Ohio Agronomy Guide, 14th Edition

Bulletin 472-05

Chapter 5: Soybean Production

By Dr. Jim Beuerlein and Dr. Anne Dorrance

NOTE: Material in this chapter related to pesticides may not be valid after 2005. Please contact the county office of Ohio State University Extension or the Agronomy Team web site at www.agcrops.osu.edu for current information.

The major objective of a crop production system is the interception, fixation, and storage of sunlight energy. There are many components of a system that will accomplish that objective. The most important are early planting; narrow rows; productive varieties that resist disease; control of weeds, insects, and diseases that rob energy from the system; and availability of soil nutrients in adequate amounts. Other inputs to the system must not limit energy fixation or slow the process. The effects, interactions, and relationships of various inputs of an efficient soybean production system are discussed here.

Variety Selection

Most soybean varieties have genetic yield potentials well over 100 bushels per acre. A variety’s adaptability to the environment and production system where it will be used sets the yield potential of the production system. The quality of the weather during the growing season and the stresses from weeds, diseases, and insects determine what the crop yield will be. A variety’s performance in a previously conducted yield trial is a measure of its performance in that particular environment and production system and does not assure satisfactory performance under a different set of conditions.

When a group of varieties is tested for yield over a range of environments, their rank order commonly changes, which indicates that some varieties are better adapted to a specific environment than others. Therefore, it is best to select varieties with characteristics that will help them perform well in the cultural system and environment to be used, rather than on their yield record alone. For example, if excessive growth and lodging are problems, then select varieties that are medium to short in height with good standability. If the field has a history of Phytophthora, then select a variety with a resistance gene plus a high partial resistance rating to address that problem. The selection of medium or small seed when using a grain drill will improve metering and stand uniformity.

Alternatively, one can select the varieties that performed best at a test site that is similar to the field for which a variety is being selected, or select a variety that has performed well over several test sites that vary widely in yield potential. Maturity information should be used to select varieties that mature at different times to allow for timely harvest and high test weights. Generally, each 10-day delay of planting in May delays maturity three to four days in the fall. For best yields in wide rows, select full-season varieties with a bushy growth habit. Growth habit is not important in narrow rows. Fitting the variety to the environment is superior to selecting a variety and hoping the environment and weather will fit it.

Variety Performance Trials

Figure 5-1. Ohio Soybean Variety Performance Trial plots.

The purpose of the Ohio Soybean Performance Trials is to provide an unbiased evaluation of variety characteristics and performance to facilitate the selection of varieties appropriate for particular production sites and systems. Field trials are conducted at six locations representing the diverse production regions of Ohio, and several laboratory tests are conducted at the Ohio Agricultural Research and Development Center (OARDC) in Wooster, Ohio. Data is collected on yield, lodging, relative maturity, seed size, plant height, oil and protein content, and resistance to Phytophthora root rot. The data for up to 200 entries are published each December as Horticulture and Crop Science Series 212 entitled Ohio Soybean Performance Trials. Details of testing and evaluation procedures are included in the bulletin, which is available, free of charge, from county Extension offices and on the Internet at: www.agcrops.osu.edu .

Disease Control

Figure 5-2. Ohio Soybean Performance Trial Bulletin showing test regions and test sites.

Early Season and Seed-Borne Diseases

Phytophthora root and stem rot is the most serious soybean disease in Ohio and is present everywhere soybeans are grown. Damage to the crop by Phytophthora is most prevalent in fields with poor drainage, high number of years with soybeans, and reduced tillage systems. Varieties are susceptible at all stages of growth. Saturated soil with a temperature above 60°F provides the ideal conditions for infection. Susceptible varieties should not be grown in poorly drained soils or on soils known to have a history of the disease. Seed of varieties with good partial resistance (score of 2 to 4) should be treated with a fungicide (Apron XL or Allegiance FL) that aids in the control of Phytophthora damping off. Varieties with Rps resistance gene(s) should also be treated to control Phytophthora damping off because these Rps genes are not effective in every field nor across the whole field. Planting early, well before soil temperatures reach 60°F, often allows varieties with high levels of partial resistance to escape early infection if the soil does not become saturated.

Pythium and Rhizoctonia root rots are also common in Ohio, and most varieties are susceptible. Damage to plant stands is greatest on poorly drained soils and during seasons of high rainfall. Pythium is controlled by Apron XL and Allegiance FL seed treatments, and seedling infections of Rhizoctonia are controlled by a number of seed treatment materials (see Table 5-1). Also see Ohio State University Extension Bulletin 639, Seed Treatment for Agronomic Crops, or the Internet at www.ohioline.osu.edu/b639/b639_17.html for additional information.

Phomopsis seed rot can be severe when rainfall occurs intermittently during grain drydown and harvest. The longer soybeans are in the field after ripening, the greater the incidence of seed rot. Harvesting soon after the soybeans mature (15% to 20% moisture) decreases the amount of seed damage. Using varieties with a range of maturities allows for a more timely harvest of each field. Many varieties are resistant to Phomopsis seed rot; if Phomopsis develops in a variety, look for a different variety for future years. Fungicides that are currently labeled for soybeans have little activity for Phomopsis and are generally not economical. Crop rotation and tillage are excellent management tools for this seed-rot pathogen as it survives on old crop residue.

Phomopsis seed rot can reduce overall germination in certain seed lots and Phytophthora, Pythium, and Rhizoctonia can kill seeds and seedlings after they are planted. One of the management tools for these seed and soil-borne plant pathogens is to use fungicide seed treatments. Table 5-1 lists the efficacy of many of these compounds. Note, no one fungicide is highly effective for all pathogens. Choosing a mix of several compounds will provide broad spectrum control. Seed treatments are best used on fields with poor drainage, a history of stand establishment problems, and reduced tillage systems.

Mid-Season to Late-Season Soybean Diseases

Soybean cyst nematode (SCN) now exists in production fields throughout Ohio. In some fields, the population of SCN is currently quite high (> 10,000 eggs per cup of soil). Populations of SCN may take eight to 10 years from introduction to reach damaging levels throughout a field. In a variety test in west central Ohio on a fertile, dark-colored soil, varieties resistant to SCN yielded more than 50 bushels per acre, whereas those susceptible to SCN yielded from 24 to 39 bushels per acre. Although these studies were conducted in problem fields, the estimated yield loss from SCN in other Midwestern states is 8% to 12%.

In the vast majority of fields in Ohio, SCN causes no above-ground symptoms. The only difference that growers will see is that yields may be five to 10 bushels less than fields with similar yield potential. In more severe situations, where SCN populations are high, injury is easily confused with other crop production problems, such as nutrient deficiencies, injury from herbicides, soil compaction, or other diseases. The first field symptoms are usually detected in circular to oval patches of stunted, yellowed plants. Symptoms are most evident in late July or August when plants are under drought stress or in fields with low fertility. When populations of nematodes are high, the symptoms may even occur under normal to optimal growing conditions. Affected areas of a field may increase in size each year in the direction of tillage. In these affected areas, SCN females can often be found feeding on the roots.

SCN is best managed with crop rotation, rotating non-host crops such as wheat, corn, alfalfa, or red clover and rotating sources of SCN resistance. Never plant a SCN-resistant variety without checking your SCN population levels first. When a non-host crop is planted, SCN populations will decline by as much as 50% annually. SCN resistance is measured by a reduction in the number of females that feed on roots, but a few females will reproduce. Thus, over time, populations will adapt to these sources of resistance and reproduce in increasing numbers. Sources of resistance that are currently available to soybean producers include PI88788, PI437654 (Hartwig came from this source), and Peking.

To determine what your SCN levels are, soil samples should be taken from the top four inches of soil. Each field should be divided into sections not exceeding 10 acres, and each section should be sampled by taking 15 to 20 subsamples in a zigzag pattern. This level of sampling is necessary to obtain relatively accurate counts of the nematode population and to make meaningful recommendations for control. The soil samples should be moist, but not wet; packaged in double plastic bags; and protected from becoming too warm. Mail samples to the C. Wayne Ellett Plant and Pest Diagnostic Clinic, The Ohio State University, 110 Kottman Hall, 2021 Coffey Road, Columbus, OH 43210. The telephone number for the clinic is 614-292-5006.

Phytophthora stem rot will continue to infect plants throughout the growing season. This late-season phase of the disease can only be found in fields where heavy rains or saturated soils have occurred and with varieties having ineffective Rps genes and low levels of partial resistance. If the Rps genes are effective against the P. sojae population, then no disease will develop; however, if they are no longer effective, then stem rot will develop. We have found from a number of years and locations that varieties with high levels of partial resistance (scores 3.5 to 5.5) rarely develop stem rot. One reminder is that not all seed companies use the same scoring system; at The Ohio State University, 3.5 means very little root rot, and 9 indicates that all plants are dead.

Sclerotinia stem rot is present throughout most of Ohio and may be severe (50% of plants in a field infected) when wet weather occurs prior to and during flowering. Varieties with resistance to Sclerotinia have fewer numbers of plants infected but all are susceptible to some degree. Stem symptoms first appear as water-soaked lesions followed by cottony growth and, eventually, black irregular-shaped sclerotia which resemble mouse droppings in form. Wide rows (30 inches) aid in control by permitting air to move through the canopy to dry plant leaves and the soil surface but also reduce yield due to less sunlight fixation. Reduction in plant populations (160,000 to 180,000) and planting in 15-inch rows can reduce the overall incidence of Sclerotinia stem rot without negatively impacting yields.

Brown stem rot can severely reduce yield. This fungus enters the plants through the roots and slowly colonizes the stem and the xylem, where it interferes with water transport. The disease symptoms develop after flowering and are identified by an internal browning of the stem in August. Foliar symptoms are rarely seen in Ohio, but the leaves of infected plants may suddenly wilt and dry 20 to 30 days before maturity and drop from the plant. Crop rotation is an excellent control for this disease.

Sudden Death Syndrome (SDS) is another late-season disease that appears to always be associated with soybean cyst nematode and areas of the field with very poor drainage. Symptoms are very similar to brown stem rot in that brown spots develop in the leaves between the veins, and this is surrounded by a bright yellow chlorosis. In SDS, the roots are very degraded along with the crown. One of the key diagnostic tools is the color of the pith, which remains white and healthy with SDS and is brown and decayed with brown stem rot. This fungus survives in the soil for long periods of time, so to prevent rapid build-up of the pathogen, crop rotation and improving soil drainage are key.

Crop Rotation

Crop rotation is the most effective pest-control practice available to crop producers. The sequence of crops grown in a field affects the productivity of each crop. Research from most Midwest states indicate that a soybean crop following a crop other than soybeans will usually yield about 10% more grain, on average, than when soybeans follow soybeans. Many of the crop disease and insect problems currently experienced in Ohio are due to short crop rotations or no crop rotation. If all our crops were produced in a four-year crop rotation, yield loss to disease and insects would be near zero rather than at the 8% to 12% we currently experience.

Figure 5-3. Effect of length of crop rotation on percent crop yield.

The effect of crop rotation on yield has been thoroughly investigated by most Land-Grant Universities. Researchers at the University of Wisconsin conducted a crop rotation study with corn and soybeans. Averaged across 29 years by location environments, corn and soybeans in their corn-soybean rotation resulted in 13% and 11% greater yield than the respective monoculture. Kansas State University conducted a 20-year crop rotation study in which the soybean yields were 20% greater when rotated with wheat or grain sorghum than without rotation. Results of a Canadian crop rotation study showed that soybean yields from a wheat-corn-corn-soybean rotation were 7.1% higher than in a corn-corn-soybean rotation. Results of a 10-year crop rotation study conducted in northern Ohio indicated that continuous corn yielded only 89% as much as corn in a corn-oats-hay rotation, and corn in a corn-soybean rotation yielded 94% of the corn-oat-hay rotation.

Economics often dictates crop sequence, but where choices are available, soybeans should follow crops other than soybeans, sod, or legume crops like alfalfa. Corn or other grass crops can make good use of the nitrogen left by legume crops and sod. The effect of the length of a crop rotation on yield can be seen in Figure 5-3.

Tillage

Tillage disrupts soil aggregates, and repeated disruption destroys soil structure. It also causes a long-term decline in soil organic matter, which further destabilizes soil structure. Tillage disrupts the continuity of large soil pores and restricts the movement of water through the soil profile, creating soil drainage problems. Repeated use of tillage tools operating at the same depth or when the soil is too wet results in the formation of compacted zones which restrict both water movement and root development.

Secondary tillage operations performed to prepare fine seedbeds usually cause the formation of impermeable crusts on light-colored silt loam soils such as Blount and Crosby. These crusts reduce seedling emergence, air exchange, and water intake, all of which reduce yields. When thick crusts form, disrupting them by rotary hoeing or cultivation often improves yield, particularly in dry years.

Tillage is one of the largest out-of-pocket expenses used for crop production and often does not generate enough yield to make the tillage profitable. While no-tillage can reduce production costs and increase profits, it also creates problems that producers must solve with proper management of the other inputs and production practices. Some of these problems are colder, wetter soil at planting, more root rot disease, slower emergence and growth, dealing with crop residues and the diseases they contain, and more. There are times when tillage is warranted and will likely be profitable. Here is a partial listing of some of the situations when tillage may be needed:

1. Use tillage when inadequate soil drainage leads to serious yield loss due to root rot diseases, poor stand establishment, or late planting.
2. Use tillage to bury crop residue and thus reduce pathogen and insect survival that can infect a following crop.
3. Use tillage as a prelude to land leveling, rock removal, and for the incorporation of soil amendments, such as lime or very high rates of fertilizer.
4. Use tillage to mitigate compacted soil layers or zones that interfere with water movement into and through the soil, which may delay planting, harvesting, and other field operations.

Table 5-2 contains the cost to perform various tillage operations and the yield increases needed to pay for those operations.

Producing Soybeans Without Tillage

Growing soybeans in Ohio without tillage has become both practical and profitable and often reduces or eliminates some tillagerelated problems. Time savings accrued by eliminating tillage can be invested in earlier and more careful planting or the planting of more acres. Maintenance of crop residue on the soil surface reduces soil crusting, which can lead to better and more uniform seedling emergence; improved yields on some soils; and reduced needs for rotaryhoeing, cultivating, and replanting. In addition, notillage systems do not bury weed seeds, reducing the germination potentials of some species, particularly largeseeded broadleaf weeds. Finally, use of notillage systems can prolong the life of surface drainage improvements, particularly on flatter fields.

When planting notillage soybeans, growers should pay attention to soil drainage; planting procedures; crop rotation options; and disease, insect, and weed control. While the improper management of any of these factors will reduce yields in tillage systems, their effects can be much more adverse with no tillage. Problems created by removing tillage from the crop production system and important adjustments that must be made to offset those negative effects are presented here:

1. Cooler soil temperatures slow germination, emergence, and early growth.

   Because the soil is warmer at the surface than at the 1.5-inch planting depth, the solution is to plant shallow (one inch), but in moist soil. The warmer soil temperatures at shallow depth will enable seeds to germinate and emerge earlier and, in effect, produce a closed leaf canopy and get to the reproductive stage sooner. The use of narrow rows (7.5 inches) will compensate for the slower early growth associated with no-till production. Good seed-to-soil contact and the use of high-quality seed treated with the appropriate fungicides promote the rapid emergence of a healthy crop. Slower planting speed will allow the planting tool to space seed more uniformly in the row and at a more uniform depth, so that seeding rates can be reduced and production costs can be lowered.

2. Root rot diseases are much more severe due to a wetter and cooler soil environment.

   Two actions can increase plant stands and improve root health:

   • Select varieties with high levels of Phytophthora partial resistance that will give a good level of protection against all strains of Phytophthora.
   • Treat the seed with Apron XL or Allegiance to protect the seedling from Phytophthora and Pythium root rot until the partial resistance mechanism takes effect just after emergence.

   Another strategy is to use soybean varieties that have one or more Rps resistance gene(s) for control of Phytophthora root rot. Other broad spectrum fungicides will control other diseases that damage the root system and lower stand counts. Do not use no-till culture on poorly drained fields, and do not plant when the soil is too wet for shallow tillage. Tillage or planting operations on a wet soil compact the soil particles, which inhibits the proper development of root systems and thus reduces yield. For additional information on controlling soybean diseases, see Profitable Soybean Disease Management in Ohio, Bulletin 895, at: ohioline.osu.edu/b895/ .

3. Heavy crop residue and more dense soil can interfere with proper seeder function and lead to poor distribution, poor placement of seed, and lack of adequate depth control.

   Spreading crop residue evenly when harvesting will help keep the field surface uniformly covered with residue and at uniform moisture so the entire field is ready to plant at the same time. Due to its fineness, wheat residue keeps the soil colder and wetter than other residues because it provides nearly 100% cover and its light color reflects sunlight. Remove wheat straw when possible and partially incorporate the stubble with a disk to promote its degradation.

   Try not to plant on old corn rows since old corn roots interfere with depth control and seed placement. Maintain a down pressure of at least 200 pounds on each row opener to penetrate hard soil areas and use a good depth-control mechanism to maintain the proper seeding depth in soft soil. A residue cutting colter will prevent hair pinning of residue into the seed furrow, which interferes with seed placement, and it will also loosen some soil that the furrow closers can use to cover seed and improve seed-to-soil contact.

Rhizobium Inoculation

Fifty-eight soybean inoculation trials were conducted between 1995 and 2004. During that time, the average yield increase due to inoculating was about 2.0 bushels per acre. The cost of inoculating an acre of soybeans is $2 to $3, depending on the product and the rate used. If soybeans are worth $7 per bushel, the per-acre profit for inoculating soybeans would be about $11 to $12 per acre or a return on investment of more than 300%.

Dry and liquid formulations of the same product appear to perform similarly. Once the carrier of the inoculum dries on the seed, the bacterial cells start dying. Inoculated seed should be planted as soon as possible after treatment (12 hours or less), so the bacterial cells will remain moist and survive long enough to infect soybean roots following germination.

When applying a fungicide or using fungicide-treated seed, be sure the fungicide has dried before applying inoculum to the seed. Currently (2004), most inoculation products may NOT be mixed with fungicides and applied to seed together. One exception is that liquid formulations of inoculum may be mixed with ApronMaxx RTA fungicide and applied together. Work is underway to develop formulations of additional fungicides that can be mixed and applied with inoculation materials. Development of inoculation materials that can be applied up to several weeks before planting is also underway.

When loading a drill or a planter using an auger, liquid or dry inoculation materials should be added to the seed as it enters the auger for thorough application. When loading a planter or drill from bags, fill the seed box to a depth of three inches and scatter an appropriate amount of inoculum over the seed and mix thoroughly. Continue to add seed in six-inch layers, treating each until the box is filled. With some dry materials, it may be necessary to slightly moisten seed to increase adherence. A few small specks of inoculum on each seed is adequate. At the recommended use rate, there will be more than 500,000 bacterial cells on each seed. Excessive amounts of inoculum on seed can reduce seed metering by up to 35%. Seeding equipment should be calibrated using the treated seed to be planted. Some seeding-rate monitors allow a continuous check of seeding rates, so adjustments can be made to the seeding rate if and when necessary.

When soybeans are planted in a field for the first time, it is not uncommon for even the most ideal inoculation procedures to be less than adequate for producing enough nitrogen for a good crop. When the nodules are insufficient to supply adequate nitrogen, it will be necessary to supply some nitrogen to the crop. In this event, one application of 75 pounds actual nitrogen as urea can increase yields by 8 to 12 bushels per acre. This supplemental nitrogen should not be applied until flowering, which is usually late June and July depending on variety maturity, date of planting, and the weather. To assure the establishment of a reliable inoculation for future years, it is advisable to grow soybeans in a new field for two successive years and to inoculate the seed thoroughly both years.

For satisfactory nitrogen fixation in eastern Ohio where soils tend to be more acid, the pH in the plow layer should be above 6.5, and the percent base saturation of calcium and magnesium should be greater than 40% and 10%, respectively. On fields where the lime requirement is very high, a shallow incorporation (two to four inches) of two to four tons of dolomitic limestone will aid in the establishment of bacterial colonies on the root system. Dolomitic limestone should be used whenever magnesium levels are lower than 10% base saturation.

Planting Date

Figure 5-4. Effect of planting date on soybean yield.

The date of planting has more effect on soybean grain yield than any other production practice. The results of numerous research studies conducted in Ohio since 1970 are presented in Figure 5-4. In southern Ohio, soybeans should be planted any time after April 15 when soil conditions are suitable. In northern Ohio, planting should begin the last few days of April, if soil conditions are satisfactory. Yield loss resulting from delayed planting ranges from one-fourth bushel to more than one bushel per acre per day, depending on the row width, date of planting, and plant type.

Regardless of planting date, row width, or plant type, the soybean crop should develop a closed canopy (row middles filled in) prior to flowering or by the end of June, whichever comes first. Generally, when planting in early May, rows must be less than 15 inches apart to form a canopy by late June (Table 5-3). An early canopy results in high yields because more sunlight is intercepted and converted into yield than when row middles do not fill in until late in the growing season. Assuming a half bushel per acre per day yield loss with delayed planting, a 10-day delay in planting 300 acres would decrease total production by 1,500 bushels, which is worth approximately $10,500.

Adequate, vigorous stands are sometimes more difficult to obtain with early planting. Seed treatments, good seed-soil contact, and reduced seeding depths, however, aid in establishing vigorous stands. Herbicide programs must provide weed control for a longer time until the crop is large enough to suppress weed growth through competition. Narrow rows provide the needed competition for weeds sooner than wide rows.

Late Planting

Late planting reduces the cultural practice options for row spacing, seeding rate, and variety maturity. The row spacing for June planting should be no greater than 7.5 inches. Appropriate seeding rates for the first half of June are about 200,000 to 225,000 seeds per acre. For the last half of June, 225,000 to 250,000 seeds per acre is recommended, and in early July, the recommendation is 250,000 to 275,000 seeds per acre.

Relative maturity (RM) has little effect on yield for plantings made during the first three weeks of May, but the effect can be large for late plantings. During the first half of June, a four-day delay in planting delays physiological maturity about one day. In the last half of June, it takes a five-day planting delay to delay physiological maturity one day. As planting is delayed, yield potential goes down, and there is concern about whether late maturing varieties will mature before frost.

When planting late, the rule of thumb is to plant the latest-maturing variety that will reach physiological maturity before the first killing frost. The reason for using late-maturing varieties for late planting is to allow the plants to grow vegetatively as long as possible to produce nodes where pods can form before vegetative growth is slowed due to flowering and pod formation. More nodes equates to more pods and more yield. Late-maturing varieties are needed that will mature before getting frosted, but since the first frost date is unknown, we use a narrow range of maturity that will not be damaged by frost occurring at the normal time.

The recommended relative maturity ranges in Table 5-4 assume normal weather and frost dates, so varieties with those relative maturities should mature before frost and produce maximum possible yields when planted on the dates indicated. Varieties with an earlier relative maturity will mature earlier but will produce reduced yields.

Row Spacing

Figure 5-5. Effect of row spacing on soybean yield.

Most Ohio soybeans are planted in narrow rows (seven to 15 inches). Soybeans grown in narrow rows produce more grain because they capture more sunlight energy, which drives photosynthesis. Within limits, as sunlight interception increases, so does yield. Researchers have learned that the peak demand for the products of photosynthesis occurs during the reproductive stage. Therefore, the row width should be narrow enough for the soybean canopy to completely cover the interrow space by the time the soybeans begin to flower (June 20 to July 10). The row widths that will accomplish that goal will vary with soil type, planting date, weather conditions, and, in some cases, variety.

The later in the growing season soybeans are planted, the greater the yield increase due to narrow rows. The response to narrow rows is also greater for short varieties and when growing conditions cause plants to grow slowly or be short. The effect of row spacing on the yield for early-planted soybeans can be seen in Figure 5-5. Planting systems using precision seed metering to achieve uniform seed spacing within the row plus uniform depth of seed placement usually produce higher yields than planting systems with less uniform seed spacing and variable depth of planting.

Skip-Row Planting Systems

Figure 5-6. A soybean field with 15-inch rows and skip rows spaced 10 feet apart to accommodate application of pesticides.

Modern production systems rely on postemergence herbicide application for weed control and postemergence insecticide application for insect control. This reliance necessitates the use of large equipment for pesticide application in soybean fields during the growing season. Many modern applicators are equipped with narrow tires so that crop damage is minimal early in the season when plants are small, but plant damage increases as plant size increases. Skiprow planting patterns (Figure 5-6) enable the application of pesticides through most of the growing season without causing crop damage. Skiprow systems usually consist of narrow rows with strategically placed wider row spacing to accommodate application equipment tires.

Yield loss due to these wider row middles is usually too small to measure (0.05 bushels per acre). For example: A 60-foot spray boom covers 96 rows spaced 7.5 inches apart. Leaving two of the 96 rows unplanted would reduce the yield by 2.1% times the yield difference between 7.5-inch and 15-inch rows. This reduction amounts to, at most, two bushels per acre or 2.5 pounds of grain, which is worth about 30 cents. The yield loss due to running over two rows of soybeans in August is usually $4 to $8 per acre, depending on the yield potential, value of grain, and working width of the sprayer. For example, 2.1% times 50 bushels per acre is 1.05 bushels per acre worth $7.35 if we lose all the grain from the two rows run down in August. Additional advantages of skip-row planting systems are the elimination of application skips and overlaps, timely applications, and the adoption of GPS and automated guidance systems. When custom application is planned, consult the applicator for acceptable widths of skip rows, their spacing, and frequency across the field.

Plant Population

When the soybean crop is planted in rows spaced 7.5 inches apart, the effect of plant population on yield is very small over the normal range of seeding rates and for any particular set of conditions. For a crop planted before May 20 in narrow rows, final populations of 100,000 to 120,000 plants per acre are adequate for maximum yield. Final populations for mid-June plantings should be in the range of 130,000 to 150,000 plants per acre. Final populations for early July plantings (double crop) should be greater than 180,000 plants per acre. Final population is a function of seeding rate, quality of the planting operation, and seed germination percentage, and depends on such things as soil moisture conditions, seed-soil contact, disease pressure, fungicide seed treatments, and more. Final harvest stands are typically 60% to 80% of the seeding rate when high-quality seed is used, and there are few impediments to stand establishment.

Some seed is dead when planted; other seeds may not have the vigor needed to emerge and grow rapidly; some will be lost to disease and insects prior to emergence; some emerged plants will be killed by disease; and some will not grow fast enough to compete adequately for sunlight and thus perish during the growing season. The effect of soybean seeding rate on yield and harvest population for two very different environments can be seen in Figure 5-7. For both environments, there were six test sites containing two varieties with four replications each, or a total of 48 plots for each of the six seeding rates. Plant growth and size was much greater in the good environment than in the poor environment. The increased plant size and competition between plants resulted in fewer plants surviving to harvest, and there was little response to increased seeding rates. For this environment, the most profitable seeding rate was about 150,000 seeds per acre, which resulted in a harvest population of 110,000 plants per acre.

Figure 5-7. The effect of soybean seeding rate on harvest population, yield, and profit for good and poor growth environments.

In the poor growth environment, there was less vegetative growth, and the reduced competition between plants resulted in increased harvest populations. Yields were much lower and generally decreased as the plant population decreased. In the poor growth environment, the most profitable seeding rate was just over 200,000 seeds per acre and resulted in a harvest population of about 180,000 plants per acre. A good rule-of-thumb for seeding rate is: Where soybean plants grow to 40 inches tall, plant 125,000 seeds per acre; where they grow to 30 inches tall, plant 175,000 seeds per acre; and where they grow to only 20 inches tall, plant 225,000 seeds per acre.

Replanting

Sometimes, plant stands are reduced by disease, herbicide injury, hail, insects, and flooding. If crop insurance covers the damage, consult the insurance agent before taking action. When all plants of a field are lost, it is realistic to replant if adequate growing season remains for the crop to mature. Areas of fields may be replanted while leaving the remainder of the field as is, and areas of inadequate stands can be thickened by inter-planting additional seed. If the stand loss is random or erratic throughout the field, a stand count should be taken to determine the number of plants remaining. For dark soils, do not inter-plant more seed unless the number of plants per foot of row is less than 45% of the recommended seeding rate for the date on which replanting could be accomplished (see Table 5-5). For light-colored soils, do not inter-plant unless the number of plants per foot of row is less than 60% of the recommended seeding rate for the date on which replanting could be accomplished.

For example: A stand count reveals that for most of the field there are about 1.5 plants per foot of 7.5-inch row. The date is June 10 and a replanting can be made on June 15 when the recommended seeding rate for 7.5-inch-wide rows is 2.8 seeds per foot of row. If the soil in the field is dark in color and good vegetative growth is anticipated, then replanting would likely not be profitable. However, for a field with light-colored soil or where plants will likely be small, inter-planting may be warranted, and the inter-plant rate should be 1.65 seeds per foot of row. That replant rate is the difference in the current stand and the recommended seeding rate for the date on which the inter-seeding could take place, plus about 10% to compensate for the plants killed while inter-planting. For example:

• The recommended seeding rate for June 15 is 2.8 seeds per foot of 7.5-inch row.
• There are currently 1.5 plants per foot of row.
• 2.8 seeds per foot - 1.5 plants per foot = 1.3 plants per foot of row needed
• 1.3 seeds per foot x 110% = 1.43 seeds per foot of row or about 100,000 seeds per acre
• or 40 pounds of seed if there were 2,500 seeds per pound.

If low plant populations are due to root rot diseases, the guidelines for replanting also include planting a variety with disease resistance genes or partial resistance plus the use of a fungicide seed treatment containing either Apron XL or Allegiance. See Profitable Soybean Disease Management in Ohio, Extension Bulletin 895, which is available from county Extension offices or on the Internet at: ohioline.osu.edu/b895/ .

Planting Depth

One inch to 1.5 inches is the ideal planting depth where tillage is used. Where tillage is used, the soil should be free of large clods to ensure good seed-soil contact and good seed coverage. Shallow planting (Ύ to one inch) in late April promotes more rapid emergence than deeper planting. However, be aware of the increased exposure to herbicides, which may damage young seedlings. In late April soil temperatures at one-inch depth are three to eight degrees warmer than at two-inch depth. After May 15, the air temperatures are higher, and the probability of crusting increases.

It is a poor practice to plant deeper than one to 1.5 inches, because a crust may form above the seed and reduce emergence. It takes the combined pressure of many plants to break through the crust. In the process, many of the hypocotyls are broken, and the seedlings do not emerge. When planted at a one-inch depth, the seed is more likely be inside the crust layer. As the seed swells in the germination process, the soil crust is broken, and a higher percentage of plants emerge. On some crusting silt loam soils, deep planting results in 25% to 50% mortality during emergence. Where soil crusting is a problem, no-till planting and crop residue are preferred. Adequate crop residue prevents the formation of soil crust and aids in stand establishment. Three-fourths-inch to one-inch seeding depth is ideal for no-till seeding.

Fertilization Recommendations

For optimal yields on mineral soils with subsoil pH greater than 6.0 (generally western Ohio), the pH range should be maintained between 6.2 and 6.8. On mineral soils with subsoil pH less than 6.0 (generally eastern Ohio), the range should be higher (6.5 to 6.8).

Lime should be added to soybean fields when pH levels drop below the optimal range. A soil test will be necessary to calculate lime requirements. Lime may be applied anytime for recommendations of two tons or less. Fall applications will allow time for lime to raise the soil pH before spring planting. Split applications will be required for recommendations larger than four tons per acre—half before plowing and half after plowing. Regardless of the recommendation, no more than eight tons of lime should be applied in one season. Lime application rates for no-till fields should be one half of recommendations given for a tilled field sampled to an eight-inch depth.

Nitrogen (N)

Soybeans, like other legumes, have the ability to form a symbiotic relationship with nitrogen-fixing bacteria. These bacteria can fix adequate atmospheric nitrogen to produce a yield of 50 to 60 bushels per acre. Little benefit has been obtained from adding nitrogen to wellnodulated soybeans. Soybeans adjust to early-applied nitrogen by fixing less nitrogen from the atmosphere; applications after flowering have not shown a consistent or predictable yield advantage.

Soybeans also do not respond to starter nitrogen (most soils have the ability to provide adequate nitrogen until the Bradyrhizobia bacteria infects roots and forms nodules). Bacterial infection occurs soon after emergence, and nitrogen fixation begins as early as Growth Stage V2 (second trifoliate leaf).

Yield-limiting deficiencies of nitrogen are uncommon in soybeans. Deficiencies may occur temporarily during extended cool and/or wet soil conditions after planting. These short-term situations should not lower yields, and nitrogen fixation will quickly resume with warmer temperatures and drier soils. Deficiencies seldom occur later in the growing season. However, disease, such as soybean cyst nematode, or extended hot and dry weather may limit the ability of plants to absorb nutrients and produce symptoms that resemble nitrogen deficiency.

Nitrogen fertilizer may be necessary the first time soybeans are planted in a field, even when seed inoculation is used. If the crop does not have a dark green color by early July, 75 pounds of nitrogen per acre should be applied as urea. To ensure a reliable source of inoculation in new fields, soybeans should be grown for two years, and the seed inoculated each year.

Phosphorus (P)

Soybeans require relatively large amounts of phosphorus. It is not unusual for a 60-bushel-per-acre crop to contain 48 pounds of phosphate (P₂O₅) in the grain. Although phosphorus is taken up throughout the growing season, the period of greatest demand occurs during pod development and early seed fill (Growth Stages R3 – R5). Deficient plants seldom exhibit specific leaf symptoms. Generally, phosphorus-deficient plants will be stunted, a symptom easily confused with disease and environmental stress symptoms. Plant and soil tests are the most reliable methods to ensure against phosphorus deficiency. Soil-test phosphorus levels should be maintained between 15 and 30 parts per million. Phosphate recommendations are based on the yield potential of the field and the corresponding phosphorus levels from a recent soil test (Table 5-6).

Potassium (K)

Soybeans require large amounts of potassium. It is essential for vigorous growth, yet never becomes a part of protein molecules and other organic compounds. Potassium is not involved extensively in biological activities in the soil. Most of the total plant potassium will be in the seed at maturity (1.4 lbs per bushel). Deficiencies are not common but are easy to recognize by yellow leaf margins.

Soil Cation Exchange Capacity (CEC) may affect potassium availability, so the critical level increases as the CEC increases. The critical level for soybeans (ppm) is 75 + (2.5 x CEC). For soils low in potassium, recommendations are designed to provide more potash than crop removal, so that soils will build up above the critical level in four years. Potash should be applied annually until soil test potassium is above the critical level. Once above the critical level, recommendations are made to replace soil potassium removed by the crop. These recommendations are slightly above the critical level to account for soil sampling or analytical variation. Depending on the CEC, the range to maintain soil-test potassium levels for optimum soybean production is between 100 and 180 ppm. Potash recommendations are given in Table 5-7. These recommendations are dependent upon a field’s yield potential, CEC, and soil-test level.

Calcium (Ca) and Magnesium (Mg)

Soybeans require a minimum exchangeable soil-test level of 200 and 50 ppm (400 and 100 pounds) of calcium and magnesium, respectively. In most cases, these requirements are automatically met when soils are maintained at the proper soil pH. Soybeans will grow well over a wide range of calcium to magnesium ratios and should not need additional calcium as long as the proper pH is maintained and soil calcium levels are higher than magnesium. Soils naturally low in magnesium (eastern and extreme southern Ohio) should be limed with dolomitic limestone. Dolomitic lime is an economical source of magnesium and still contains generous amounts of calcium.

Sulfur (S)

Soybeans use large amounts of sulfur. A crop yielding 60 bushels per acre contains about 25 pounds of sulfur, 15 pounds of which is in the grain. Soils with more than 1.0% organic matter usually supply adequate sulfur for high yields. Deficiencies generally occur during cool wet weather on sandy soils and/or soils low in organic matter. Soil tests are not reliable in predicting crop response to sulfur. A continuing plant-analysis program is the best guide to confirm the need for additional sulfur. If a sulfur need is identified, several suitable materials, such as gypsum, potassium sulfate, or potassium sulfate magnesia, will correct the deficiency.

Manganese (Mn)

Even though manganese deficiency in soybeans is not a widespread problem, its occurrence is more common than the other micronutrient deficiencies. Deficiencies are more likely to occur in glacial lakebed, glacial outwash, peat, and muck soils. Soil pH is the most important factor affecting manganese availability (it becomes less soluble at higher pH levels), but other factors such organic matter, soil type, and weather may magnify the problem. On silt loams and clayey soils, manganese deficiency seldom occurs below pH 6.8. It may occur on sandy soils that are high in organic matter with a pH as low as 6.2. Muck and peat soils occasionally are deficient at a pH as low as 5.8. Pale yellow to nearly white leaves with distinct green veins is the most visual symptom of manganese deficiency. Deficiency symptoms will first appear on younger leaves. In more severe cases, the plants will become stunted.

Manganese may be banded for wide-row soybeans, but narrow rows require foliar applications. Generally, when the plants have two or three trifoliolate leaves (Growth Stages V2 or V3), a foliar application of four to eight pounds of manganese sulfate will usually correct minor deficiencies. Multiple applications may be needed when both the surface and subsoil have high pH values.

Manganese fertilizers should probably not be mixed with herbicides such as glyphosate to prevent the loss of weed control. Producers should examine the herbicide label to confirm that the product selected will not interfere with the activity of the herbicide. Spraying at the optimal time for weed control and using the manganese chelate product, EDTA, may lower the potential for antagonism between fertilizer and herbicide.

Insect Control

Insecticide application may be required at any time throughout the development of the crop. At planting time, seed treatments may be warranted for protection against seed maggots. Early-planted fields may require control of over-wintering bean leaf beetles if severe stand loss appears likely. Prior to flowering, soybeans can tolerate up to 40% defoliation by insects without an economic loss in yield. Soybeans become more susceptible to defoliation from flowering through pod-fill, and defoliation should not be allowed to exceed 15% during that time.

Once pods have set, pod injury by bean leaf beetle may be a problem. If pod injury is detected, a rescue treatment is warranted when injury of 10% or more of the pods appears imminent. Insecticides recommended for the control of several soybean insects are presented in Table 5-8. For additional information on controlling soybean insects, see Extension Bulletin 545, Insect Pests of Field Crops, available at county offices of OSU Extension or on the Internet at: ohioline.osu.edu/b545/index.html.

Weed Control

Specific chemical weed control recommendations can be found in the Weed Control Guide for Ohio Field Crops, Extension Bulletin 789, available at all county OSU Extension offices and on the Internet at: ohioline.osu.edu/b789/index.html .