As harvest wraps up, it’s time for growers to get started planting cover crops. However, it’s important to consider the potential impact of residual activity of any herbicides used on previous crops grown in fields where covers will be seeded.

Herbicide carryover is a common problem and can affect different parts of each field in various ways, leading to uneven cover crop establishment. Areas in a field where herbicide carryover may be most prominent include field entrances and edges, sprayer turnaround areas, eroded hills, and high and low pH soils. Zones in a field with high or low moisture and low organic matter can also be particularly affected by herbicide carryover.

Some herbicides require a field bioassay to be completed before cover crops can be planted, so be sure to refer to herbicide labels before seeding covers.

South Dakota State University (SDSU) recently distributed two fact sheets entailing guidelines for growers who need to consider the residual effects of herbicides for upcoming cover crops.

Here are some interesting findings from the fact sheets:

  • Any forage restrictions listed on a herbicide label should especially be followed if a cover crop will be grazed or harvested for feed.
  • In a 2-year study done by SDSU, research showed that soil with a pH over 7 and a lack of microbial breakdown are leading causes for herbicides to cause carryover.
  • Oats and crimson clover planted after wheat treated with herbicide seemed to show the best performance when soil moisture levels were low. Cover crops such as flax, radish and pearl millet seemed to high levels of loss no matter how much moisture was in the soil. Rapeseed, field pea and sunflower establishment varied depending on the specific type of herbicide used prior to those covers being planted.
  • Cover crops grown after herbicide-applied corn such as mustard, winter pea, Winfred, radish, flax, rye, annual rye and oats all had varying levels of loss, depending on which herbicide was used.

Doing your research and conducting soil testing can make a big difference in whether or not you have to deal with herbicide carryover impacting your next crop.