The unusually warm weather Iowa has had through March will create additional weed challenges for a number of farmers this spring. Winter annuals in no-till fields will likely accumulate much more biomass prior to planting time than normal, and therefore use more soil moisture, tie up more nutrients and potentially interfere with crop planting and crop establishment. In addition, weeds such as horseweed (marestail) will grow more rapidly and reach growth stages that are difficult to control much sooner than in a "normal" spring.
That's how Iowa State University Extension weed management specialists Bob Hartzler and Mike Owen are sizing up the situation during this last week of March 2012. They offer the following weed management advices and recommendations.
Applications of burndown herbicides in early April may be of benefit this year
Due to these potential problems, applications of burndown herbicides in early April may be beneficial and improve the control of winter annual and early spring annual weeds. An additional benefit of earlier application dates for the burndown treatment is minimizing the risk of including 2,4-D at the higher rates (i.e. 2 pts/A of LV-4) in the program. Of course, there is the important assumption that planting dates are not moved proportionally earlier.
Many farmers will want to include preemergence herbicides with these early spring burndown treatments. While this may provide a clean seedbed at planting and crop emergence, the longevity of weed control is likely to be shortened significantly. The magnitude of this reduction will depend on the time period and weather encountered between application and planting, and the herbicide rate.
Keep in mind that the rates of many preemergence herbicide products have been reduced due to the reliance on postemergence products, primarily glyphosate. If applications are going to be made a few weeks earlier than normal, you should carefully evaluate the product rates in order to maximize the contribution of the preemergence herbicide(s) to residual weed control after crop emergence.
Preemergence herbicide products need to be used in a way that controls the target species of weeds
Preemergence herbicides are a key component of herbicide resistance management. But to be effective, they need to be used in a manner that results in significant control of the target species of weeds. Very early applications of preemergence herbicides or reduced rates per acre will greatly reduce their effectiveness on late-emerging weeds such as waterhemp, or large-seeded species such as giant ragweed.
Many preemergence herbicide products specify split applications where a portion of the product is applied early and a remainder is applied at, or shortly after planting. This approach could be beneficial this year where an extended period of weed control may be needed due to early applications resulting from prevailing weather conditions.