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Weed Management on Prevented Planting Acres

Aaron Hager
May 29, 2019
Recommended citation format: Hager, A.. "Weed Management on Prevented Planting Acres." Department of Crop Sciences, University of Illinois at Urbana-Champaign, May 29, 2019. Permalink

Persistent wet field conditions have increased the likelihood that many farmers will opt to take the prevented planting option through their crop insurance policy.  Even though no crop will be planted, weed control practices still should be implemented to reduce seed production from summer annual weed species.  Any weed seed produced in 2019 will add to future weed control costs.  The old weed science adage “One year’s seedling equals seven years weeding” reinforces the need to adequately manage weeds on prevented planting acres.

Many species of winter annual weeds already have flowered and soon will produce seed.  Additionally, many summer annual weed species have emerged and are growing rapidly.  We suggest the focus of weed management on prevented planting acres should be on summer annual weed species.  Several options exist that could be used singly or in combination to keep weeds under control.

Tillage.  Tillage implements that significantly disturb the soil (tandem disk, field cultivator, etc, but not vertical tillage implements) can effectively eliminate summer annual weeds.  Generally, tillage is more effective when weeds are small and soils are not overly wet.  Large weeds that escape a tillage pass can be very difficult to control later in the growing season.  While usually effective at controlling established weeds, keep in mind that tillage can stimulate germination and emergence of additional weeds.  Multiple tillage operations likely will be needed before a killing frost to prevent summer annual weeds from producing seed.  Fuel consumption/cost and potential for soil erosion are additional factors to consider when using tillage to control weeds on prevented planting acres.

Mowing.  Repeated mowing can help suppress weed growth, but might not prevent seed production of all summer annual species since some seed could be produced from plants that regrow or from tillers present on grasses below the height of cutting.  Adjust the mower to cut as close to the soil surface as possible.  Utilizing mowing followed by tillage likely would be more effective in reducing seed production than mowing alone.  Alternatively, if vegetation is quite large, mowing that precedes tillage by several days might improve the effectiveness of the tillage operation in reducing seed production.

Herbicides.  Non-residual herbicides can control many summer annual species, but will miss any plants/species that are resistant to it.  Combining glyphosate with 2,4-D or dicamba would provide more consistent control of emerged waterhemp, marestail and giant ragweed than glyphosate alone.  Waterhemp’s extended emergence duration will require at least two to three herbicide applications before the first killing frost.  We do not recommend applying soil-residual herbicides as they are unlikely to maintain sufficient weed control in the absence of a planted crop.

Cover crops.  A well-established grass cover crop (such as rye, wheat, sudangrass, etc.) can be quite effective in limiting emergence and growth of summer annual weed species.  It is advisable to control any emerged weeds before seeding the cover crop.  Tillage or non-residual herbicides can be used prior to seeding, but you should allow several days between herbicide application and seeding for the herbicide to control existing vegetation. Drilling cover crop seed likely will result in a good stand that can be very competitive with weeds and also help scavenge soil nitrates.  A growth regulator herbicide (2,4-D, dicamba, etc.) could be applied after the cover crop has emerged to control broadleaf weeds if needed.  Without vernalization, rye or wheat plants are unlikely to produce viable seeds by the end of the growing season but might still provide suppression of fall-emerging winter annual weed species.  Be sure to plant weed-free cover crop seed, which might require cleaning bin-run wheat seed.  Many references on cover crop establishment are available, including one published by the Illinois Nutrient Research and Education Council.

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