Growers have several options when it comes to cover crop termination, but herbicides are used by 79% of growers, according to the 2021 Cover Crop Strategies benchmark study. A recent multi-state study funded by the United Soybean board evaluated herbicide options.
The study found that control of cereal rye and wheat used as a cover crop was best with glyphosate alone or in combination with 2,4-D, dicamba, Sharpen or Select. Contact herbicides like glufosinate and paraquat are often less effective on grass species because of their inability to translocate and adequately terminate the growing point the way that systemic herbicides like glyphosate can.
Glyphosate, gramoxone, and Liberty were all effective on legume cover crops such as hairy vetch and/or winter peas, especially when used in combination with 2,4-D, dicamba or Sharpen. Ninety percent of crimson clover could be controlled with glyphosate plus 2,4-D, dicamba or Sharpen when applied in early April to plants that were 4-7½ inches tall.
According to the study results, cover crops such as cereal rye can provide good control of winter annual weeds and other troublesome species like horseweed, while also providing early season suppression of summer annual weeds like waterhemp.
Grass cover crops, including annual ryegrass, cereal rye, and wheat were best terminated using Liberty and 2,4-D, followed by a mix of Liberty, Canopy and 2,4-D. However, annual ryegrass proved challenging to control except with high rates of glyphosate plus a generic clethodim product, reinforcing the need for caution when using annual ryegrass as a cover crop, as recommended by weed scientists.
The study spanned across Arkansas, Indiana, Missouri, Mississippi and Wisconsin.