Winter annuals continue to increase as tillage is reduced. Field pennycress, shepherdspurse, horseweed (marestail), chickweed, and others may be present in many fields this spring. Generally, winter annuals do not represent a major threat to corn and soybean production and thus do not require special tactics for control. However, last fall's extended warm temperatures have allowed these weeds to accumulate more biomass than typical. Some fields, particularly in the southern third of Iowa, may benefit from targeted herbicide applications to control these weeds.
There are two pertinent questions:
- Should special efforts be made to control winter annuals? and
- When should these efforts be implemented?
If winter annuals are found on more than 25 percent of the field, it is likely that some effort to control them will be beneficial. The question is whether to make a separate application specifically targeting the winter annuals or to wait and deal with them at planting. Although there is limited information available on which to base this decision, we feel that if the winter annuals are currently providing 60-80 percent ground cover there may be benefits to an early application. If present at this density the winter annuals may accumulate sufficient biomass to interfere with planter operation or negatively impact crop establishment. By allowing them to grow until planting, they also may reach growth stages that are difficult to control.
If the winter annual infestation warrants treatment, herbicide selection should be based on the species present. Options for winter annuals include Gramoxone Max, Roundup Ultra (and many other brands), Touchdown, Liberty, and 2,4-D. Residual herbicides can be included with these treatments, and many of these residuals may provide additional activity on the winter annuals (atrazine, Authority, Canopy). It is important to recognize that although the early preplant application timing may lessen the requirement for timely rainfall, there is still need for some precipitation for activation of the herbicide.
Below: field pennycress in corn stubble.
This article originally appeared on page 37 of the IC-486 (5) -- April 16, 2001 issue.