Weed Management in Soybeans

By J. A. Ferrell, G. E. MacDonald, and R. Leon

Successful weed control is one of the most important practices for economical soybean production in Florida. Losses due to weeds have been one of the major limiting factors in soybean production. Weeds compete with soybeans for light, moisture, and nutrients, with early-season competition being the most critical. Most of the yield reduction due to weed competition occurs during the first six weeks after planting; therefore, major emphasis on control should be given during this period. However, producing a good crop of soybeans is only half the battle and will not be profitable unless the soybeans can be harvested. Late-season weeds can result in inefficient equipment operation and excessive harvest losses. Weeds can be controlled in soybeans; however, this requires good management practices in all phases of soybean production. Good soybean weed control involves utilizing all methods available and combining them in an integrated weed management system.

Crop Rotations

Crop rotations may be beneficial since many of the most troublesome weeds in soybeans (sicklepod, morningglories, cocklebur, and Florida beggarweed) can be more easily controlled in a crop such as corn. If the full benefit of the rotation is to be achieved, weeds must be controlled throughout the growing season of the rotational crop. Seed produced late in the season of the corn will be available to germinate and compete with the succeeding soybean crop. The major goal of the rotational crop for weed control is to reduce the number of weed seed available for germination the following season. Other benefits of crop rotation may include reduction in insects, diseases, and nematodes.

Crop Competition

Crop competition is one of the most important, but often one of the most overlooked tools in weed control. A good stand of soybeans, which emerge rapidly and shade the middles early, is helpful in reducing weed competition. This involves good management practices such as choosing a well-adapted variety, good fertility, maintaining proper soil pH, adequate plant populations, and using row spacings as narrow as practical. Utilizing these good management practices is necessary for producing good soybean yields and is also an aid in weed control.

The plant that emerges first and grows most rapidly is usually the plant that will have the competitive advantage; therefore, everything possible should be done to ensure that the soybeans, and not the weeds, have this competitive advantage.

Cultivation

Cultivation is still a good and economical method of weed control; however, for cultivation to be effective in controlling weeds in the row, the soybeans must be taller than the weeds. The major reason for cultivation is weed control; therefore, if good weed control has been achieved with an herbicide, delay cultivation until weeds are present. Cultivate only deep enough to achieve weed control since deep cultivation may disturb soybean roots, bring weed seed to the surface, and disturb the layer of soil previously treated with an herbicide.

Know Your Weeds

Choose control methods that are effective for your specific weed problem. Generally, for preplant and preemergence applications, the weed problems must be anticipated since weeds may not have emerged at the time of application. This can best be done by observing the field in the fall and recording the weeds present and their location in the field. These "weed maps" can be very useful the next spring in refreshing your memory and making a decision on what herbicide to purchase. Before selecting your herbicides (Table 1, Table 2 and Table 3), identify your expected weed problems. Once your weed problems have been determined, Table 4 and Table 5 can be helpful in determining which herbicide is most effective for these weeds.

Herbicides

Herbicides are one of the most effective tools for weed control in soybeans (Table 1 and Table 2). Preplant or preemergence applications combined with the previously discussed management practices are important in ensuring that the soybeans have the initial competitive advantage. One of the problems often encountered during this period is lack of rain to activate surface-applied herbicides. Surface-applied herbicides require rainfall or irrigation to be effective, and for best results, moisture is needed within a week after application. Lack of moisture during this period often results in poor weed control. Incorporated herbicides are not dependent on rainfall or irrigation and have generally given more consistent weed control; however, they do require additional time and equipment for incorporation. Surface-applied herbicides can give excellent control and offer the greatest ease of application but they also carry the risk of failure if moisture is not received.

If good initial weed control is received with either a preplant or preemergence herbicide and the soybeans are taller than the weeds, then other control measures are available to extend the control throughout the season. Over-the-top applications are successful in controlling late germinating weeds and weeds not controlled by preplant or preemergence herbicides. They can also be used to aid in harvest efficiency.

Calibrate

Calibrate accurately since rates too high may injure the crop and rates too low may not provide weed control. The herbicides listed in Tables 1 and Table 2 are those that have performed well in UF/IFAS research at the rates and time of application suggested. Herbicides, like any other pesticide, should be handled with care. Store herbicides behind locked doors, in the original containers, with the label intact, and separated from seed, fertilizer, and other pesticides.

Prepack Mixes

During the past several years a great number of prepack mixes have become registered on soybeans and are legal for use within Florida. Currently, individual active ingredients recommended in this publication can be purchased prepacked in a number of combinations. To list all prepack mixes currently available would be prohibitive in this format. It is important to evaluate the weed problem to be controlled and consult this fact sheet for individual or tank mix combinations which give good activity. Upon deciding the appropriate active ingredient needed, shop around to see if the active ingredient or active ingredient combination desired can be purchased in prepack form. At times the proper ratio desired may not be available but if suitable ratios do exist many times price advantages will be noted when purchasing prepack mixtures.

A list of prepack mixes and active ingredients contained that are registered for soybeans in Florida is shown in Table 2.

Herbicide-Resistant Soybeans

Transgenic or herbicide-resistant soybeans are genetically altered to tolerate (the actual resistance is altered enzyme) herbicides that would normally kill or injure conventional or non-transgenic varieties. This genetic modification allows the use of broadspectrum herbicides over-the-top of soybeans and provides economical and efficient weed management.

The transgenic soybean varieties, which are commercially available, are Roundup Ready and Liberty Link. Using transgenic varieties can expand weed management options while providing a more convenient and effective method of weed control. The following section provides information regarding herbicide applications for Roundup Ready and Liberty Link varieties. This is not intended to substitute for herbicide labels, which should be carefully read prior to planting these varieties.

Roundup Ready Soybeans

Roundup Ready varieties are genetically altered to tolerate over-the-top applications of glyphosate, Roundup WeatherMax, Glyphomax Plus, Glyphomax, Touchdown, and several other products containing glyphosate. However, be sure to check the product label for use on all Roundup Ready soybeans. Glyphosate controls most annual grasses and broadleaf weeds. The Roundup Ready system has been most successful where soybeans are drilled in 7- to 10-inch rows, where fields are scouted early, and when timely applications are made. Producers are encouraged to keep good records of planting to avoid spraying non-transgenic resistant fields with glyphosate. Drift to adjacent crops should be avoided. Sprayers should be cleaned thoroughly and flushed prior to and after application.

Application Timing – Glyphosate can be applied anytime from the cracking stage throughout flowering. Multiple applications can be made, but should not exceed the recommended limit on the label. Application rates are dependent on weed species and size; therefore, consult label for this information. Timely application is required. The first application should usually be made 16 to 20 days after planting; repeat applications can be made if needed. In narrow-row soybeans under good growing conditions, a single application is often sufficient. Wide-row soybeans normally require either a preemergence herbicide followed by glyphosate or two postemergence applications.

Herbicide Program – Any registered soil-applied herbicide can be used on Roundup Ready soybeans. Soil-applied herbicides are generally not needed with timely postemergence applications except for fields with Florida pusley. Use of soil-applied herbicides will, however, make the timing of postemergence herbicides less critical and usually eliminate the need for a second application. Consult the manufacturer label for use rates.

Tables

Table 3. 

Weed Management in Soybeans.

Source:ufl.edu