By Dr. Mark Loux
Weed control is an essential part of all crop production systems. Weeds reduce yields by competing with crops for water, nutrients, and sunlight. Weeds also may directly reduce profits by hindering harvest operations, lowering crop quality, and producing chemicals that are harmful to crop plants (allelopathy). In addition, weeds left uncontrolled may harbor insects and diseases and produce seed or rootstocks that infest the field and affect future crops. Despite large expenditures for weed control, it is estimated that losses in U.S. crops due to weeds left uncontrolled exceed $7.5 billion annually.
Years of research have shown that good weed control within the first four to six weeks after crops are planted is critical in order to avoid a yield reduction from weeds. There are many cultural, mechanical, and chemical methods of weed control that are extremely effective, if applied at the correct time. Fields that are kept free of weeds for the first four to six weeks after planting give the crop a head start that enables it to shade out or otherwise out compete weeds that emerge later in the season.
It is important to identify specific weed problems, because different weeds vary greatly in their ability to compete with crops and reduce yields. Is 100% control of all weeds necessary? Perhaps this can be answered by stating that most successful producers design control programs to maximize profit, not just control weeds. While it is true that crops are able to tolerate a certain threshold number of weeds without suffering a yield reduction, it is first important to consider weed problems on an individual basis. There are some weeds for which 100% control may be desirable because they are particularly competitive, persistent, or difficult to control. These include some annual weeds such as giant ragweed, common cocklebur, burcucumber, or shattercane, and several perennial weeds such as Johnsongrass, Canada thistle, bindweeds, Jerusalem artichoke, quackgrass, and hemp dogbane.
The general principles of weed and herbicide management in corn, soybeans, wheat, and alfalfa are presented in sections that follow. Additional information can be found in OSU Extension Bulletin 789, Weed Control Guide for Ohio Field Crops, ohioline.osu.edu/b789/index.html
Preemergence herbicide programs have long been the mainstay of weed management in corn, due in large part to the low cost of atrazine and its broad spectrum of control. A total preemergence approach can still be effective in fields with low to moderate populations of most annual weeds. The commonly used premix of atrazine plus an acetamide herbicide (Bicep II Magnum, Degree Xtra, FulTime, and others) can be supplemented as necessary with Balance, Callisto, Python, Hornet, or simazine to improve control of weeds such as fall panicum, triazine-resistant lambsquarters, and velvetleaf. However, some of these tank-mix partners can occasionally cause crop injury, especially when corn is planted too shallow, on coarse-textured soils, and when cold, wet conditions occur after planting.
In fields with moderate to high weed populations, a preemergence plus postemergence approach will provide more consistent control with less risk of corn injury. A number of options are available for this type of program at a reasonable cost. A preemergence plus postemergence approach is especially effective in fields with giant ragweed, burcucumber, moderate to high annual populations of annual grass, velvetleaf, and triazine-resistant lambsquarters, and perennial weeds.
A number of effective total postemergence herbicide programs are also available. However, research indicates that total postemergence programs lacking residual activity should be used only in fields with low weed populations. A total postemergence herbicide program should be applied before most weeds in a field exceed four inches in height, and reinfestation with later-emerging weeds is likely if a herbicide with residual activity is not included. Using a postemergence program that has substantial residual activity allows more flexibility in the window of application. Herbicides can be applied when weeds are anywhere from two- to four-inches tall, since the residual component will control many weeds that emerge after application.
A number of broad-spectrum preemergence (PRE) and postemergence (POST) herbicides are available for use in soybeans. Almost any type of approach to herbicide management can be used in fields with low weed pressure with little risk of crop yield loss. These approaches include total PRE, PRE followed by POST, and total POST. However, the biology of some weeds that occur in soybean fields, in addition to the slow early development of no-till soybeans, can make it difficult to achieve effective weed control with a single application of PRE or POST herbicides. For this reason, a PRE followed by POST program or a two-pass POST program often provides the most consistent control.
Weeds that are especially problematic in a total PRE herbicide program, depending upon their population, include annual grasses, giant ragweed, ALS-resistant common ragweed, marestail, annual morningglory, cocklebur, waterhemp, and most perennial weeds. Another major drawback to PRE herbicides applied at planting is the narrow window of time in which at least one-half to one inch of rain must occur to move herbicide into the soil. In early May when soybeans are often planted, weeds will typically start to emerge within eight to 10 days after tillage or an application of glyphosate or paraquat. Rain is needed before these weeds emerge in order for PRE herbicides to be effective. Applying herbicides several weeks or more before planting of no-till soybeans often results in more consistent weed control initially, although herbicide activity may not last as long after planting, compared to application at the time of planting.
Weeds that can be problematic in a one-pass, total POST herbicide program, depending upon their population, include annual grasses, giant ragweed, waterhemp, lambsquarters, marestail, and some perennials (because they may be too small at the time of an early POST application). Most POST soybean herbicides should be applied before weeds exceed three to six inches in height for consistent control and to avoid crop yield loss. Glyphosate application can be timed for weed heights of four to eight inches, although velvetleaf, lambsquarters, and some other weeds are more easily controlled when less than six inches tall.
Factors to take into consideration when deciding on the proper timing of a single postemergence application include:
A planned PRE plus POST approach will provide more consistent weed control than any one-pass approach in many fields and will help solve some of the problems in management of POST herbicides. The most complete PRE plus POST program includes use of a PRE herbicide with activity on key broadleaf weeds that also provides at least some early-season control of grasses, followed by a POST treatment with activity on grass and broadleaf weeds.
In many cases, early-spring application of a PRE herbicide with 2,4-D ester in no-tillage fields can reduce or eliminate the need for glyphosate or paraquat, in addition to providing residual weed control. The PRE herbicide can control or reduce the population of some problem broadleaf weeds, such as lambsquarters, waterhemp, and giant ragweed, making it relatively easy to control any later-emerging weeds with the POST treatment. A major advantage of the PRE plus POST approach, compared to total POST, is that the PRE herbicide will often provide enough weed control to prevent major problems if weather delays the POST application, reducing the risk of yield loss. Use of the PRE plus POST approach to allow for a slightly delayed POST application can result in more consistent control of late-emerging weeds, such as foxtails, giant ragweed, black nightshade, waterhemp, and perennials.
A healthy wheat crop competes well with weeds, especially when production techniques result in an initial uniform stand and when loss of stand due to winter injury is minimal. Effective weed control and prevention of weed seed production in prior crops will reduce the risk of weed problems in wheat. Some wheat fields can benefit greatly from herbicide application, and failure to scout fields and take the appropriate measure can result in yield loss and harvesting problems in these fields. The weeds that appear above the wheat canopy late in the season, such as ragweeds and Canada thistle, can often be easily controlled with a spring herbicide treatment.
The most common weed problems in wheat include:
It is essential to apply herbicides at the correct stage of wheat growth to avoid crop injury. When wheat has not yet reached the jointing stage, any herbicide labeled for spring application to wheat can be safely applied. As the wheat growth stage advances past jointing and approaches the boot stage, herbicide choices become more limited. Most herbicides can be applied using nitrogen fertilizer solution as the carrier. This may increase the risk of crop injury, and some labels recommend reducing surfactant rates to minimize injury.
Management of weeds in forages requires a different approach than in row crops. More than 95% of the weed control in a healthy forage crop comes from the competition provided by the forage. However, to maintain a relatively weed-free forage, proper fertilization, cutting management, insect control, the use of disease-resistant varieties, and selective herbicide use are necessary to keep the forage stand competitive.
If weeds become a problem, they can compete or interfere for light, nutrients, water, and space, directly influencing yield and standability. Common chickweed infestations in alfalfa have been reported to reduce forage stand by more than 30%. Common chickweed emerges in the fall and winter and early in spring develops a thick lush mat that can compete with the first forage cutting. Once the chickweed dies in early summer, summer annual weeds, such as foxtails, lambsquarters, and pigweed, or perennial weeds, such as dandelion, can replace the dead or dying winter annual weeds and continue to reduce forage yield and quality.
Unlike most grain or fiber crops from which weeds are separated at harvest, weeds are often harvested along with the forage crop, potentially reducing quality. Reductions in quality are often in the form of lower protein content and feed digestibility. Although weeds do have some feed value, this varies among species. Dandelions can provide levels of protein and total digestible nutrients (TDN) similar to alfalfa. Control of dandelion may not necessarily improve the quality of hay, but it may be of some value in reducing the time necessary to dry the hay, since dandelion dries more slowly than alfalfa. Increased drying time may mean greater harvest losses if rain occurs too soon after harvest.
Grassy weed quality can be similar to that of the forage. In general, weedy grasses have about 75% of the quality of alfalfa. However, controlling quackgrass in alfalfa can increase forage protein levels by 4% to 7%. Weeds with woody stems or flower stalks, such as yellow rocket, white cockle, rough fleabane, curly dock, and broadleaved dock, have lower protein levels (about 50% of the quality of alfalfa), so controlling them is even more important.
When weeds are present or persist in spite of good management, herbicides can help improve forage yield and quality. Weed control at establishment or in the seedling year is most critical for maintaining a healthy forage stand. When weeds are controlled the seedling year, the forage crop seldom requires additional herbicide treatments for at least the first two years of the stand.
Weed management in forages can be divided into two phases—control in the establishment or seedling year and control in an established stand.
Managing weeds in forages begins long before crop establishment. Certain types of weeds are potentially serious problems for forages, so it is important to eliminate them in advance. In particular, perennial broadleaf weeds and grasses, such as dandelion, curly dock, Canada thistle, and quackgrass, are much easier to manage prior to planting a forage crop. In addition, biennial weeds, such as musk thistle, wild carrot, and burdock, should be eliminated before establishing forage. If these weeds are not removed before the seeding is made, they commonly persist throughout the life of the forage. The cost of controlling weeds before or at the time of seeding should be considered an investment that will be returned for the life of the stand.
Some general rules for managing weeds at establishment or in the seedling year follow:
Herbicides are needed most often during establishment, and several options exist for managing weeds in pure legume seedings. In no-till seedings, adequately controlling the existing vegetation, especially perennials, prior to planting is very important. Weed control is also very important while the forage is young and prone to competition from invading species.
The best weed control in an established forage stand is achieved by maintaining a dense healthy stand through proper fertilization, cutting management, and insect control. Controlling weeds in established forages is normally of greatest benefit in the first cutting. Weeds generally cause less yield loss in the second and succeeding harvests. Before using a herbicide in established stands, evaluate the forage to ensure that it is worth the cost of the herbicide.
Some general rules to follow before using a herbicide in established forage stands are: