Considerations for Postemergence Herbicides and Planting Cover Crops this Fall

Mark VanGessel, Extension Weed Specialist;

Most of the herbicides we use in corn and soybeans are compatible with planting cereal rye cover crop in the late summer or fall. However, use of residual herbicides such as Dual, Zidua, Outlook, or Warrant postemergence could pose a risk; particularly if the cover crop is planted prior to mid-September. Broadleaf cover crops such as forage radish or legumes are more likely to show injury from herbicides, and the sooner after application these cover crops are seeded the greater the risk for injury. We have seen stunting in the forage radish from Valor or Spartan. Postemergence applications of ALS herbicides (Group 2) or fomesafen (Reflex or FlexStar) have the greatest risk of injuring forage radishes.

Hairy vetch appears to be more tolerant of herbicide carryover than crimson clover. Halex, Zidua, and atrazine in corn and fomesafen, Warrant and ALS herbicides in soybeans are the greatest risk to crimson clover.

Irrigated fields allow more herbicide degradation by microbes and less the risk of injury. Herbicide evaluation for carryover to cover crops is still an active area of research at UD and other universities and with more research, we hope to provide detailed guidance on this topic. However, if cover crops are an important part of your cropping system, give careful thought to the herbicides you use this spring to reduce the risk of cover crop injury this fall.