Impromptu No-till Not Recommended

Chris Boerboom, Ext. Weed Scientist

What is “impromptu no-till”?  I heard this phrase for the first time last year to describe the weed management approach used in several no-till corn or soybean fields each year.  One synonym for “impromptu” is “unprepared”.  In these impromptu no-till fields, growers are unprepared for effective weed management.  Effective weed management in no-till includes 1) managing winter annuals (mustards, horseweed, prickly lettuce, etc.) and perennials like dandelions before planting; 2) burning down early emerging summer annuals like giant ragweed, foxtails, and lambsquarters; and 3) managing early season weed competition. 

In some cases, Roundup Ready crops may give growers a false belief that all three of these steps can be accomplished with a single glyphosate application after planting corn or soybeans.  In other cases, growers may be short on time because of weather delays which leads to skipping seedbed tillage and going directly to no-till planting.  In either case, appropriate plans have not been made for weed management.  Both weed control and crop growth may suffer. 

What are the problems with impromptu weed management?  Here are a couple potential examples.

° Dandelions.  Heavy infestations of dandelions are most effectively controlled in the fall.  Even if dandelions were not controlled in the fall, spring burndown treatments should still be applied before planting to control or suppress dandelions.  If not, dandelions can grow vigorously and compete with corn or soybeans as they emerge and reduce the stand of either crop.

° Early emerging summer annuals.  Giant ragweed might be the worst offender here, but many other common annual weeds like foxtails, lambsquarters, common ragweed, and velvetleaf can cause the same problem.  The situation might be that these annual weeds don’t seem dense enough to justify a burndown treatment or that they are so small that there will be time to spray them postemergence.  In either case, we need to think about the basics of weed competition.  Weeds that have already emerged before planting have a large competitive advantage.  Time wise, they are already growing before the crop has even sprouted. They may have a 1 to 4 week head start.  Because they are more competitive, their economic threshold is lower (it takes fewer of them to justify treatment even if they seem scattered).  Because they are more competitive, the time that they need to be controlled is also earlier in the season.  This is especially true with giant ragweed.  Giant ragweed may also grow so rapidly and become so large that control is difficult.  In either case, a burndown treatment at planting is recommended to control these weeds and give greater flexibility to subsequent postemergence herbicide applications.

The time and expense of burndown applications in no-till might be one reason why these treatments are skipped.  However, the benefits of controlling winter annual and early summer annual weeds and creating more flexibility for postemergence applications should provide a good return on this investment.  An additional benefit for a burndown treatment is that the application can be “cost shared” with a preemergence residual herbicide.  In other words, the application expense for a burndown treatment has already been paid.  If a preemergence herbicide is desired, it can be tank mixed with the burndown herbicide with no additional application cost.  This provides two good weed management practices in an efficient and cost effective manner.