By Bob Hartzler, Department of Agronomy
As the end of April nears with little field work accomplished, getting the crop in the ground becomes the priority for many farmers. No-till farmers may plant fields prior to killing weeds with a burndown herbicide, with the intent of returning later to control the weeds with an early postemergence application. While this strategy can be effective, it is important to realize that planting into established weeds greatly shortens the time required for weeds to impact crop yields (critical period).
When crops are planted into a weed-free seedbed, weeds typically compete with the crop for three to four weeks after crop emergence without impacting yield, but this isn’t the case when planting into established weeds. Factors that will influence the critical period include:
density of weeds
size of weeds
cultural practices (row spacing, seeding rate, etc.), and
Because of the many variables involved, it is impossible to accurately predict when weeds begin to affect yields; however, weeds present at planting may affect yields as early as the V1 stage of crop development. Thus, appropriate herbicides should be applied as soon after planting as possible to remove these weeds.
Due to the increased cost of glyphosate, an increase in the use of preemergence herbicides is anticipated in order to reduce the number of postemergence glyphosate applications. Certain preemergence herbicides cannot be applied if the crop has emerged, so be sure to determine label restrictions and crop stage of development prior to application.
Bob Hartzler is a professor of weed science with extension, teaching and research responsibilities.