Kolby Grint, Northwest Regional Specialist, Nutrient and Pest Management Program, University of Wisconsin-Madison
Managing a Cereal Rye Cover Crop for Weed Suppression in Corn and Soybean
Cereal rye (Secale cereale) is an effective cover crop in annual cropping systems because of its ability to establish in the fallow period of corn-soybean rotations, overall winter hardiness, and the high amounts of biomass production in the spring. Achieving weed suppression from a rye cover crop is dependent on the amount of cover crop biomass that the cereal rye accumulates. Despite the cooler spring growing conditions in Wisconsin that can limit cereal rye biomass accumulation, effective suppression of small-seeded weeds is still possible with proper cover crop management.
Maximizing weed suppression from cereal rye
Cereal rye grown as a cover crop in Wisconsin will likely need additional time to grow and accumulate biomass to achieve weed suppression in the spring compared to southern states. Delaying cereal rye termination until the time of cash crop planting or later by planting green has been shown to be an effective management strategy to achieve early-season weed suppression in Wisconsin corn and soybean production systems. Delaying the termination of cereal rye not only allows for more biomass production but will also increase the persistence of cereal rye residue on the soil surface by allowing the cereal rye plant to accumulate more carbon, which slows its residue decomposition. When cereal rye is terminated earlier with lower amounts of biomass accumulated the residue decomposes more quickly. To achieve dependable weed suppression of later emerging weeds from cereal rye (i.e. waterhemp), delaying termination until around the boot growth or anthesis growth stage of cereal rye is needed to allow for the maximum amount of rye biomass accumulation and cereal rye residue persistence. Later termination of cereal rye can be done chemically with glyphosate and/or using a roller-crimper. The use of a roller-crimper alone for termination requires that cereal rye plants in the field are all at full anthesis, or the termination will be unsuccessful; conversely, waiting too long will allow the rye plants to produce seed which can be problematic. When using glyphosate for chemical termination of cereal rye, the glyphosate susceptible weeds that are present at the time of termination are controlled as well. However, when established weeds that are suspected to be glyphosate-resistant in Wisconsin (i.e. giant ragweed, horseweed) are present or there is a legume cover crop species with cereal rye it is beneficial to include an effective growth regulator herbicide (i.e., dicamba, 2,4-D) with glyphosate to optimize control from the chemical termination application. Wisconsin research indicates that suppression of summer annual weeds is improved by the use of a high biomass cereal rye cover crop in corn and soybean with no effect on soybean yield under normal field conditions but lower corn yield has been found in some cases.
Should I be concerned about allelopathy?
Weed suppression from cereal rye or subsequent effects on a cash crop are sometimes attributed to allelopathic effects. Allelopathy occurs when chemicals produced by one organism (cereal rye) suppress or prevent the growth of another organism (i.e. plants, pathogens, insects). Even though cereal rye has been identified to produce allelochemicals that have weed suppressive effects in research conducted under controlled environments, there currently isn’t strong evidence that these allelochemicals are a major contributor to weed suppression under field conditions. Competition for resources (i.e. sunlight, nutrients, water) from an actively growing cereal rye cover crop followed by reduced light transmittance and physical suppression of emerging weed seedlings by residue following cover crop termination have been identified to be some of the main mechanisms of weed suppression from cereal rye. The lack of measurable plant suppressive allelopathic effects in the field is likely due to the variable characteristics of allelochemicals produced by cereal rye including their short persistence, low activity, and variable production between rye cultivars making their activity difficult to predict. There has been no documentation of allelopathy between a cereal rye cover crop and a corn/soybean cash crop in the field. Suspected cases of allelopathy in corn, such as early-season stunted plants, are likely from a reduction in nitrogen availability by cereal rye.
Weed suppression may vary between weed species
Not all weed species are suppressed by cereal rye equally. Weed suppression from cover crops is most effective for small-seeded weed species (i.e., common lambsquarters, waterhemp) whereas large-seeded weed species (i.e., giant ragweed, velvetleaf) are still able to establish successfully at troublesome levels regardless of the presence of cereal rye cover crop.