Delaying Cover Crop Burndown

Mark VanGessel, Extension Weed Specialist;

Delaying cover termination late into the spring allows for more biomass production; and more biomass is also assumed to be more benefits. But these additional benefits may come with “trade-offs”.

  • More legume biomass can mean more nitrogen fixation.
  • More cereal biomass can mean more nitrogen immobilization.
  • More biomass can mean more mature tissue that does not break down as quickly; this can be good for reducing mid-season evaporation and improving mid-season weed suppression but can increase planting challenges.
  • More biomass means more water use, thus can dry out the soil faster, which is good for a year like this but is negative in a dry spring.

When terminating cover crops later in the spring it is best to either spray the field at least 7 to 10 days before planting (planting into a brown/dead cover crop) or spray after planting (planting green). It can be challenging to cut through plant material that has started to die and plant tissue is not very firm (in science parlance it has lost its turgidity). Be aware that if allowing cover crops to advance beyond the flowering stage, the cover crop is producing viable seeds that could cause an issue in subsequent crops. Volunteer cover crops are the biggest challenge if small grains will be planted in the fall.

If planting into fields with lots of cover crop biomass or planting green, be sure you have a planter outfitted with plenty of weight and sharp cutting coulters to provide a good seed furrow. Your planter should be outfitted with a guidance system because it will be very difficult to use row markers. With planting green, apply your burndown herbicide after planting and use high gallonage to achieve good coverage of the cover crop and weeds present. We use 20 gallons per acre and that seems to be adequate.

If you are considering rolling the cover crop, we have found it be best to roll after planting. Leaving the cover crops standing for planting allows for a better seed furrow to form and less hairpinning of cover crop in the seed furrow.

There is still a need for residual herbicides for most fields with cover crops. Most cover crop stands are not uniform across the field and where the cover crop biomass is less there is a greater chance for weed emergence and faster weed growth. The benefit of the cover crop for weed management is fewer weeds present (weed density is reduced) and growth of the weeds is slower, so weeds are smaller when postemergence herbicide applications are made. So, cover crops improve overall weed control; but they do not replace herbicides.