Winter annual weeds have steadily increased in the past decade due to expanding no-till acres. In the majority of Iowa no-till fields, winter annuals do not generate sufficient biomass by planting time to interfere with planting or early-season crop growth. Because of this, our recommendation generally is to control winter annuals with at-planting burndown treatments. If the current dry period continues into the spring, it may be beneficial to control these weeds in March or early April to minimize their use of soil moisture reserves. Keep in mind that the establishment of winter annuals last summer and fall was reduced by dry conditions in many areas of the state. Thus, the only way to know whether an early herbicide application is warranted is to get out and walk the fields to determine the presence of winter annuals. Unless significant areas of the field have densities of more than 3 to 5 weeds per square foot in early March, there probably is no need for an early treatment.
This article originally appeared on page 55 of the IC-496 (4) -- March 13, 2006 issue.