Cereal rye (Secale cereale) is a popular cover crop in eastern Kansas, either alone or in a mixture, and either before corn or soybean. It has many benefits, including high biomass production which produces abundant aboveground biomass. It is great at scavenging excess N, preventing soil erosion, suppressing weeds, and over a period of years, can increase soil organic matter. Cereal rye can be mixed with a legume, such as crimson clover or hairy vetch, and brassicas, such as radishes or turnips.

The number one question related to cereal rye and corn is the subject of growth suppression. Rye is an excellent weed suppressor, particularly because it is able to outcompete weeds. Small, light-sensitive weeds such as lambsquarters, pigweed, velvetleaf, chickweed, and foxtail are often suppressed by cereal rye, as long as the cereal rye covers at least 90% of the soil surface. (Managing Cover Crops Profitably, 2009, p. 98-105). 

Allelopathy is when the release of chemicals from one plant inhibits or even kills other plants, and cereal rye does release allelopathic chemicals as it decomposes. It is thought that allelopathy affects small seeds more than large seeds, and since many weed seeds are located very near the soil surface, particularly in no-till, cereal rye has the greatest impact on small-seeded weeds, either due to allelopathy or competition.

So what about allelopathic or competition effects from cereal rye on corn? In a recent on-farm study conducted by Practical Farmers of Iowa, “Farmers reported that in 36 of 40 trials, properly managed cover crops had little or no negative effect on corn and soybean yield (and actually increased soybean yield in 4 trials).” One potential reason why there might be less effect of cereal rye on corn than some other species is that corn seeds are larger, and planted deeper, than many weed seeds (Hartzler, 2014).

Best management practices:

Still, to help alleviate any potential concerns about the allelopathic effect cereal rye may have on corn, producers should terminate it 10-14 days before corn planting. Allelopathy is not the only factor to consider when using a cereal rye cover crop prior to corn. The rye can keep soils cooler and wetter than normal when it’s time to plant corn -- if the weather has been cool and wet.

Current USDA Risk Management Agency policy is that for parts of eastern Kansas, cash crops may be planted into a living cover crop, as long as the cover crop is terminated within a certain number of days, and before the cash crop emerges (Agronomy eUpdate 436, January 10, 2014). Termination is defined as the day the cover crop is sprayed or rolled. If the farmer sprays the cover crop and there is a herbicide failure of any kind, then it is the farmer’s responsibility to go in and fix that problem.

For more information, see:

Practical Farmers of Iowa. http://practicalfarmers.org/farmer-knowledge/research-reports/2014/winter-cereal-rye-cover-crop-effect-cash-crop-yield/ 

Managing Cover Crops Profitably, 3rd edition. Can be downloaded for free at: http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition

Cereal Rye Cover Crops, Allelopathy and Corn, by Bob Harztler, Iowa State University. 4-23-2014. http://www.extension.iastate.edu/CropNews/2014/0423hartzler.htm

Changes in cover crop termination guidelines by USDA. eUpdate 436. 1-10-2014. https://webapp.agron.ksu.edu/agr_social/eu_article.throck?article_id=129