Weed suppresion using cover crops

Cover crops can be an important part of integrated management of weeds in field crops

The over-reliance on one form of weed control has resulted in the selection of weeds resistant to herbicides. Glyphosate, a non-selective broad-spectrum herbicide, has been the dominate weed management tool in glyphosate-tolerant corn, soybean, and cotton.

Postemergence-only glyphosate programs simplified weed control but resulted in a system dominated by one mode of action across millions of crop acres. It replaced important practices including rotating herbicide modes of action and use of soil residual herbicides.

In 2005, Palmer amaranth was confirmed resistant to glyphosate in Georgia (less than a decade after the introduction of glyphosate-tolerant crops). Palmer amaranth resistance has since spread across the South and Midwest crop producing regions of the United States. In South Carolina, Palmer amaranth is resistant to several herbicide families including glyphosate (group 9), ALS-inhibitors (group 2), and DNA-inhibitors (group 3).

Other states have reported Palmer amaranth populations resistant to glufosinate (group 10), triazines (group 5), synthetic auxins (group 4), PPO-inhibitors (group 14), and HPPD inhibitors (group 27).

Cultural control

Cultural practices, such as cover crops, can suppress weed emergence through shading of the soil surface and reduce reliance on herbicides. The key is to select a cover crop species that produces a high biomass by the time of termination.

cereal rye cover crop
(Left) Rye cover crop planted in the fall and (Right) terminated and rolled rye cover crop after cotton planting — photos courtesy Clemson University IPM Program

Cereal rye is a winter-planted cover crop that can produce high levels of biomass (several tons per acre). Biomass production is dependent on the planting date with the earlier planted rye (October-November) yield higher than later planted rye (December-January).

Some weed species require a light cue for germination. Typically, small-seeded weeds like Palmer amaranth and large crabgrass will not germinate if the soil surface is covered by a thick layer of biomass from the terminated cover crop. If some seeds manage to germinate, the incidence of the weed populations will be significantly reduced compared to a bare-ground soil surface.

Studies have been conducted over the past several years by Dr. Mike Marshall, field crop weed specialist at the Edisto Research and Education Center, focusing on using cover crops as part of the overall weed management plan.

palmer pigweed vs. cover crop
(Left) Postemergence glyphosate based only herbicide system in cotton with no cover crop and (Right) cotton planted into a terminated cover crop and use of multiple herbicide modes-of-action and residuals during the season.

Results from the trials showed that cover crops significantly reduced Palmer amaranth germination and subsequent populations in cotton. In addition to the cover crops, a diverse herbicide program was also used that included multiple modes of action and the use of soil residual herbicides at planting and with every postemergence application (overlapping residuals).

Cover crops are an important cultural practice in managing weeds in field crops. With herbicide resistance a growing concern, these cover crops can help reduce the selection pressure on the herbicides that still work.

Preserving herbicide effectiveness is critical because the crop protection industry is not producing new herbicide modes of action every year for growers to use when an herbicide fails on a given weed species, especially Palmer amaranth.

Clemson University Integrated Pest Management Program contributed this article.

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