Weeds compete with corn plants for limited resources such as water, nutrients, light, and space. Weeds have also been shown to alter the growth of corn seedlings even before the competition for resources begins. Weeds growing very near corn plants can change the quality of light reflecting off the weed foliage which may negatively impact yield potential. This may help explain yield losses that are often more than expected from early in-season competition when the demand for light, water, and nutrients is limited.
Early season weed competition causing corn yield loss can begin soon after planting. The critical period varies widely depending on field conditions, and there are no simple guidelines to accurately predict for all situations. Testing conducted in glyphosate-tolerant corn over two years across multiple sites and states suggested that the optimum timing for initial glyphosate application to avoid yield loss was when weeds were less than 4 inches tall, no more than 23 days after planting, and before the V4 stage of corn growth. Remember these are only guidelines, and it is important to understand that timely early season weed control is critical to help protect corn yield potential.
Fields should be scouted prior to post herbicide applications to accurately determine the growth stage of corn plants and weeds. Herbicide product labels need to be followed relating to crop growth stage and other recommendations. Environmental conditions should be considered relating to how they may affect post-applied herbicides. Overall, the use of pre- and postemergence herbicides in sequential applications or in tank mixtures can be the best approach to weed control in corn. To help protect corn yield potential, weeds should be controlled during the growing season with a mix of herbicides and multiple sites of action that can help prevent the development and spread of resistant weeds.
For additional information regarding postemergence weed control, please click here.
RoundupreadyPLUS.com is an additional source for season long weed control information.