Cover Crop Mixtures (Mountains, Ridge & Valley, Piedmont)

cover-mixtures-mtn

Quick Look:

  • Soil health. Cover crop mixtures may increase microbial community diversity, but research in agricultural production systems in the Southern region is limited. A diverse microbial community may help increase soil carbon, solubilize micronutrients, reduce pathogens and increase resilience to extreme weather events.
  • Functions. Cover crop mixtures can provide several functions, for example, both residue for weed suppression and nitrogen fixation. There are tradeoffs and mixtures may not maximize any particular functions.
  • Yields. The data on this is not consistent. Cash crop yields will vary depending on soils, irrigation, cover crop mixture, and frequency of cover crop use within a rotation. It may take multiple years of integrating a mixture into a rotation before a cash crop yield increase is seen. Some farmers report increased profitability do to decreased inputs and increased soil moisture.
  • Species selection. Cover crop should be matched to your goals and your planting windows. Before corn, you should choose early-maturing varieties of the species. Before cotton, later maturing varieties should be used.
  • Brassicas. Plan how and when you will terminate when using brassicas. At least two consecutive nights below 20oF are needed for winterkill. Radishes and other brassicas may not winterkill in the Piedmont and may not be a good fit before late-planted cash crops such as cotton.
  • Grain/legume mixtures. Research in North Carolina and other southern states indicate grain/legume mixtures work well in many agricultural production systems. The grain provides longer lasting residue in a no-till system and helps slow down nitrogen release from the legume.
  • Termination. Glyphosate alone will not terminate most mixtures. Consider herbicide residuals and consult local Extension. For organic production, a mixture with varieties that have similar maturity dates will help provide a good kill with roller/crimpers. Best termination with roller/crimpers is obtained at 2/3 bloom with legumes and soft dough stage with grains.
  • Costs. Although mixtures can be more expensive than single species, with careful thought you can create a mixture that is comparable in cost to a cereal rye monoculture.

There is a lot of interest in cover crop mixtures. Some growers report improvements with their use. For example, soil physical properties such as infiltration can increase, which can increase yields in years with extended dry periods. Interest in cover crop mixtures has been driven by a desire to mimic natural ecosystems such as a prairie. In many natural ecosystems, there is high biodiversity in vegetation that contributes to the stability of the ecosystem. Agro-ecosystems have much less biodiversity. Planting a mixture of covers crops increases the biodiversity of farming systems, which may increase soil health and increase the number of benefits a cover crop can provide.

Different cover crop species have different root structures. These access different areas in the soil and release a variety of substances (root exudates) that microbes use as food, or make some plant nutrients such as phosphorus more available. The assumption is, if there are many different types of plants with different root exudates and structures, there will be a more diverse microbial community. This will increase soil carbon, solubilize nutrients, resist pathogens and be resilient to disturbance. There is research to support this assumption, but most of it has not been conducted in soils and climate of the Southern United States.

Cover crop mixtures can provide a variety of benefits. For example, in a no-till or strip-till system, grains in the mixture provide a longer lasting residue that helps with reducing erosion, reducing soil temperature, preventing crusting, adding carbon, and weed suppression. Legumes fix nitrogen that may be available to the following cash crop. Brassicas can scavenge nitrogen, may alleviate compaction, and provides early pollen as well as nectar for pollinator and beneficial insects. A cover crop mixture will provide some of all these benefits; however, research has shown that the mixture may not maximize any of these functions. For example, a cover crop mixture would not provide as much long-lasting residue for weed suppression as a cereal rye cover crop. In multi-species cover crops the legume component is diluted and nitrogen credits can be quite low (15-20 lbs/ac). Whether or not a mixture is right for your operation will depend on the benefits you want the cover crop to provide.

One question is whether cover crop mixtures will increase yields. The data on this is not consistent. Cash crop yields will vary depending on soils, irrigation, cover crop mixture, and frequency of cover crop use within a rotation. It may take multiple years of integrating a mixture into a rotation before a cash crop yield increase is seen. However, some farmers report increased profitability due to increased soil moisture and reduced inputs for the subsequent cash crop.

The University of Tennessee conducted one of the few studies of complex cover crop mixtures in the South. This research evaluated the effect of cover crop mixtures on yield in a no-till corn-soybean rotation using a no cover crop control, a wheat monoculture, a cereal rye monoculture, a cereal rye/hairy vetch mixture, a cereal rye/crimson clover mixture and a 5-species mixture containing cereal rye, oats, daikon radish, purple top turnip, and crimson clover. After three years, soybean yields were greater in the 5-species cover crop mixture treatment than the other treatments due to higher soil moisture levels that were attributed to increased infiltration rates. There was no difference seen in soil organic matter between the cover crop treatments during this relatively short time frame (three years). All the cover crop treatments, except cereal rye, had greater soil inorganic nitrogen than the no cover control, and most had greater potentially mineralizable nitrogen than the control. The soils where the study was conducted were silt loams. These finer textured soils are likely to respond relatively quickly to cover cropping, because the greater amount of clay in the soil tends to protect soil organic matter from decomposition.

When selecting cover crop varieties for mixtures, try to select varieties that mature at a similar time. This will help prevent having one species setting viable seeds before you are ready to terminate the cover crop. For example, before corn, you may want to use an early-maturing grain such as FL401 (cereal rye), oats (Legend) with brassicas and an early maturing crimson clover (AU Robin or AU Sunrise). For later planted cash crops such as cotton, later maturing varieties should be used. Forage variety trials are good source of information for varieties that perform well in your state. Usually forage variety trials report heading date for grains.

You should think carefully about when to include brassicas in your mixture. Brassicas do not reliably winterkill in the Piedmont, because at least two consecutive nights below 20oF are needed for reliable winterkill. Brassicas are likely to winterkill in the Mountains and Ridge & Valley. Brassicas grow quickly in the winter and flower in late February to early April. This means these may produce viable seed by the time the cover crop is terminated for a late-planted crop such as cotton in the Piedmont. Brassicas could be a good fit before early planted cash crops such as corn, since the cover crop mixture would be terminated much earlier.

One type of cover crop mixture that has been used extensively is grain/legume mixtures. Grain/legume mixtures such as cereal rye and crimson clover can provide close to the same amount of biomass as a grain monoculture. This mixture can slow down the nitrogen release from the legume to better synchronize nitrogen with cash crop. In no-till or strip-till systems, the grain also provides a longer lasting residue than a monoculture legume that helps suppress weeds and well as protect the soil. Most farmers using a grain/legume mixture plant ½ to ¾ of the full legume seeding rate and ¼ to ½ of the full grain seeding rate. The proportion of grain versus legume biomass obtained by planting a mixture depends on the soil, planting date, and climate conditions as well as the seeding rate. If a grain/legume mixture is planted in soils with a high inorganic nitrogen content, the grain will dominate the mixture. If the grain/legume mixture is planted in low inorganic nitrogen soils, the legume will typically dominate the mixture, because legumes can fix their own nitrogen. Legumes will also tend to produce more biomass when planted at the optimum time – typically September in the Piedmont, Mountains, and Ridge & Valley. Legumes usually need enough time in the fall to produce good roots before they can create aboveground biomass. Grains, particularly cereal rye, can grow when planted later in the fall. Extremely wet or cold conditions in the fall and winter can also change the proportion of grains to legumes. Very high seeding rates can suppress legume biomass. Planting a small-seeded legume at the same depth as the grain, which can happen when both are mixed in the large seed box, can also reduce legume biomass.

Termination of complex mixtures can present challenges. When designing a cover crop mixture for your farm, should think carefully about the following cash crop. Termination of cover crop mixtures with herbicides usually requires a combination of glyphosate and other herbicides for a complete kill. More information on terminating cover crop with herbicides can be found here. You should consult your local Extension for specific recommendations in your area.

Mixtures can be expensive to plant. Some example seed costs from demonstration projects in Georgia in 2017 are given in Table 1. These are only for example as seed costs can vary each year with crop performance. However, with careful planning, you can create a mixture comparable in cost to a cereal rye monoculture. An economically viable 6-species mixture that grows well before corn in Georgia is cereal rye (18 lbs/ac), oats (12 lbs/ac), vetch (3.5 lbs/ac), crimson clover (5.5 lbs/ac), daikon radish (1.75 lbs/ac), and rapeseed (1 lbs/ac). 

Cover Crop Mixtures Table
Additional Resources:
Contributors:

Julia Gaskin, Extension Specialist, University of Georgia; Nicholas McGhee, Agronomist, Jimmy Carter Plant Materials Center; Nathan Lowder, Soil Health Specialist, NRCS; Peyton Sapp, Burke County Extension, University of Georgia; Dr. Forbes Walker, Assoc. Professor, University of Tennessee Knoxville; Dr. Dara Park, Assoc. Professor, Clemson University