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 may be beneficial since many of the most troublesome weeds in soybeans 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 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 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.
Herbicides are one of the most effective tools for weed control in soybeans. 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.