Weeds and Unplanted Fields

Chris Boerboom and Jerry Doll

Several areas of the state have fields that will not be planted with any summer crop because of continuously wet conditions. Weeds are having a field day in these sites and the potential for huge increase in the weed seed bank is great unless action is taken. Here are our suggestions.

To control annual weeds, tillage may seem to be the cheapest and easiest option, but there are drawbacks to fallowing fields with tillage. Tillage may cause soil compaction if the soil is still wet and will leave the soil exposed to erosion from subsequent summer rains. Tilling for weed control will not prevent later flushes of weeds so additional tillage will likely be needed during the summer. This will add to the cost.

A herbicide burndown treatment is an alternative to tillage for fallowing. In most cases, 0.75 lb ae/a of glyphosate (1 qt/a of most brands) would probably be adequate. Lower glyphosate rates may not provide sufficient weed control considering the size and maturity of many of the broadleaf weeds. Fields could be sprayed as soon as they start to dry, which should be this week. Even with chemical fallow with glyphosate, additional weeds will emerge because glyphosate lacks residual soil activity. To avoid respraying, the field could be no-till planted with a cover crop to suppress the summer weeds. Oats would be an inexpensive cover crop to consider. Rye would be another option, but remember that it is a winter annual and would grow into next year. Another option to consider is to plant a summer seedling of alfalfa. Of course, alfalfa doesn’t fit into everyone’s operation and some fields may not be suited for alfalfa production.

For fields with perennials like quackgrass, wirestem muhly, hemp dogbane, bindweeds and Canada thistle, the first step should be to apply 0.75 lbae/acre of glyphosate (1 qt/a of most brands) as soon as you can drive across the field. All perennials are in the prime stage (late bud to full flower) to be very susceptible to this rate of glyphosate. Adding a pint of 2,4-D to this mix may improve the kill of tall and flowering annual broadleaves like common lambsquarters and can only add to the control of perennial broadleaves as well.

The one perennial weed not well controlled by this approach is yellow nutsedge and it could be very prolific in some of these fields as it thrives in wet soil conditions. In fields with nutsedge, plan to do moderate tillage 7 to 10 days after application or add a low rate of Permit to the glyphosate to enhance the nutsedge control. Keep in mind the plant back restrictions for Permit if you use it.

June 2001