day hybrid would be, on average, 3 to 4.5 points drier than a fuller season 112-day hybrid if they were planted the same day (6 “days” multiplied by 0.5 or 0.75).

The relationship between days to maturity and kernel moisture is usually dependable when comparing hybrid maturities within a single seed company. However, because there are no industry standards for the days-to-maturity rating system, grain moisture comparisons of similar hybrid maturities from different seed companies may vary considerably. Days-to-maturity ratings are satisfactory for pre-season hybrid maturity selection when length of the growing season is usually not an issue. For delayed planting or replanting hybrid selection needs, growers need more absolute descriptions of a hybrid’s growing season requirements to manage the risk of a killing fall frost to late-planted corn.

Growing-Degree-Day (GDD) Maturity Rating System

The growing-degree-day (GDD) maturity rating system is based on heat units. It is more accurate in determining hybrid maturity than the days-to-maturity system because growth of the corn plant is directly related to the accumulation of heat over time rather than the number of calendar days from planting. The GDD system has several advantages over the days-to-maturity system. The GDD system provides information for choosing hybrids that will mature reliably, given a location and planting date; allows the grower to follow the progress of the crop through the growing season; and aids in planning harvest schedules.

The GDD calculation method most commonly used for corn in the United States is the 86/50 cutoff method. Growing degree days are calculated as the average daily temperature minus 50.

GDD = (Tmax + Tmin ÷ 2) - 50

If the maximum daily temperature (Tmax) is greater than 86°F, 86 is used to determine the daily average. Similarly, if the minimum daily temperature (Tmin) is less than 50°F, 50 is used to determine the daily average. The high cutoff temperature (86°F) is used because growth rates of corn do not increase above 86°F. Growth at the low temperature cutoff (50°F) is already near zero, so it does not continue to slow as temperatures drop further. Growing degree days are calculated daily and summed over time to define thermal time for a given period of time. The cumulative GDDs associated with different vegetative and reproductive stages is shown in Table 4-4.

Each corn hybrid requires a certain number of accumulated GDDs to reach maturity. Seed corn dealers have information on specific hybrids. Table 1-1 (see Chapter 1) lists average GDD accumulation for several Ohio locations from several dates in May to the 10% frost date at the particular location (late September). To monitor GDD accumulations during the growing season, the grower should follow the Weekly Crop and Weather Summary, a bulletin provided by the USDA-Ohio Agricultural Statistics Service (Room 608 Federal Building, 200 N. High Street, Columbus, Ohio 43215-2408). This information is also available online at: www.nass.usda.gov/oh.

As with any system, the GDD system has several shortcomings. Growing-degree-days ratings of hybrids with similar days-to-maturity ratings don’t always agree, especially if the hybrids are from different companies. Some seed companies start counting GDDs from the day of planting, while others begin from the day of emergence. When this occurs, similar maturity hybrids may vary by 100 to 150 GDDs—the average GDDs required for emergence. Some companies use entirely different mathematical methods to calculate GDDs. Although most companies use the “86/50 cutoff method” described earlier, others use different methods to calculate GDDs. Also, under certain delayed planting situations and stress conditions, GDD requirements for maturity may be reduced significantly.

Yield Potential and Stability

Choose hybrids that have produced consistently high yields across a number of locations and/or years. The Ohio Corn Performance Tests indicate that hybrids of similar maturity may vary in yield potential by as much as 40 bushels per acre or more. Choosing a hybrid because it possesses a particular trait, such as big ears, many kernel rows, deep kernels, prolificacy, or upright leaves, does not ensure high yields; instead, look for stability in performance across environments.

The superior performance of corn hybrids has virtually eliminated the use of open-pollinated varieties. Recently, we compared two open-pollinated varieties with hybrids in the Ohio Corn Performance Test. The average yield for the open-pollinated varieties was 83 bushels per acre and 29% lodged stalks. The average yield of the hybrids was 133 bushels per acre with 8% lodging—a 50-bushel or 60-percent yield advantage for hybrids over open-pollinated varieties.

Growers may want to consider planting herbicide-resistant or -tolerant hybrids in soils where residual levels of certain herbicides may be too high for normal corn growth. Seed companies have used various genetic techniques to insert resistance or tolerance to imazethapyr plus imazapyr (Lightning), glufosinate ammonium (Liberty), and glyphosate (Roundup) into existing corn hybrids which are characterized as Clearfield (or imidazolinone-tolerant), Liberty Link, and Roundup Ready hybrids, respectively. Planting these tolerant/herbicide-resistant hybrids may allow growers to use herbicide formulations normally used on soybeans or other crops. These herbicide resistant/tolerant hybrids offer new weed control options that involve fewer applications and use of more environmentally benign chemicals.

Corn hybrids highly resistant to European corn borer injury and western corn rootworm also are available. These resistant hybrids contain a gene from bacteria that produces the insecticide known as Bt. Planting Bt corn hybrids may eliminate the need for soil insecticide treatments (rootworm) and postemergent insecticide applications (corn borer), which are less effective and potentially harmful to nontarget beneficial insects.

Review the results of state, company, and county performance trials before purchasing hybrids. Because weather conditions are unpredictable, the most reliable way to select superior hybrids is to consider performance during the last year and the previous year over a wide range of locations and climatic conditions. When using University performance trials results, two years of data from several locations is usually adequate; test summaries for three or more years may exclude new hybrids with better performance potential.

On-farm strip tests are not reliable in hybrid selection because they cannot predict hybrid performance across a range of environmental conditions. However, on-farm hybrid tests can be useful in evaluating various traits, such as lodging, drydown, harvestability (ease of shelling, ear retention, etc.), disease resistance, and staygreen.

Ohio State University Extension conducts corn performance tests across Ohio. Test results are published each year in a bulletin entitled Ohio Corn Performance Test, Horticulture and Crop Science Series 215, and is also available online at: agcrops.osu.edu .

Stalk Quality and Lodging

Hybrids with poor stalk quality should be avoided for grain production even if they show outstanding yield potential. This trait is particularly important in areas where stalk rots are perennial problems or where field drying is anticipated—conditions that often lead to lodging (stalk breakage below the ear following crop maturation). If growers have their own drying facilities and are prepared to harvest at relatively high moisture levels (above 25%) or are producing corn for silage, then standability and fast drydown rates are less critical selection criteria.

Traits associated with improved hybrid standability include resistance to stalk rot and leaf blights, genetic stalk strength (a thick stalk rind), short plant height and ear placement, and high staygreen potential. Staygreen refers to a hybrid’s potential to stay healthy late into the growing season, after reaching maturity, and should not be confused with late maturity. Resistance to European corn borer conferred by the Bt trait can also enhance stalk quality by limiting entry points in plant tissue through which fungal pathogens can invade the plant. However, the Bt trait will do little to minimize stalk rot and lodging in a hybrid characterized by below-average stalk quality.

Another stalk-related problem, green snap or brittle snap, has started to appear in recent years. Corn plants are more prone to snapping during the rapid elongation stage of growth. According to studies in Iowa, Minnesota, and Nebraska, the V5 to V8 stages (corn approximately 10 to 24 inches in height) and the V12 stage through tasselling are the most vulnerable stages. Vulnerability to green snap damage does vary among hybrids. However, all hybrids are at risk from wind injury when they are growing rapidly prior to tasseling. The use of growth regulator herbicides, such as 2,4-D or Banvel, has also been associated with stalk brittleness, especially if late application or application during hot, humid conditions occurs. Once tassels begin shedding pollen, green snap problems generally disappear.

Disease Resistance and Tolerance

Hybrids should be selected for resistance or tolerance to stalk rots, foliar diseases, and ear rots, particularly those that have occurred locally. Seed dealers should provide information on hybrid reactions to specific diseases in Ohio (Table 4-21).

Grain Quality

The protein and oil composition of corn grain is a major factor in determining grain feeding value. Although the grain market does not include this factor in price determination, growers who feed livestock may use this information to reduce feed costs and optimize diets. Because hybrid genetics significantly affect the protein and oil content of corn grain, there is interest in using compositional data in selection of hybrids to plant for feed use, as well as other end uses. For feed, protein content is of primary interest, whereas for processing uses, oil content is of interest. Corn grain is typically 8% protein and 3.6% oil (on a 15.5% moisture basis).

Table 4-8 shows averages and ranges for the grain protein and oil content of hybrids tested at different locations in Ohio from 2001 to 2003. The protein and oil content of grain is reported as protein and oil percentages at 15.5% grain moisture. Although significant differences among hybrids for oil and protein were found in each test, protein and oil levels varied considerably from test to test. Some normal dent corn hybrids produced primarily for grain exhibit elevated protein and oil levels. Environmental conditions (temperatures, rainfall) and cultural practices (nitrogen fertility, plant population) can affect grain composition, especially grain protein. Additional information on hybrids developed for special grain composition characteristics is in the Specialty Types of Corn section in this chapter.

Date of Planting

The recommended time for planting corn in northern Ohio is April 15 to May 10 and in southern Ohio, April 10 to May 10. Approximately 100 to 150 GDDs are required for corn to emerge. In central Ohio this number of GDDs usually accumulates by the last week of April or the first week in May. Improved seed vigor and seed treatments allow corn seed to survive up to three weeks before emerging if soil conditions are not excessively wet. An early morning soil temperature of 50°F at the one-half to two-inch depth usually indicates that the soil is warm enough for planting. Corn germinates very slowly at soil temperatures below 50°F. Short-term weather forecasts should be monitored to make the best decision on early planting. After April 25, planting when soil moisture conditions allow is usually safe. The latest practical date to plant corn ranges from about June 15 in northern Ohio to July 1 in southern Ohio. Plantings after these dates yield no more than 50% of normal yields.

Planting should begin before the optimum date if soil conditions will allow the preparation of a good seedbed. Growers should have the equipment capability to plant more than half of their corn acres prior to the optimum planting date; this should allow planting all the corn acres prior to the calendar date when corn yields begin to quickly decline. Ohio corn producers usually cannot perform field operations during all days of their optimum planting date range due to spring rains and cool weather conditions that limit soil drying. On average, during the optimal corn planting time in Ohio, only one out of three days is available during which fieldwork can occur.

Table 4-9 shows the effect of date of planting in Columbus. In central Ohio, yields decline approximately 1 to 1.5 bu/day for planting delayed beyond the first week of May. Grain yield and test weight were increased by early plantings, whereas grain moisture was reduced, thereby allowing earlier harvest and reducing drying costs. Early planting generally produces shorter plants with better standability. Delayed planting increases the risk of frost damage to corn and may subject the crop to greater injury from various late insect and disease pest problems, such as European corn borer and gray leaf spot.

Corn should be planted only when soils are dry enough to support traffic without causing soil compaction. The yield reductions resulting from mudding the seed in may be much greater than those resulting from a slight planting delay. No-tillage corn can be planted at the same time as conventional, if soil conditions permit. In reality, however, planting may need to be delayed several days to permit extra soil drying. Planting a full-season hybrid first, then alternately planting early-season and mid-season hybrids, allows the grower to take full advantage of maturity ranges and gives the late-season hybrids the benefit of maximum heat unit accumulation. Full-season hybrids generally show greater yield reduction when planting is delayed compared with short- to mid-season hybrids. Planting early hybrids first, followed by mid-season, and finally the full-season hybrids spreads the pollination interval for all the corn acres over a longer time period and may be a good strategy for some drought-prone areas with longer growing seasons.

Planting hybrids of different maturities reduces damage from diseases and environmental stress at different growth stages (improving the odds of successful pollination) and spreads out harvest time and workload. Consider spreading hybrid maturity selections between early-, mid-, and full-season hybrids—for example, a 25-50-25 maturity planting, with 25% in early- to mid-season, 50% in mid- to full-season, and 25% in full-season. Planting a range of hybrid maturities is probably the simplest and most effective way to diversify and broaden hybrid genetic backgrounds.

When corn planting is delayed past the optimum dates or if a crop needs to be replanted, it may be necessary to switch hybrid maturities. In most delayed planting situations, however, full-season hybrids still perform satisfactorily and reach physiological maturity (black layer formation) when planted as late as the last week of May. Hybrids planted in late May or early June mature at a faster thermal rate (require fewer heat units) than the same hybrid planted in late April or early May.

Recent research from Ohio and Indiana indicates that the required GDD units from planting to kernel black layer decreases with delayed planting. For each day that planting was delayed after May 1 the reduction in GDD requirement was about 6.5 GDDs. A hybrid rated at 2,800 GDDs with normal planting dates (such as late April or early May) may require only 2,605 GDDs when planted on May 30. Therefore, a 30-day delay in planting may result in a hybrid maturing in 195 fewer GDDs (30 days times 6.5 GDDs per day).

Other factors concerning hybrid maturity need to be considered when planting is delayed. For plantings in late May or later, the dry-down characteristics of hybrids should be considered. Although a full-season hybrid may still have some yield advantage over shorter season hybrids planted in late May, the full-season hybrid could have significantly higher grain moisture at maturity than earlier maturing hybrids. In addition, there will be less calendar time for field drying, and drying costs will be higher. Later planting dates generally increase the possibility of damage from European corn borer (ECB) and may warrant selection of ECB Bt hybrids, if suitable maturities are available.

Seeding Depth

The appropriate planting depth varies with soil and weather conditions. For normal conditions plant corn 1.5 to two-inches deep to provide frost protection and allow for adequate root development. Shallower planting often results in poor root development and should be avoided in all tillage systems. In April, when the soil is usually moist and the evaporation rate is low, seed should be planted shallower—no deeper than 1.5 inches. As the season progresses and evaporation rates increase, deeper planting may be advisable. When soils are warm and dry, corn may be seeded more deeply—up to two inches on non-crusting soils. Seed press wheels can help ensure good seed-soil contact, which is especially important as temperatures increase to 70°F or 80°F.

Row Width

Over the past 30 years, row spacing in corn has steadily decreased as plant populations have increased. Since the early 1970s, average row spacing in Ohio decreased from about 35 inches to about 30 inches in 2004. This reduction in row spacing coincided with an increase in average plant population from approximately 18,000 plants/A to more than 26,000 plants/A. During the past decade, there has been considerable interest in narrowing row spacing even further, and many studies have been performed comparing corn planted in narrow rows (row spacing 22-inches or less) and conventional 30-inch row spacing.

Although narrow row systems are often perceived as a proven method for increasing yield and profitability, recent studies on narrow-row corn production have produced mixed results. Some of the inconsistency may be related to latitude with narrow rows in the north central region of the United States exhibiting the largest yield increases (5% or more) over 30-inch rows. This advantage diminishes sharply moving southward with average yield advantages for narrow rows of less than 2% in the central Corn Belt and less further south. Results of a Michigan State University study conducted in 1998-99 showed that corn grain yields increased by 2% and 4% when row width was narrowed from 30 inches to 22 inches and 15 inches, respectively. However, in recent Ohio State University research, the yield advantage of narrow rows over 30-inch row spacings has been smaller (usually less than 2%) and less consistent.

Some growers are considering “twin rows” as another row spacing configuration that may offer some of the yield increases associated with narrow row corn. In the typical twin row system, two rows are placed 7-inches apart on 30-inch centers, although other twin row configurations are used. Twin rows make it possible to create narrow rows without changing the row configuration of other equipment, and to avoid costs associated with equipment conversion to a narrow row system. Staying on 30-inch centers allows growers to use the same corn header and tractor tire spacing used in 30-inch corn production. In several Ohio studies, a twin row system has performed as well or better than the 30- and 15-inch row spacings, but results of evaluations of twin row corn in other states have not been conclusive.

Potential yield gains from narrow rows must be balanced against the investment for new equipment and higher input costs associated with narrowing row spacing. Greater interest in increasing equipment use efficiency by using the same planter or drill for soybean, sugar beet, and corn may warrant adoption of narrow row systems for corn. Producers in northern regions that also grow soybeans and sugar beets in 22-inch rows often find it more efficient to use this same row spacing for corn.

Plant Populations and Seeding Rates

When corn is produced for grain in Ohio, recommended plant populations at harvest (or final stand) can range from 20,000 to 30,000+ plants/A, depending on the hybrid and production environment. Hybrids differ in their response to high plant population with some exhibiting stalk lodging at the upper end of the plant population range. Populations for corn silage typically exceed those for grain by 2,000 to 4,000 plants/A. Seed companies specify a range in final stands for the various corn hybrids they market, and these seeding rate guidelines should be followed closely to maximize crop performance.

Yield potential of the production environment (i.e., soil productivity, etc.) is the primary factor determining hybrid yield response to increasing plant population. Seeding rate adjustments should be made on a field-by-field basis using the average yield potential of a site over a three- to five-year period as the major criterion for determining the appropriate plant population. When determining the realistic yield potential for a site over a five-year period, it may be appropriate to ignore the highest and lowest yields, which may have occurred during years that were unusually favorable or unfavorable for corn performance.

Higher seeding rates are recommended for sites with high yield potential that have high soil fertility levels and water holding capacity. On very productive soils which may average yields of 175 bu/A or more (such as a drained, Kokomo silty clay loam), final stands of 30,000 plants/A or more may be required to maximize yields. On soils averaging 150 bu/A, final stands of 26,000 to 28,000 plants/A may be needed to optimize yield. Lower seeding rates are preferable when droughty soils or late planting (after June 1) limit yield potential. On soils that average 120 bu/A or less, final stands of 22,000 plants/A may be adequate for optimal yields.

Hybrid response to high population can be limited by stalk lodging, which often increases at higher plant density. Some hybrids that have shown positive yield response to higher populations cannot be grown at high plant densities because of the increased risk of lodging at harvest. Lodging reduces yields and slows the harvest operation. Therefore, it is essential that hybrids planted at high seeding rates possess superior stalk quality for standability. Hybrids should also have resistance (or the best levels of tolerance available) to fungal leaf diseases (such as gray leaf spot and northern corn leaf blight), which contribute to stalk lodging problems and stalk rots (such as Anthracnose and Gibberella).

If a grower plans to rely extensively on field drying that can delay harvest, there may be little benefit from using high plant populations above 30,000 plants/A. We recently completed a study that evaluated effects of plant population (24,000 to 42,000 plants/A) and harvest dates (early-mid October, November, and December) on the agronomic performance of four hybrids differing in maturity and stalk quality (Table 4-10).

Although the hybrids exhibited similar yield potential when harvested early (early/mid October), differences in yield became evident with harvest delays, which could be attributed to differences in stalk quality. Yield differences among plant population were generally small on the first harvest date, but with harvest delays, major yield losses occurred at the higher plant populations, especially 42,000 plants/A, due to increased stalk lodging. Grain moisture averaged about 24% on the first harvest date, 18% on the second harvest, and 17.5% on the third harvest date. After the first harvest in early/mid October, stalk lodging increased to as much as 80% for certain hybrids at high plant populations, resulting in yield losses of nearly 50% by mid December.

Final stands are always less than the number of seeds planted per acre. Cold, wet soil conditions, insects, diseases, cultivation, and other adversities will reduce germination and emergence. Generally, you can expect from 10 to 20 percent fewer plants at harvest than seeds planted. To compensate for these losses, you need to plant more seed than the desired population at harvest. Many seed companies recommend over-planting by 10% to 15%.

To calculate your own planting rate, consider the following formula:

Planting Rate = Desired Population per Acre ÷ (Germination x Expected Survival)

Germination is the percent seed germination shown on the seed tag (converted to decimal form). Expected survival is the percent of seedlings and plants that you expect to reach harvest maturity under normal conditions (converted to decimal form). Ninety percent survival (or 10% plant mortality) is about average. If you are planting very early when the soil will likely remain cool for several days following planting, you may want to reduce expected survival by 5%. A similar approach should be followed when planting no-till, especially in heavy residues.

Example:
Target stand at harvest - 26,000 plants per acre
Seed tag indicates 95% seed germination
Assume 90% survival (10% plant mortality)
Planting rate = 26000 ÷ (0.95 x 0.90) = 30,409 seedsper acre
According to the formula, you should consider a planting rate of approximately 30,400 seeds/A to achieve the desired final stand of 26,000 plants/A.

Uneven plant spacing and emergence may reduce yield potential. Seed should be spaced as uniformly as possible within the row to ensure maximum yields and optimal crop performance —regardless of plant population and planting date. Corn plants next to a gap in the row may produce a larger ear or additional ears (if the hybrid has a prolific tendency), compensating for missing plants. These plants, however, cannot make up for plants spaced so closely together in the row that they compete for sunlight, water, and nutrients. Crowding often results in barren plants or ears too small to be harvested (nubbins), as well as stalk lodging and ear disease problems.

Although uniformity of stand cannot be measured easily, studies have indicated that reduced plant stands will yield better if plants are spaced uniformly than if there are large gaps in the row. As a general guideline, yields are reduced an additional 5% if there are gaps of four to six feet in the row and an additional 2% for gaps of one to three feet. Studies at Purdue University suggest that corn growers could improve grain yield from four to 12 bushels per acre if within-row spacing were improved to the best possible uniformity (depending on the unevenness of the initial spacing variability).

The most effective way to improve planter accuracy is to keep planting speed within the range specified in the planter’s manual. Additional considerations for improving seed placement uniformity are listed here:

• Match the seed grade with the planter plate.
• Check planters with finger pickups for wear on the back plate and brush (use a feeler gauge to check tension on the fingers, then tighten them correctly).
• Check for wear on double-disk openers and seed tubes.
• Make sure the sprocket settings on the planter transmission are correct.
• Check for worn chains, stiff chain links, and improper tire pressure.
• Make sure seed drop tubes are clean and clear of any obstructions.
• Clean seed tube sensors if a planter monitor is being used.
• Make sure coulters and disk openers are aligned.
• Match the air pressure to the weight of the seed being planted.

Uneven emergence affects crop performance because competition from larger, early emerging plants decreases the yield from smaller, later emerging plants. The primary causes of delayed seedling emergence in corn include soil moisture variability within the seed depth zone, poor seed-to-soil contact resulting from cloddy soils, inability of no-till coulters to slice cleanly through surface residues, worn disk openers, and maladjusted closing wheels. Other causes include soil temperature variability within the seed zone, soil crusting prior to emergence, occurrence of certain types of herbicide injury, and variable insect and/or soil-borne disease pressure.

Based on research at the University of Illinois and the University of Wisconsin, if the delay in emergence is less than two weeks, replanting increases yields less than 5%, regardless of the pattern of unevenness. However, if one-half or more of the plants in the stand emerge three weeks late or later, then replanting may increase yields up to 10%. To decide whether to replant in this situation, growers should compare the expected economic return of the increased yield with both their replanting costs and the risk of emergence problems with the replanted stand.

Use Tables 4-11 and 4-12 to determine the number of kernels dropped or the plant population per acre. Table 4-13 includes the bushels of corn obtained for various numbers of ears harvested at different ear weights.

Making Replant Decisions

Although it is not unusual that 10% to 15% of planted seeds fail to establish healthy plants, additional stand losses resulting from insects, frost, hail, flooding, or poor seedbed conditions may call for a decision on whether or not to replant a field. The first rule in such a case is not to make a hasty decision. Corn plants can and often do outgrow leaf damage, especially when the growing point is protected beneath or at the soil surface (up until about the six-leaf collar stage). If new leaf growth appears within a few days after the injury, then the plant is likely to survive and produce normal yields.

When deciding whether to replant a field, assemble the following information: original planting date and plant stand, earliest possible replanting date and plant stand, and cost of seed and pest control for replanting. If the plant stand was not counted before damage occurred, providing that conditions for emergence were normal, estimate population by reducing the dropped seed rate by 10%. To estimate stand after injury, count the number of living plants in 1/1,000 of an acre (Table 4-12). Take counts as needed to get a good average —one count for every two to three acres.

When the necessary information on stands, planting, and replanting dates has been assembled, use either Table 4-14 or 4-15 to locate the expected yield of the reduced plant stand by reading across from the original planting date to the plant stand after injury. Then, locate the expected replant yield by reading across from the expected replanting date to the stand that would be replanted. The difference between these numbers is the percentage yield increase (or decrease) to be expected from replanting.

Here’s how these tables might be used to arrive at a replant decision (Table 4-15 will be used in this example). Let’s assume that a farmer planted on May 9 at a seeding rate sufficient to attain a harvest population of 30,000 plants per acre. The farmer determined on May 28 that his stand was reduced to 15,000 plants per acre as a result of saturated soil conditions and ponding. According to Table 4-15, the expected yield for the existing stand would be 79% of the optimum. If the corn crop was planted the next day on May 29 and produced a full stand of 30,000 plants per acre, the expected yield would be 81% of the optimum. The difference expected from replanting is 81 minus 79, or 2 percentage points. At a yield level of 150 bushels per acre, this increase would amount to three bushels per acre which would probably not justify replanting costs.

Tables 4-14 and 4-15 show the effects of planting date and plant population on the final grain yield for the central Corn Belt. Table 4-15 is a newer chart, developed by Dr. Emerson Nafziger at the University of Illinois, that includes earlier planting dates and higher optimum plant populations. Table 4-14 is based on older data from the 1970s, but it still provides a reasonable assessment of potential yield losses, especially for planting dates in June. Grain yields for varying dates and populations in both tables are expressed as a percentage of the yield obtained at the optimum planting date and population.

Keep in mind that replanting itself does not guarantee the expected harvest population. Corn replant decisions early in the growing season will be based mainly on plant stand and plant distribution. Later in the season as yields begin to decline rapidly because of delayed planting, calendar date assumes increased importance.

Fertility Recommendations

A good nutrient management program is key to high-yield corn production. Instituting the best management techniques to ensure adequate nutrient availability throughout the growing season can pay real dividends at the end of the year and minimize the adverse effects of nutrient runoff and leaching on the environment. Considering the importance of nitrogen (N), phosphorus (P), and potassium (K), the discussion will cover these primary soil fertility concerns.

Nitrogen

Timing and Sources

Nitrogen (N) applications for corn production should consider all risks and weigh all possibilities, particularly with nitrogen prices being high. Fall application of nitrogen is not recommended, but if N is to be applied in the fall, make certain that soil temperatures are below 50°F and that anhydrous ammonia is used. Do not apply N fertilizers that contain nitrate in the fall; the risk of loss is high due to leaching. Application of N in the spring is more efficient and less susceptible to loss. Nitrogen stabilizers may be used for early spring application, but the benefit of such compounds is questionable under certain growing conditions (fine textured soils subject to denitrification can show benefit—in wet years). Application of sidedress N is a good alternative to preplant applications of N. In-season applications move fertilization away from the busy planting period and are closer to actual crop uptake of N. Sidedressing also minimizes the risk of N loss, especially on poorly drained fine-textured soils, which are subject to denitrification, and coarse-textured soils, which are susceptible to leaching. The main risk of in-season application is the possibility of delayed application due to wet conditions.

When selecting a nitrogen source, remember that a pound of N is a pound of N; make selections based on risk and cost. For example, it would be risky to apply urea to the surface of no-till ground due to the potential loss of N by volatilization. Surface dribble-banding of liquid N or subsurface injection are better alternatives. This is not to say that urea is not a good source of N, but in this instance, there are better options. Always consider the cost of the material as well as the field environment that will be encountered to get the most efficient use of fertilizer N.

Rates

Current nitrogen recommendations for corn production based on yield potential are presented in Table 4-16. Rates should be adjusted for high organic soils (> 20% organic matter) by using the pre-sidedress nitrate soil test or flat rate decreases of at least 40 lb per acre. Application rates for late planted corn should also be adjusted to reflect the decrease in yield potential. To determine a reasonable yield potential, consider the yield average of your last five corn crops at that specific field.

Phosphorus and Potassium

Application Methods

Phosphorus (P) and potassium (K) are a little easier to handle than N when it comes to application methods. Phosphorus and potassium are not subject to the same loss mechanisms as N, thus application concerns are not as restrictive. The main loss mechanism for P is runoff. Utilization of conservation practices that minimize the risk of soil runoff to surface waters is adequate for good P management. P and K can be applied either broadcast prior to planting or banded (near the row or over the row [pop-up]) as a starter when planting. If applying starter in a band two inches to the side and two inches below the seed, the total amount of salts applied (N + K2O) should not exceed 100 lb per acre. If starter is applied over the rows with the seed (not recommended due to potential salt problems), the total salts (N + K2O) applied should not exceed 5 lb per acre for low cation exchange capacity (CEC) soils or 8 lb per acre for high CEC soils. The benefit of starter fertilizers increases when soil-test levels and soil temperatures are low and when soil surface residues are high. Soils that have moderate to high levels of soil test P and K show little to no benefit from starter fertilizer.

Sources

Little difference exists between commonly used forms of phosphorus and potassium with regard to nutrient uptake. Ortho- and poly-phosphate formulations perform equally well, even though the crop takes up the ortho form (poly forms convert to ortho forms rapidly). It should be mentioned that if dry formulations of P are to be applied in contact with the seed, monoammonium phosphate (MAP) is a somewhat safer form of P to apply than diammonium phosphate (DAP). DAP produces more ammonia (NH3) which is toxic to germinating seeds. When banding MAP, DAP, or ammonium polyphosphate (APP), do not exceed more than 40 lb N per acre. If soil-test P and K are high on no-till soils, then only N should be applied as a starter, unless 40 to 60 lb N per acre have been applied preplant.

Rates

Soil test levels below the critical value are considered deficient and warrant application of fertilizer (Table 4-17). Current recommendations for P and K are presented in Tables 4-18 and 4-19. Buildup and maintenance recommendations are designed to increase soil-test levels to the critical value or maintain current soil-test levels. Considering it takes between 8 to 20 lbs of P2O5 and 5 to 10 lbs of K2O (added or removed) to change the soil test level by one unit (depends largely upon soil texture), soil-test levels above the critical value will be adequate for crop production for at least a few years (depending upon the soil-test level).

For comprehensive information on corn fertilization and soil fertility management, consult Extension Bulletin E-2567, Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa, available online at: ohioline.osu.edu/e2567/index.html .

Crop Rotations

The corn-soybean rotation is by far the most common cropping sequence used in Ohio. This crop rotation offers several advantages over growing either crop continuously. Benefits to growing corn in rotation with soybeans include more weed control options, fewer difficult weed problems, less disease and insect buildup, and less nitrogen fertilizer use. Corn grown following soybeans typically yields about 10% more than continuous corn.

No-till cropping systems, which leave most of the prior crop residue on the surface, are more likely to succeed on poorly drained soils if corn follows soybean or meadow rather than if corn follows corn or a small grain, such as wheat. Table 4-20 shows the influence of corn rotation on corn response to tillage and soil type. On the poorly drained Hoytville silty clay loam, where corn follows soybean or meadow, yield differences between no-till and plowed are greatly reduced. Crop rotation with soybeans had much less effect on corn response to tillage on the well-drained Wooster silt loam. This yield advantage to growing corn following soybean is often much more pronounced when drought occurs during the growing season.

Corn Pest Management

Weed Control

A number of factors need to be considered when developing weed control programs for corn, including soil type, weeds, weeds present, crop rotation, and budget. No single control program effectively handles the various weed problems that arise under different environmental conditions. Weeds are the major pest control problem in corn production in Ohio. Extension Bulletin 789, Weed Control Guide for Ohio and Indiana, provides comprehensive information on managing weeds, including herbicide label updates. This bulletin is available online at: agcrops.osu.edu/weeds/documents/2005WeedControlforOHandIN.pdf .

Insect Control

Field corn is a host crop for a complex of pest species that may cause economic injury throughout the growing season (Figure 4-1). (Figure 4-1 is available at: ohioline.osu.edu/b827/b827_69.html.) Despite the number of different pests that feed on corn, the average field generally exhibits minimal pest problems because the normal activity of most pest populations is too low to inflict significant injury to warrant attention. However, economic levels of pest injury do occur, and economic losses resulting from sudden outbreaks of pest activity can be prevented if the farm manager is aware of the biology of corn pests, monitors fluctuations in pest population activity, and implements timely corrective action.

Figure 4-1. Key periods of corn insect pests.

Management of pest populations on corn requires a combination of preventive and timely responsive actions based on the risks associated with various cultural practices and pest activity observations collected through periodic field inspections. The text in the next section reviews the complex of invertebrate pests that may impact field corn, summarizes the ecological factors associated with outbreaks of specific pests, and discusses available pest management options.

Planting Time Decisions

When planting corn, there are basically four pest-management options:

• If potential loss from rootworm exists, then an application of a granular or liquid soil insecticide, the use of a seed-applied insecticide, or the use of a Bt hybrid for rootworm may be warranted.
• Application of an insecticide in a herbicide tank mix applied as a pre-plant or pre-emergence treatment to prevent stand loss.
• Application of a seed treatment (either planter box or seed-applied insecticide) to prevent insect damage to seeds.
• Use of no treatment to prevent pest losses.

If pest populations threaten to cause stand loss, using a soil insecticide or seed-applied insecticide may be warranted. If corn rootworms threaten to attack corn root systems, using a soil insecticide, high rates of seed-applied insecticide, or Bt hybrid for rootworm at planting may be warranted. Although the use of soil insecticides at corn planting time represents the leading use of insecticides in Ohio, the Midwest, and North America, 75% to 80% of the corn in Ohio is planted without the use of a soil insecticide because of limited pest pressure and extensive annual crop rotation. A review of the conditions that warrant the use of a soil insecticide is presented here.

Pest activity that may cause stand losses and can be prevented by appropriate selection of soil or seed-applied insecticide treatments includes seedcorn maggots, cutworms, grubs, and wireworms. Each of these pest problems is often associated with unique conditions and should not be considered a widespread problem applicable to all corn-growing habitats.

Seedcorn maggots may be a problem when high levels of organic matter are associated with climatic conditions delaying seed germination and seedling emergence. Most problems will occur when organic matter, such as an old alfalfa field or a cover crop, is incorporated into the soil in early spring. Either applying a soil insecticide or a seed treatment may prevent seedcorn maggot injury. In general, if corn is planted into a high organic habitat that is soil incorporated and where adverse climatic conditions may result in delayed emergence, then preventive treatment should be implemented.

Cutworm problems are generally associated with the black cutworm, which is an annual migrant from the South. Infestations of black cutworm depend on whether significant populations are immigrating from the South, and whether a certain field is an attractive location to the immigrating populations of cutworm. In general, most corn fields under conventional or minimal tillage having minimal weed problems are not attractive to migrating cutworms and do not require preventive treatment. No-tillage corn plantings tend to have a higher incidence of cutworm infestations, but it is questionable whether routine preventive treatment for cutworm in such habitats is needed. Stand loss resulting from cutworms may be prevented by applying soil insecticides having a high level of efficacy against cutworm or by applying insecticide as a tank mix in pre-plant or pre-emergence herbicides. The tank mix treatment may also prevent problems associated with common stalk borer, which most soil insecticide treatments do not prevent and which is difficult to control with rescue treatments.

Wireworm and grub infestations associated with stand loss in corn are relatively uncommon, but such problems should be expected when corn follows pasture, sod, forage, or fallow ground. In these production environments, the establishment of specific grasses may facilitate development of wireworm or grub populations with life cycles extending beyond a year. In such situations, using a soil or seed-applied insecticide having efficacy against wireworms and grubs is recommended. A planter box seed treatment may be regarded as a minimal treatment against potential wireworm damage to seed, but using a soil insecticide at planting provides extended protection of both the seed and the seedlings.

Reducing losses resulting from corn rootworm on continuous corn or first-year corn rootworm fields is the predominant objective of most soil insecticide applications, high rates of seed treatment, or Bt hybrids for rootworm. In general, most Ohio growers routinely apply soil insecticide to all continuous corn plantings. Although applying a soil insecticide to continuous corn planting is a routine practice, many continuous corn fields do not have rootworm activity warranting preventive treatment. The potential for rootworm injury can be predicted by monitoring adult rootworm activity in the field programmed for planting of continuous corn or using sticky traps in soybean fields where first-year corn rootworms may be a threat the following year. However, adult rootworm activity is rarely monitored in continuous corn. Routinely applying soil insecticides, high rates of seed treatments, or Bt hybrids for rootworm on continuous corn may be justified, however, because the combination of stand loss prevention plus root injury from rootworm may generate a benefit exceeding the cost of application. However, if either stand loss or rootworm injury is lacking, then the benefit derived from these treatments may not outweigh the cost of treatment.

Planting corn without any treatment has been the norm for most Ohio corn producers. In general, corn planted without any insecticide treatment is first-year corn under either conventional or minimal tillage without the threat of damage from first-year corn rootworm. Planting no-till, first-year corn acreage, or continuous corn acreage without a soil insecticide, increases the risk of injury resulting from soil pests. Selection of the optimal pest management practice at planting time depends, in part, on a grower’s preference for minimizing risk and on personal experience related to losses attributed to pest problems.

Early Stand Loss Prevention

Depending on the degree of preventive treatments taken at planting, corn stands may be vulnerable to infestations that reduce stand establishment. If the early stand loss is a result of seedcorn maggot, wireworm, grubs, or early cutworm infestation, timely response with rescue treatments may not be feasible. However, if a successful stand has been established and significant injury resulting from cutworm, stalk borer, slugs, or armyworm is detected, then application of a timely rescue treatment may prevent further stand losses. Timely detection and response to early pest infestations of corn depends on implementation of a program of field inspection to detect pest problems in their early stages of development. In general, as tillage decreases, the potential for early pest problems increases. Significant armyworm and slug infestations are primarily no-tillage corn production problems.

Corn planted no-till into either old hay stands or cover crops of rye is extremely vulnerable to armyworm infestations. Under these conditions, armyworm may cause significant defoliation. Such infestations, however, may be easily controlled with a rescue treatment if the infestation is detected in a timely manner. Any corn planted no-till into a grassy habitat should be monitored weekly and semi-weekly if armyworm activity is detected. Rescue treatment is warranted whenever the level of defoliations appears to be resulting in a loss of stand.

Slug problems tend to increase if acreage remains under no-tillage over an extended period. If defoliation is significant, rescue treatment with approved baits is a viable option. However, if slugs continue to be a problem on a yearly basis, implementation of minimum tillage will tend to reduce the overall problem.

Mid- (V8) and Late- (V12) Whorl Pest Problems

As corn stand is established, the relative incidence of severe pest problems tends to decline. However, periodic inspections should be conducted to detect pest problems. The primary pest problem of mid-whorl corn is infestation by first-brood European corn borer. Less than 5% of corn stands exhibit significant infestations of corn borer, and few fields ever have infestations warranting corrective action. However, if an infestation is found where one or more larvae can be prevented from boring into the stalks, then rescue treatment may be warranted. If the farm or field has a history of significant European corn borer problems, then the use of a Bt hybrid with activity against corn borer might be warranted. If planting is made in late May or early June, research has shown a yield increase when a Bt hybrid for corn borer is planted at that time.

In general, the development of a borer in a stalk leads to a 5% reduction in yield of the infested plant. Therefore, if whorl infestations and larval presence are detected at significant levels, timely application of a rescue treatment may be warranted.

No relationship has been found to exist between corn borer incidence and tillage. Although no-tillage corn production allows corn stubble with the overwintering larvae to remain in the field, corn borer activity in no-tillage corn is equivalent to that of conventional and minimal tillage corn.

Another insect problem, which becomes evident during the mid- to late-whorl stage of corn development, is root lodging resulting from corn rootworm. Although the problem cannot be corrected once it occurs, July is the optimal time to evaluate rootworm impact on continuous corn. If rootworm activity is significant, root-lodged corn plants will be evident. Root systems should then be inspected to confirm and evaluate the degree of rootworm activity.

Tassel and Silk Stage Pest Activity

As corn enters the tassel and silk stages, a few pest problems may occur that may possibly disrupt successful pollination and adversely influence yield. These pest problems include the corn leaf aphid and various beetles feeding on the silks.

Corn leaf aphid problems are generally associated with dry conditions. If abundant, colonies of corn leaf aphids may secrete sufficient honeydew to disrupt the release of pollen from the tassels. Abundant corn leaf aphids on 70% or more of the stand may warrant rescue treatment, but the activity of beneficial predators usually takes care of such infestations when they occur.

Successful pollination may be affected by excessive clipping of silks by abundant populations of corn rootworm beetles and Japanese beetles. Application of rescue treatments may be warranted when beetle populations are clipping silks to less than a half-inch or when five or more beetles are present per plant.

Pre-Harvest Corn Pest Activity

As corn ears mature, two pest problems may appear that may influence ear formation and warrant attention. The most common late-season pest problem is infestation of the ears and stalk by the second brood of the European corn borer. Early detection of significant second brood corn borer infestations is difficult except when preceded by abnormal levels of adult moth flight activity.

If significant infestations of second brood corn borer are detected early, rescue treatment may be warranted to prevent excessive stalk injury and ear infestation. When severe infestations of second brood corn borer activity are detected in the stalks, fields should be flagged for early harvest; significant lodging may occur if harvest is delayed. If the farm or field has a history of significant European corn borer problems, then the use of a Bt hybrid with activity against corn borer might be warranted. If planting is made in late May or early June, research has shown a yield increase when a Bt hybrid for corn borer is planted at that time.

Another pest commonly observed in the ears during the late summer is the corn earworm. Although this pest is regarded as an important pest of sweet corn, it is not generally regarded as a significant pest of field corn warranting rescue treatment.

For more information on managing insect problems and the chemicals labeled for corn insects, see Extension Bulletin 545, Insect Pests of Field Crops, available from the county offices of OSU Extension and on the Internet at: ohioline.osu.edu/b545/index.html.

Disease Control

Major corn diseases in Ohio include leaf blights, stalk rots, ear rots, and kernel rots (Table 4-21). Although some diseases can be controlled by a single practice, such as planting a resistant hybrid, most diseases require a combination of practices to ensure that economic damage is kept to a minimum. Once a disease has been identified, its management depends on understanding its cause(s), the factors that favor disease development, which plant parts are affected, and when the disease organisms are spread. A summary of management practices to prevent yield losses in corn from diseases in Ohio follows:

1. Plant high-quality seed, treated with a fungicide seed treatment, in a well-prepared seedbed (see Table 5-1.) Plant seed 1.5 to two inches deep at rates recommended by the seed company to ensure proper plant populations. When populations are excessively high, the stress caused by plant-to-plant competition may increase stalk rot and lodging.
2. Plant high-yielding hybrids with resistance to leaf blight, ear rot, and stalk rot diseases. Several potentially destructive diseases now cause only minor losses because of the widespread use of resistant hybrids. Review the level of resistance available in hybrids offered by your seed dealer before ordering seed for planting.
3. alanced fertility is the key to vigorous, well-developed plants. High rates of nitrogen, especially when excessive in relation to potassium, favor the development of stalk rot and some leaf diseases. Use recommended levels of N, P, and K based on soil tests.
4. Crop rotation and destroying corn residues by tillage reduce the numbers of disease organisms surviving in the field. However, reduced tillage should be practiced to conserve energy and to protect soil from loss through erosion. When corn is planted after corn, especially under reduced tillage production, these disease-management practices are lost. Other disease-management practices, such as growing highly resistant hybrids, become essential to compensate for this loss.
5. Improve soil drainage in poorly drained soils. This reduces water stress and reduces losses from seedling blights, root and stalk rots.
6. Control insects and weeds in and around fields. Insects such as rootworm and stalk borer create wounds that serve as entry points for fungi causing anthracnose stalk rot. The corn flea beetle serves as the vector of Stewart’s leaf blight bacterium. Some weeds act as reservoirs for corn pathogens. In southern Ohio, eradicate Johnsongrass to eliminate the reservoir for corn viruses and their insect vectors.
7. Survey fields for damage from leaf blight disease by tasseling. Fungicide application may be justified in commercial corn production fields only if susceptible hybrids are grown. Popcorn and inbreds grown for seed production are generally more susceptible to leaf diseases than dent corn and should be scouted for leaf diseases regularly. Visit the Ohio Field Crop Disease web site at www.oardc.ohio-state.edu/ohiofieldcropdisease/ for updated information on fungicide use on corn.
8. Survey fields in the fall prior to harvest to determine the incidence of stalk rot. A rapid and easy technique to determine the incidence of stalk rot is the squeeze method. Grasp the base of the stalk above the brace roots and squeeze the stalk between the thumb and first two fingers. Stalks with significant rot will crush easily. Those fields with the greatest percentage of rotted stalks should be harvested first to avoid losses resulting from lodged corn.
9. Proper adjustment and operation of the combine or picker reduces harvesting losses in the field with stalk-rotted, lodged corn. Some equipment companies have attachments for the combine header to help pick up lodged corn.
10. For long-term storage, dry shelled corn to 13% to 14%. Ear corn to be cribbed should be dried to 20% moisture. Maintain cool and dry storage conditions to prevent storage molds from developing.

For more information on recognizing and managing corn diseases and on mycotoxins associated with moldy grain, visit the Ohio Field Crop Disease web site at: www.oardc.ohio-state.edu/ohiofieldcropdisease/ . These printed bulletins are available at OSU Extension offices: Bulletin 802, Corn Disease Management in Ohio, and Bulletin 639, Seed Treatment for Agronomic Crops.

Harvesting

Harvest date should be determined by crop maturity, not by the calendar. Plan to harvest fields with potential lodging or harvest loss problems (such as stalk rot or deer damage) first. All field shelled corn with more than 15% moisture must be dried for safe storage. The ideal kernel moisture level at which to harvest for dry grain storage is 25%. Corn normally dries approximately 0.75% to 1% per day during favorable drying weather (sunny and breezy) during the early, warmer part of the harvest season —from mid-September through mid-October in central Ohio. By late October to early to mid-November, field dry-down rates usually drop to probably no more than 0.5% per day. By mid- to late November the rate drops to 0.25% per day, and after Thanksgiving, drying rates are negligible (Table 4-10).

Dry-down rates can also be estimated in terms of Growing Degree Days (GDDs). Generally it takes 20 to 30 GDDs to lower grain moisture each point from 30% down to 20%. In September, accumulation of GDDs averages 10 to 15 per day. In October, the accumulation drops to five to 10 GDDs per day. These estimates are based on generalizations, however, and some hybrids may vary considerably from this pattern of dry-down.

Monitoring harvest losses is an important part of the harvesting process. Ear corn losses from in front of the combine (preharvest losses) should be subtracted from the total harvest loss estimate. The loss of one normal-sized ear per 100 feet of row translates into a loss of more than one bushel per acre. An average harvest loss of two kernels per square foot is about one bushel per acre. Keep in mind that most harvest losses occur at the gathering unit. A recent Ohio State study found that approximately 80% of the total machine loss is caused by corn never getting into the combine.

Drought-induced stalk lodging and insect problems reduce the yield potential of many corn fields if harvesting is delayed much beyond maturity (Table 4-10). Ear drop damage may be high in some years as a result of extensive European corn borer damage. Estimates of harvest losses based on long-term average data at Purdue University indicate that losses increase by 1% to 2% for each week of harvest delay. Ear damage by corn borers and other insects may also increase the potential for grain quality problems caused by ear molds.

Shelled grain weights can be adjusted using a grain shrink table (Table 4-22). Shrink represents both the moisture loss and a 0.5% dry-matter loss encountered during dry and grain handling. To estimate the amount that a given wet weight of corn will lose during the drying process, multiply the wet weight by the shrink factor from the table. For example, assume that one ton (2,000 lb) of shelled grain at 25% moisture will be dried to 15.5% moisture. Drying and handling losses are 2,000 lb x 0.1174, or 235 lb. This results in 2,000 lb minus 235 lb, or 1,765 lb of grain at 15.5% moisture. Monitor debris and cracked corn in the grain as harvesting progresses. Debris and cracked corn lower grain quality and increase the potential for spoilage of stored corn. Table 4-23 shows USDA grade requirements for shelled corn.

Test Weight

Test weight of corn determines the weight of a bushel volume (1.244 cu ft) of grain. Test weights determined on dry (15.5% moisture) corn indicate whether the grain crop reached full maturity. Low test weights indicate immaturity. If bushel test weight of mature corn is determined at harvest when grain moistures are greater than 15.5%, the test weights will be biased downward. In other words, as corn grain dries, test weight increases. Differences in test weight influence USDA grading of shelled corn (Table 4-23). The adjustments in test weights do not apply if grain contains more than 10% broken kernels, was damaged by drought or disease, was harvested when immature, or was dried at air temperatures of 180°F or higher.

Ear Corn

Ear corn can be cribbed safely when the grain moisture is 21% or less. However, with cold weather and narrow (four-foot), well-ventilated cribs, corn may be stored when grain moisture is several percentage points higher. Use Table 4-24 to convert ear corn yields to shelled corn equivalents. For example, four tons (8,000 lb) of ear corn at 21% grain moisture is equivalent to 8,000 divided by 77.6 or 103 bushels of shelled corn.

Corn Silage

Corn harvested for silage yields one-third more feed nutrients per acre than corn harvested for grain. Corn in the full dent stage produces 50% more feed than in the milk stage and 100% more feed than in the silking stage. Corn harvested in the milk or silking stage results in poorer quality silage because of its high moisture content.

One of the most important steps in producing quality corn silage is to harvest at the proper moisture. The storage structure determines the proper moisture level at which to harvest.

Desired moisture levels for different structures are as follows:

• Sealed airtight silos—55% to 60%
• Bag silos—60% to 70%
• Upright silos—62% to 68%
• Trench silos—65% to 70%.

Ideally, moisture levels in the silage should be monitored at harvest to prevent harvesting the crop outside the desired range. If moisture testing is not feasible, then estimate the crop moisture by the stage of crop development.

Kernel milk line can serve as an indicator of whole plant moisture levels. As kernels start to dent, a separation between kernel starch and milk can be seen. The firm starch is deposited in the crown (outer) area of the kernel, and the milk occupies the basal area of the kernel. This appears as a whitish line separating the two areas. As the crop matures, this kernel milk line moves down the kernel, and the whole plant moisture declines. When this line reaches the midpoint of the kernel, 90% of the final kernel dry weight has been achieved, and silage yields reach a maximum. At this point, the stover part of the plant has good digestibility, and the moisture is usually in the desired range for storage in airtight silos.

A higher moisture level and a slightly earlier harvest are recommended for bunker silos (full dent to one-quarter milk line) and upright conventional silos (one-quarter to one-third milk line). Harvest time can be predicted by monitoring the progression of the milk line. When the milk line reaches the base of the kernel, a black layer forms and the crop is physiologically mature.

Silage harvest should not be delayed beyond the black layer point because the silage gets too dry, the kernels tend to harden, and the digestibility of the stover declines rapidly. The desired chopping length for corn silage is 5/8 to ¾ inch. If silage is harvested when the crop moisture is lower than desired, consider chopping finer than normal to promote good packing and to minimize air pockets in the silage. For more information on producing silage, consult North Central Regional (NCR) Publication 574, Corn Silage Production: Management and Feeding.

Specialty Types of Corn

The type of corn most widely planted in Ohio and across the United States is yellow dent. High grain and silage yield potential, high feed value, and availability of adapted superior hybrids account for the widespread use of yellow dents. Yellow dents have the highest content of carotene (vitamin A) of the cereal grains. Other types of corn include flint, pop, waxy, and sweet. Each type of corn has unique kernel characteristics that determine its use and how it is grown.

Because most specialty corn hybrids (including white dent, waxy, high oil, and popcorn) are grown under contract, it is advisable to identify a market before planting. Also, some specialty corn processors specify certain hybrids and cultural practices they want growers to use. Contracts for growing specialty hybrids usually offer a premium over the yellow dent price to compensate for the lower yield potential and the special handling required to ensure high grain quality. Most specialty corns must be grown in isolation from conventional corn fields to prevent cross pollination. More information on growing specialty corns for identity preserved (IP) grain production is available online at: http://www.oardc.ohio-state.edu/hocorn/. Some of the more widely grown specialty corns are discussed here.

White Corn—White corn types are equal to yellow types in carbohydrate content but are deficient in vitamin A. White types are grown primarily for direct human consumption in foods such as breakfast cereals and grits. Yields of white hybrids generally are not competitive with yellow dent yields. Typical high-yielding yellow dent hybrids produce 5% to 10% more grain yield than the best white hybrids. White corns must be grown in isolation from yellow dents to prevent cross-pollination. A mixture of white and yellow downgrades either type at the market. Long-term performance data for white corn hybrids is available online at: www.agcrops.osu.edu.

Waxy Corn—The carbohydrate or starch granules of regular dent corn consist of approximately 75% amylopectin and 25% amylose. Waxy corn has nearly 100% amylopectin. Waxy corn was initially recognized as a valuable source of industrial starch. The stability and clarity of amylopectin starch make it highly suitable as a food thickener. Waxy corn has also been considered as a potential animal feed. Feeding trials with waxy corn hybrids have occasionally shown a benefit, but they have not been consistent. Changes in feed efficiency and production have been insignificant, but usually in favor of the waxy corn hybrids. Yields of waxy corns are generally lower than those of yellow dent corn.

Growers should have some estimate of the relative yield performance of specific waxy hybrids before growing any substantial acreage. Waxy corns must be grown in isolation from other corn types to maintain purity standards for industrial use. Keeping the harvested grain separated by 12 to 16 border rows from the rest of the field is considered adequate isolation; 5% contamination with regular corn pollen is considered the upper limit.

High-Lysine Corn—Although normal dent corn contains about 9.4% protein, the quantity of two essential amino acids, lysine and tryptophane, is below nutritional requirements for humans and nonruminant (single-stomached) animals, such as pigs and chickens. High-lysine corn corrects this deficiency and may be advantageous in swine feeding rations. Adapted varieties of high-lysine corn are limited in number and have generally been lower in grain yield than normal dent varieties. The softer kernels of high-lysine corn are more vulnerable to breakage at harvest, which can lead to higher incidence of kernel or ear rot. Production fields must be grown in isolation from other corn fields to ensure protein quality. Cross pollination results in normal dent corn. A separation distance of 300 feet is recommended.

High-Oil Corn—High-oil corn (HOC) is attractive as a livestock feed because it has greater energy value than normal yellow dent corn and can replace more expensive dietary sources of fats and proteins. HOC contains 50% to 100% more oil than normal yellow dent corn, which averages about 4% oil on a dry weight basis. Feeding trials with HOC indicate that it has improved feed efficiency and results in increased rate of gain over conventional corn.

The TopCross grain production system, currently the most widely used method of producing HOC in the United States, involves the planting of a blend of two types of corn. One type, referred to as the grain parent, is a cytoplasmic male sterile (produces no viable pollen) elite hybrid that comprises 90% to 92% of the seed in the blend. The second type, comprising 8% to 10% of the seed in the blend, is a high oil pollinator. The high oil pollen contains genes that cause production of kernels with larger than average embryos. Because most of the oil and several essential amino acids are in the embryo, the increased embryo size of HOC results in greater oil content and enhances grain protein quality.

Following the introduction of the TopCross system, HOC production in the United States increased from less than 50,000 acres in 1992 to more than one million acres in 1999. However, since then, HOC production has dropped sharply with acres planted in 2002 estimated at less than 500,000. A major factor contributing to this decline has been the availability of cheaper sources of oils and fats which compete with HOC as an energy source in livestock feed rations. In TopCross HOC production, the minimum isolation distances (between HOC and normal corn fields) recommended by seed companies range from 60 to 200 feet.

Popcorn—Popcorns are essentially small-kerneled flint corns and are among the most primitive of the surviving races of corn. Kernels contain a hard endosperm with only a small portion of soft starch. Popcorns are generally either pearl or rice types. Pearls have smooth, rounded crowns, and rices are pointed. Heating the kernel turns the moisture inside the soft starch in the center into steam that explodes the kernel inside out. The greater the expansion, the higher the quality. Hybrids differ as to kernel quality, which also includes flavor, tenderness, absence of hulls, color, and shape. Popcorn is one of America’s most popular snack foods. American consumers eat, on average, 17 billion quarts of the healthy treat each year. Popcorn is not only high in fiber, potassium, and vitamins, it is also low in calories and fat. A cup of unsweetened popcorn contains between 31 and 55 calories.

Ohio ranks in the top five of 25 popcorn-producing states. Ohio harvests about 67,000 acres of popcorn annually. Van Wert County leads the state in popcorn production with more than 30% of those harvested acres and accounts for 3% of all popcorn produced in the United States.

Popcorn hybrids usually yield less than half of normal dent hybrids. To achieve maximum quality, minimize mechanical damage and dry with low heat to a moisture of 13.5%. Overdrying and kernel damage result in reduced popping volume. Handling and quality are extremely important aspects of popcorn production. From an agronomic standpoint, popcorn must be planted to mature before frost, and herbicide programs must be labeled for popcorn. Use a fertility program for a 150 bushel per acre yield goal and pay extremely close attention to potassium fertility to guard against poor stalk quality and lodging. Popcorn production fields do not have to be isolated from other types of corn, even if the popcorn is not dent-sterile. However white and yellow popcorn should not be planted within 0.5 mile of each other to minimize cross-pollination.

Most popcorn in the United States is grown under contract for processors and companies. Growers producing popcorn commercially generally follow cultural practices and plant popcorn hybrids as specified by these companies. Information on popcorn production from Extension and university sources is available online.

• NCH-5, Popcorn Production and Marketing
• Popcorn Production, University of Nebraska
• Alternative Field Crops Manual, Popcorn, University of Wisconsin
• Agricultural Marketing Resource Center-Popcorn

Isolation Requirements for Identity Preserved (IP) Non-GMO Corn Production

Managing pollen drift is an important consideration in the production of specialty corns and non-GMO corn as IP grain crops. Corn is a cross-pollinating crop in which most pollination results from pollen dispersed by wind and gravity. Although most of a corn field’s pollen is deposited within a short distance of the field, with