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Cover Crops' Role in Weed Suppression and Water Quality

Much has been written about the positive benefits gained from inserting cover crops into a grain production system. The below cited articles deal with two of the perceived advantages from their inclusion.

The recent vacating of dicamba herbicide product registrations by the U.S. Circuit Court has created a real dilemma for soybean producers who were planning to use this recently developed technology to help manage problematic weeds that have evolved resistance to multiple herbicide classes. This puts added pressure on the remaining herbicides in the chemical control toolbox, and on non-chemical control methods such as cover crops. A recent article titled “Weed Suppression with Cover Crops: It’s all about Biomass“ by Iowa State Univ. Agronomists Hartzler and Anderson provides a summary of just what it takes for a cover crop to be effective as a weed management tool. Pertinent points from that article follow.

•   One perceived benefit from cover crops is providing a cultural method to manage/suppress problematic weeds.

•   Cereal rye has the most potential for suppressing weeds prior to and immediately following planting because it accumulates more biomass than other cover crop species.

•   Three cover crop treatments were used: 1) no cover crop; 2) early termination when rye was 6-8 in. tall and had produced 900 lb./acre of biomass; and 3) late termination when rye was flowering and had amassed 10,000 lb./acre biomass.

•   The low biomass achieved in treatment 2 reduced waterhemp growth more than it did emergence.

•   The high biomass achieved in treatment 3 dramatically reduced both emergence and growth of waterhemp. Weed emergence was delayed more than two weeks compared to that in the treatments described in 1 and 2 above (the authors note that the 10,000 lb./acre rye biomass is greater than typically achieved in Iowa).

•   The authors concluded that 1) two important factors influencing rye biomass production are planting date and termination date, and 2) cover crops such as cereal rye that are used to aid in weed suppression should be managed to maximize their biomass accumulation prior to termination. (Click here for a recent blog post that confirms the importance of biomass accumulation of cover crops for benefits to be realized. Results cited in the White Paper on this website also confirm that high-residue cover crops in combination with PRE herbicides can be part of an effective strategy for controlling herbicide-resistant (HR) Palmer amaranth in the Midsouth).

The movement of nitrate from crop production fields into groundwater and aboveground waterways has been and continues to be a matter of environmental concern. A recent article (J. Environ. Qual. 2020; 49:38-49; https://doi.org/10.1002/jeq2.20007) titled “Midwestern cropping system effects on drainage water quality and crop yields” by Iowa State Univ. scientists/specialists provides evidence of the effectiveness of a cover crop in reducing nitrate transport from crop production fields. Pertinent points from that article follow.

•   Nutrient loss from subsurface drainage systems is of particular concern in Midwest crop fields.

•   Treatments of interest in an 8-year study conducted in Iowa were a corn-soybean rotation (both crops present each year) with and without a cereal rye cover crop that preceded each crop each year.

•   Urea-ammonium nitrate (UAN) was applied to the corn crop in the rotation each year.

•   Rye biomass growth preceding the soybean (697 lb./acre) and corn (389 lb./acre) crops was not significantly different across the 8 years of the study. (Rye biomass growth preceding both crops in the rotation was relatively low. Thus, the results from this study are likely influenced by this fact.)

•   Over the 8 years of the study, average nitrate-N concentration in subsurface drainage water was 30% lower from the cover crop treatment. Thus, the cover crop was effective in reducing nitrate N losses in drainage water.

•   The presence of the rye cover crop was associated with significantly lower corn (13% reduction) and soybean (7% reduction) yields across the 8 years of the study.

•   The authors concluded that a cereal rye cover crop showed the potential of reducing nitrate N losses through subsurface drainage, while also negatively affecting both corn and soybean yields.

•   The study identifies use of a cover crop as a management tool that can affect downstream water quality.                                             

Results from the two above-cited articles further confirm summary points included in the White Paper on this website. Selected points from that article follow.

•     The first step when deciding to use cover crops is to define the purpose for their inclusion so that subsequent input and management decisions support that purpose; e.g., is the purpose to control herbicide-resistant weeds, remedy soil compaction, protect highly erodible soil, scavenge soil nutrients left from a preceding crop, increase soil organic matter, or provide N to a following crop? This will be important for deciding cover crop species/types to use since a one-cover-crop-fits-all approach likely will not provide the intended result regardless of the intent.

•     Identify the proper planting window for the selected cover crop species so that emergence and stand establishment are optimized.

•     Select a termination time that optimizes growth to realize the intended benefit from the cover crop species.

•     Cereal rye appears to be the best cover crop species for suppressing HR weeds, especially Palmer amaranth.

•     With any cover crop, establishment of a suitable cover is paramount. This requires the proper species selection for the latitude, as well as suitable environmental conditions for emergence and subsequent growth of the selected cover crop species. Biomass yield of the cover crop likely will depend on this.

•     No single plant species or mix of species used as a cover crop will provide complete control of problem weeds such as HR Palmer amaranth.  Rather, any species or mix of species that is used for proven suppression of problem weeds should be viewed as one component of a weed control strategy that is intended to effectively manage weeds.

Composed by Larry G. Heatherly, June 2020, larryheatherly@bellsouth.net