Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
With brassica cover crops in full bloom and crimson clover starting to bloom, remember why you planted the cover crop in the first place.
Most benefits of cover crops are maximized by allowing cover crops to develop later into the spring. As cereal rye gets taller, the stems develop more lignin and are more resistant to breaking down and they last longer on the soil surface. The more “lignin-ified” the tissue becomes, the longer it provides weed suppression and the better it will prevent moisture evaporation from the soil. The longer the legumes are allowed to grow, the more nitrogen being produced for the cash crop. Be aware that by allowing cover crops to reach 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.
While more cover crop biomass is better for some objectives, later terminated cover crops can pose some challenges. At this time of year, cover crops are using a lot of soil water. They can deplete soil moisture quickly, leaving a dry seedbed for the cash crop. In addition, by depleting the soil moisture, the cover crops themselves will undergo stress and maybe more difficult to control. So, if you have not killed your cover crop yet, be sure to look at the weather forecast for rain, and do not put yourself into a difficult situation by allowing the cover crop to use all the soil moisture if there is a poor chance of rain. On the other hand, you may consider delaying terminating the cover crop until rain comes and replenishes soil moisture.
We have done some work with planting green, meaning not killing the cover crop until after planting. This has potential, particularly if the cover crop was planted late and an additional 10 to 14 days of growth will provide substantially more biomass. 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. 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.
Planting is best if the cover crop is either dead or not sprayed yet. Cover crops that are in the process of dying can be challenging to plant into because the stems are harder to cut with a coulter. We typically plant into a standing cover crop and have been very successful in getting a good crop stand. In addition, we typically do not roll our cover crop when planting soybeans. We plant in 15-inch rows and all the planter units are on a single tool bar. This single line of planter units knocks over most of the tall rye. However, if 15-inch planter units are on two tool bars, I have seen more of the rye to remain standing after planting.
There is still a need for residual herbicides for most fields with cover crops. 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 in fields with heavy weed pressure, they do not replace herbicides.