Weed management is one of the most important and sometimes one of the most expensive components of a soybean production program. Herbicides are a necessity in profitable soybean production. They are, however, only one component of a weed management program. Weed management is most successful when crop rotation, crop competition, cultivation, and judicious herbicide use are combined in a planned and coordinated program.
Herbicides should be used only as needed and only after careful consideration of the weed problems and the impact the weeds will have on production. Your county Cooperative Extension Center has computer software that can help you determine when weed populations exceed the economic threshold and, hence, help you determine when a postemergence herbicide application is justified. In addition, your Extension agent has software to help you determine the most appropriate soil-applied or postemergence herbicide or combination of herbicides to control the complex of weeds you have. This software is available for individual growers also. You can now consult the North Carolina WebHADSS (Herbicide Application Decision Support System) for help with herbicide decisions.
Crop rotation should be an integral component of a weed management program. Crop rotation allows the use of different types of herbicides on the same field in different years. This can prevent the buildup of problem weeds and help to keep the overall weed population at lower levels. More important, crop rotation and proper selection and rotation of herbicides are key components in a strategy to avoid evolution of weed resistance to herbicides. Some soybean herbicides may persist long enough to damage certain rotational crops. Before using any herbicide, check the rotational restrictions.
Crop competition is an effective but often underused weed control tool. The basic strategy is to follow practices that result in rapid soybean growth and canopy closure so as to provide maximum shading of weeds. Soybeans should lap in the row middles as soon as possible and certainly before blooming begins. Planting in narrow rows is the most effective means of achieving rapid canopy closure. Other practices that enhance crop competition with weeds include good seedbed preparation, use of high-quality seed, proper variety selection, early-season disease and nematode management, proper fertilization and liming, and early planting.
Soybeans obviously can be grown without cultivation. Cultivation, however, is an effective and economical way to supplement control achieved with herbicides. Applying herbicides in a band over the row and cultivating the row middles can substantially reduce weed control costs. In fields with a light weed infestation, cultivation alone may be sufficient. Cultivation should be shallow (1 to 2 inches) to avoid damaging crop roots and to avoid breaking through any residual herbicide layer and bringing up untreated soil and weed seed. To be effective, shallow cultivation must be done while the weeds are small. Weed control is the only benefit received from cultivation except where special soil problems such as crusting or poor drainage exist.