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Single- vs. Multi-Species Cover Crops

Two hot-button issues in crop production that have arguably garnered the most attention in the past couple of years are herbicide-resistant (HR) weeds and cover crops. It seems that every information venue–e.g. popular press, blogs–has devoted a significant amount of coverage to these subjects because they affect/can affect every soybean acre in the country. Numerous articles in crop production journals have reported and are still reporting results from studies that deal with various aspects of both issues.

As previously stated in a White Paper on this website and in numerous other articles, producers who desire to use cover crops as a component of their production system must first decide the purpose of cover crops inclusion; e.g., help control HR weeds, remedy soil compaction, scavenge soil nutrients left from the previous crop, increase soil organic matter, provide nitrogen (N) to the following crop, prevent erosion. This will or should dictate cover crop species that are chosen.

In an article titled “Effect of Multispecies Cover Crop Mixture on Soil Properties and Crop Yield” (Agric. Environ. Lett. 2:170030, 2017) published in Dec. 2017, authors Chu et al. report results from a 3-year study conducted in West Tenn. They evaluated soybean yield and soil properties following single-, double-, and multi-species cover crops that were grown for 3 years.

The cover crops treatments were: 1) wheat; 2) cereal rye; 3) cereal rye and hairy vetch; 4) cereal rye and crimson clover; 5) a multi-species mix of cereal rye, oats, daikon radish, purple top turnips, and crimson clover; and 6) no cover crop(s). Cover crops were drill-seeded soon after harvest of either corn (2013, 2015) or soybeans (2014, 2016). Soybean yield and all soil properties were measured in Oct. 2016. Major findings from the study follow.

•   Gravimetric soil moisture content was significantly higher for the multi-species cover crop mix compared to the no-cover control. Soil moisture content in all other cover crops treatments was not different from the control.

•   Soil inorganic N was highest in the cereal rye/hairy vetch treatment. The no-cover control and cereal rye treatments had the lowest inorganic N at the time of sampling.

•   The multi-species cover crop mix and the cereal rye/crimson clover treatments had the highest potentially mineralizable N (PMN) and the control treatment had the lowest PMN.

•   Soil organic carbon (SOC) did not differ among treatments, and SOC values after 3 years were comparable to those at the beginning of the study in 2013. The authors attributed this lack of a favorable response of SOC to cover cropping to the study’s short duration and climatic conditions that favor accelerated SOC mineralization at this southern US location.

•   Soybean yield of 67.7 bu/acre following the multi-species cover crop mix was greater than yield from all other treatments. Yields of soybean following all other cover crops treatments were similar to each other and to the no-cover control, which was about 59 bu/acre. Soybean yield following the cereal rye treatment was 58.0 bu/acre.

•   The authors concluded that their findings indicate that beyond the first few years, cover cropping with a mixture of diverse species could positively affect crop productivity.

These results provide support for the following general conclusions regarding use of cover crops in Midsouth soybean production systems.

•   Short-term cover cropping may not provide significant soil or crop benefits; i.e., many of the positive effects that will result from inserting cover crops into a production system likely will only be realized when cover crops have been used continually for a period longer than 3-5 years.

•   Increased soil N following legume cover crops or cereal/legume mixes may only be important for a following crop such as corn. It is not likely that this is an important attribute for a following soybean crop.

•   The positive attributes realized following legume cover crops or cereal/legume mixes may not be compatible with situations where HR weeds are present and a cover crop such as cereal rye is needed to manage those weeds. The increased biomass from such a cover crop is a major reason it is used on sites that have HR weeds.

•   Results from cover crops studies must be evaluated with regard to the properties of the study site. For example, in the above-cited study, HR weeds were apparently not a problem, so the positive effects of legume cover crops or mixes that contain legume species that were realized in that study can be transferred to similar sites without concern for management of HR weeds.

•   If HR weeds are present, then a more likely cover cropping plan for a corn-soybean system will be to use a cereal cover crop such as rye prior to the soybean crop, and a legume or legume/cereal mix prior to the corn crop. This plan assumes that an every-other-year cereal rye crop will be sufficient to provide significant management of HR weeds that may be present. This is a facet of cover cropping that should be investigated further.

A final note. Refer to the list in the second paragraph above. It is imperative that producers first decide their goal/expected outcome from using cover crops, and then select the species or species mix that most likely will meet that goal or achieve the intended outcome.

Composed by Larry G. Heatherly, May 2018, larryheatherly@bellsouth.net