Profitable crop production depends on effective weed control. Weeds can reduce crop yield potential by competing for water, nutrients, and light. Some weeds release toxins that inhibit crop growth, and others may harbor insects, diseases, or nematodes that attack crops. Weeds can also reduce harvest efficiency and produce seed that can impact future crops.
Not all weeds compete with crops equally (Table 1).1 Broadleaf weeds tend to be more competitive than grasses in corn. For example, the predicted weed densities per 40 feet of row required to cause a 10% yield loss in corn would be 80 foxtails compared to 40 pigweeds and only 10 cockleburs. Canopy closure of the crop can limit the competitive ability of weeds. However, broadleaf weeds are better able to avoid the shading effects of the canopy and compete longer into the growing season. Early-germinating weeds are generally more competitive than weeds that emerge later in the growing season. Fields that experience moisture stress are also at greater risk of yield losses from weed competition. Heavier soils that hold moisture better can tolerate higher populations of weeds that may impact crop yield potential.
Late-emerging weeds can include those that emerge after control tactics have been implemented. The impact of late-emerging weeds decreases rapidly if weeds emerge 3 weeks or more after crop emergence. The competitiveness of late-emerging weeds is strongly influenced by how quickly the crop canopy develops. Stress factors that reduce crop growth can increase the impact of late-emerging weeds. These weeds are at a competitive disadvantage to the crop due to their delayed emergence but are still capable of causing economic losses.
Late-emerging weeds should not be ignored because they are capable of producing significant quantities of seed. If weeds have not initiated seed set at the time of herbicide application, seed production should be eliminated or reduced. However, if the fruiting structures are visible, it is unlikely that the application will reduce weed seed production or viability of the seeds. The other benefit of late-season weed control is harvest efficiency, and there are situations where these applications may be worthwhile.
Effective weed control in field crops requires the use of multiple management techniques including cultural methods as well as herbicides. Growing the same crop year after year and utilizing the same weed control techniques encourages the development of problem weeds and potentially, herbicide resistance. Utilizing crop rotation, tillage, and a diversified herbicide portfolio helps reduce this problem.
Identifying weeds properly is an essential first step in a weed management plan. This guide provides photos and identifying characteristics of grass and broadleaf weeds common to the crop’s growing region.
Click below on Grasses, Sedges or Broadleaves to identify and learn about the weeds on your farm.
General Weed Management Considerations
It is important to start the season clean with tillage or a burndown herbicide application. Allowing weeds to be present at planting because of skipping or delaying the burndown application will give weeds a competitive advantage with the crop.
The use of preemergence herbicides at planting can reduce the risk of early-season competition by reducing densities of weeds that can emerge with the crop.
The use of strictly postemergence herbicide application programs can by risky. Weeds grow rapidly, and weather can delay postemergence applications which can result in loss of yield potential. Surveys have shown that where weeds are managed with only postemergence herbicides, application timings often occur too late to protect the full yield potential of corn from early weed competition.2
The use of preemergence and postemergence herbicides in sequential applications or in tank mixtures can be the best approach to weed control. Weeds should be controlled throughout the season to protect yield potential, using a mix of herbicides and multiple sites of action that can help to prevent the development and spread of resistant weeds.
1VanGessel, M. Weed management in row crops: application to corn production - competitive index factor chart. Northeast IPM Module number 10. http://northeastipm.org.
Schaefer, K., Mueller, D., Sisson, A., Hartzler, B., Anderson, M., Jha, P., and McGrath, C. 2019. Weed identification. Field Guide. 2nd Edition. Iowa State University Extension and Outreach.
Everman, W., Sprague, C., Gower, S., and Richardson, R. 2014. An IPM Pocket Guide for Weed Identification in Field Crops. Bulletin E-3081. Michigan State University Extension.