On-farm research by Practical Farmers shows benefits of letting rye cover crop grow for longer before soybeans

After a fantastic fall for cover crop growth, cereal rye is up and growing around the state. Over the next month, it will be time to turn that green carpet brown and plant the cash crops. But when is the optimal termination date? Currently, many farmers play it safe by terminating two to three weeks before planting time, hoping to limit the chance that a cover crop might decrease the yield of their subsequent cash crop.

Some farmers, however, are finding they can let their cereal rye cover crop continue to grow right up to - and even after - soybean planting time. These farmers have been conducting on-farm research with Practical Farmers of Iowa to determine the best time to terminate cover crops ahead of soybeans.

Results from the first two years of this study show that farmers may be able to delay termination date without affecting yield, thus extending the time they’re benefiting from having a growing cover crop. In addition to benefits such as reduced erosion and improved water quality, cover crops can decrease weed pressure and hold nitrates in the soil for the cash crop to use later - both of which can translate into more cost savings for the farmer.

Read the full report, “Cereal Rye Cover Crop Termination Date Ahead of Soybeans,” at http://bit.ly/Cereal-Rye-Cover-Crop-Termination-Date-Ahead-of-Soybeans.

In 2014 and 2015, three Practical Farmers members conducted strip trials to compare two cereal rye termination dates in soybeans. The farmers - all of whom are experienced cover crop growers located in north-central Iowa - included Jeremy Gustafson of Boone, Jack Boyer of Reinbeck and Bob Lynch of Gilmore City. Each farmer no-tilled or drilled soybeans following either an early (two weeks prior) or late (a day before or after soybean planting) cereal rye termination date. Two of the three farms found no statistical difference in soybean yields from a delayed termination.

Boyer says one of the most interesting results on his farm was how allowing the rye to grow bigger helped to suppress waterhemp - a weed that has developed resistance to a range of herbicides, and can be particularly hard to control. Because he grows seed corn for DuPont Pioneer, which is harvested earlier than field corn, Boyer was able to get the rye established earlier in the fall of 2014. As a result, by early May 2015, he had over 4,000 pounds per acre of rye biomass in his research strips.

Early in the 2015 season, Boyer noticed the strips with cover crops - in both the early and late termination treatments - had far fewer weeds than those without. As a result, in early July he decided to spray just the no-cover strips with a post-emergence herbicide. By late August, he says the cover crop areas “continued to be as clean as or cleaner than the no-cover areas,” and that the cover crop areas remained mostly free of weeds through the end of the season. Thanks to the cover crop’s weed-suppressing effect, Boyer had higher farm profitability that growing season because he needed less post-emergence herbicide.

And because the project results have given Boyer the impression that “with soybeans, you have considerable flexibility with the cover crop termination date,” he sees herbicide cost-savings as another potential benefit of planting a cover crop.

Gustafson also noticed lower weed pressure affecting his soybeans in the strips with delayed termination as a result of the extra cover crop biomass. He says that’s an added benefit, on top of the rye’s soil and water quality benefits. “We wanted to see how big we could let the rye go. We think that the longer you let the cover crop grow, the more chance you have at weed suppression.”

Delaying cover crop termination even a few days can make a big difference in biomass production, because those extra days of growth happen during optimal growing conditions for the rye plant. This rapid growth was evident to Gustafson when he collected biomass samples as part of the study. When terminated earlier, the cereal rye produced 178 pounds per acre of biomass, compared with 2,684 pounds per acre of biomass when terminated later. Despite the extra plant biomass on the ground at planting time, soybean yields in the two treatments were virtually the same: 62 bushels per acre in the strips terminated early and 64 bushels per acre in the strips terminated late.

Keeping the soil covered is one critical benefit of delaying termination. During April and May, when heavy rains are common, soil is very susceptible to being lost. Because soil temperatures are rising - and soil is usually wet - spring is also a time of year when soil microbes are especially active and producing nitrates.

If plants aren’t growing on the landscape, those nitrates can leach into groundwater or run off into streams, damaging water quality. Keeping the soil covered as long as possible gives the cover crop more time to capture those nutrients so they are available later in the growing season when the cash crop needs them. Besides helping to nourish the cash crop, this nutrient capture protects water quality.

“Why do I want to pay for nutrients and fertilizers that I just end up losing off of my ground?” Gustafson says of cereal rye’s ability to scavenge nutrients from the soil.

Both Gustafson and Boyer plan to continue this research project for a third year. For Gustafson, the results of the first two years were so convincing that he decided to delay terminating rye and “plant green” on 40 soybean acres last year that were not part of the project. This year, he says he intends to plant an additional 200 acres that way.