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Herbicide Residual Effects on Cover Crops after Corn Silage

Written collaboratively by Gared Shaffer, Paul O. Johnson, David Karki, Anthony Bly, Ruth Beck and Sara Bauder

According to USDA NASS, in 2019, South Dakota had 340,000 acres of corn harvested for silage. Silage is an important feed source for livestock producers in the state. However, silage harvesting removes residue and leaves soil exposed and susceptible to erosion. There are steps that producers can take to protect and improve the soil for future crops. Soil health principles that will help keep soil healthy and productive include: keeping soil covered, limiting soil disturbance, having a living root in the soil and increasing plant diversity.

In the last ten years, crop producers in South Dakota have shown increased interest in cover crops. In South Dakota, there is usually time in the season to plant a cool season cover crop after corn silage harvest. Planting a cover crop can improve soil health by providing protection to soil, increasing plant diversity and maintaining a living root for a longer timeframe.

As farmers consider whether to plant a cover crop after corn silage harvest, they often wonder what potential impact the residual activity of the herbicide(s) used in corn will have on the new cover crop. Herbicide carryover, a common problem in modern farming, is usually not uniform across a field. This can lead to uneven establishment of cover crop species. Heavy herbicide carryover areas include field entrances and edges, sprayer turnaround areas, eroded hills, and high and low pH soils depending on active ingredients. Some herbicides may show worse effects in areas with high or low moisture levels, extreme soil pH, and low organic matter. Another contributing factor could be higher herbicide rates which can lengthen the carryover time after herbicide application. However, there could be other reasons for a decreased cover crop stand that producers need to be aware of, such as moisture deficit, high surface residue, weed pressure, and planting errors (seed depth, rate, and planting time).

Before planting cover crops, some herbicides require a field bioassay to be completed.

To perform a bioassay:

  1. Collect a representative soil sample from the field or area in question
  2. Mix the sample together and place it in a container (around a gallon of soil)
  3. Plant seeds of desired cover crop species in the container of soil
  4. As a control, plant some seeds in a container of soil from an adjacent or nearby area which did not receive the herbicide application
  5. Observe each container for one week for seed germination and growth

If satisfactory growth is established, when compared to the check, then the cover crop may be planted. One critical thing to remember is that the reliability of a bioassay is only as good as the representative soil sample.