For crop production to be profitable, effective weed control is a must. With weed pressure, crop yield potential can be limited as weeds compete with crops for nutrients, water and light. Weeds can produce seed that can impact future crops and reduce overall harvest efficiency. Some species of weeds can create conditions for nematodes, insects or diseases to attack crops or even release toxins that can limit crop growth.
Not all weeds pose the same threat to crops (Table 1).1 When considering weed threats in corn, broadleaf weeds are often more competitive than grasses. For instance, to cause a 10% yield loss in 40 feet of row, the predicted weed density would be 80 foxtails compared to 40 pigweeds and only 10 cockleburs. Typically, canopy closure can harm weeds' competitive ability. But broadleaf weeds are largely unaffected by the shading effects of the canopy, and as a result, they can compete longer into the growing season. Weeds that germinate early are typically more competitive than weeds that emerge later in the growing season. Moisture stress can also be a factor in weed competition. Heavier soils are better at holding moisture, which allows them to tolerate higher populations of weeds that can hurt crop yield potential.
Late-emerging weeds include weeds that emerge after control tactics have been implemented. The impact of these weeds drops off quickly if they emerge three weeks or more after crop emergence. The competitiveness of these weeds is heavily influenced by the development of crop canopies. As a result, stress factors that reduce crop growth and affect the canopy development can create an ideal environment for late-emerging weeds. These weeds are at a competitive disadvantage to the crop due to their delayed emergence but can still have a negative impact on yield potential.
Despite being less competitive than broadleaf weeds, late-emerging weeds are still capable of producing large amounts of seed. At the time of herbicide application, if weeds have not initiated seed set, their seed production will be reduced or even eliminated. But if fruiting structures are visible during application, it is unlikely that the application will reduce the weed’s seed production or the viability of its seeds. Late-season weed control can also improve harvest efficiency, so there are situations where an application may be a good idea but is situational for each operation.
Using multiple management techniques, including cultural methods and herbicides, is the best way for farmers to achieve effective weed control. Growing the same crop in back-to-back seasons and relying on the same weed control techniques creates an ideal environment for the development of problem weeds and potentially resistant weed species. Ideally, farmers should use crop rotation and a diversified herbicide portfolio to help reduce their likelihood of pressure.
An important part of an effective weed management program is being able to identify weeds.
1 VanGessel, M. Weed management in row crops: application to corn production - competitive index factor chart. Northeast IPM Module number 10. http://northeastipm.org. 2 Fickett, N.D., Boerboom, C.M., and Stoltenberg, D.E. 2013. Predicted corn yield loss due to weed competition prior to postemergence herbicide application on Wisconsin farms. Weed Technology 27:54-62.
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.
