On paper, adding a cover crop to a farming production system looks simple enough: plant a small grain such as cereal rye as soon as the cash crop harvest is done; let it grow, then terminate it prior to planting in the spring. But farmers and agronomists know it’s not that simple.
As more producers adopt cover crops, researchers continue to look for answers to arising questions. Entomologist Justin McMechan is one of the experts who receives regular questions from farmers using cover crops.
“Typically, I get questions like, ‘I found a certain insect in my cover crops. Should it be here and how do I control it?’’’ he said. “In our research, we are finding a lot of beneficial insect activity in cover crop systems, which is great.”
McMechan, an assistant professor at the University of Nebraska, is conducting research on how cereal rye and wheat cover crops impact pests and weed pressure. An expert on insects, he’s working with agronomists and Extension specialists at seven sites on privately owned and University farms in Nebraska. The Nebraska Soybean Board and the North Central Soybean Research Program (NCSRP) fund the studies.
“Our research goals are two-fold. We were interested in what sort of pest pressure might be in the cover crops that producers should be aware of and what types of beneficial insects are living there,” McMechan said. “Once this is known, we can help growers avoid the use of tank-mixing insecticides unless they are appropriate when they are terminating their cover crops.”
Cover crops are being adopted across the Midwest for several reasons. Some farmers are using cover crops for grazing livestock as a supplemental feed source, others are using them to reduce soil erosion, build soil organic matter and reduce weed pressure, which is part of this study. In Nebraska, farmers using cover crops seem to be able to get into their fields a little sooner, McMechan said, especially if there is a wet spring.
“One of the most important things we’re finding in the cover crops is a lot of spiders, a generalist predator. And with them, there are lot of aphids — a food source for spiders, lady beetles and other insects,” he said. “This type of activity is great because now we have this abundant food supply for predators that are sitting there in case any early-season pests show up.”
The soybean aphid is the species damaging to the soybean plant, but they aren’t finding them in their cover crop acres, McMechan said. Most of what he is finding is bird cherry oat aphid, a non-pest species found in soybean and corn systems.
Cover crop and yield impacts
A disadvantage to using cover crops is justifying the costs of seed, fuel and machinery for planting and termination. Farmers are hoping cover crop benefits translate to increases in yield.
McMechan is working with Chris Proctor, a Nebraska Extension educator on weed management, in several areas of cover crop research. Some of their studies include cover crops and weed pressure and yield response of different soybean maturity groups based on cover crop planting and termination dates.
For the termination project, cover crops were sprayed 14 days prior to planting soybeans, terminated at planting, and five days post-planting. The team measured the amounts of cover crop biomass at each termination date across all their sites.
“Our studies ranged from a few hundred pounds of biomass per acre, which is hardly noticeable, to almost 5,000 pounds per acre at one location last year,” McMechan said. “Cover crops are certainly suppressing weeds and, in 2019, we didn’t see any significant reductions in yield for soybeans, which means the cover crops were not negatively impacting the system.”
They found a little less soybean biomass earlier in the season after planting in the locations where there was a lot of cover crop biomass, but McMechan advises farmers not to panic.
“Cash crop and cover crop interaction is complex,” McMechan said. “But soybeans are a really forgiving crop when it comes to compensating for yield throughout the course of the season. They will come back.”