Cover crops are thirsty, but they also protect the soil surface and provide other benefits. How does this push-pull relationship play out in a drought year?
Dan Smith, nutrient and pest management outreach specialist at the University of Wisconsin, dug into their 2023 cropping systems study for some answers in June. Corn plants from the full tillage treatment were taller, but the roots and soil were powder dry. Some soil moisture was retained in no-till, but it was still very dry. “What surprised me the most,” stated Smith, ”was that there was soil moisture and good soil structure in the top 2-inches of the planting green treatment.” In this treatment, corn was planted into 12-inch-tall cereal rye. Rather than stealing all of the moisture, the rye helped retain more soil moisture instead.
Deciding when to terminate a cover crop is challenging. If the goal is to maximize biomass, competition could compromise both the cash crop and soil moisture in a dry year. Wisconsin studies looked at three rye termination times in both corn and soybeans: 2 weeks before, at planting, and 2 weeks after planting. Corn grain and silage yields were reduced when rye was terminated at and after planting. However, when rye was terminated two weeks before planting, yields were equal with those in conventional tillage and no-till. As a result, Smith recommends terminating the rye before planting to avoid competition in a dry year.
Soybeans yielded only slightly less at high levels of biomass. Soybeans seem to be impacted less by cover crops and soil moisture concerns than corn is.
In a similar planting green study in soybean in Minnesota, yields were significantly reduced with late rye termination. However, Dr. Anna Cates, state soil health specialist, suspects it was due to planting and other issues rather than the biomass itself, since little had been produced. In five of six study sites, preplant termination protected soybean yield.
In heavy residue systems, planter issues can follow, particularly in a dry year when planting at the right depth is challenging. However, unless a planter is severely worn out, Smith doesn’t recommend buying a new one for cover crops. Instead, he suggests making planter adjustments, such as upgrading the double disc openers and closing wheels. Other tools, like hydraulic down pressure, air down pressure, variable air ride and row cleaning wheels may also be helpful.
Because the gauge wheels ride on top of the residue, the planter seeding depth may need adjustment. To make sure it is operating correctly and rows are getting closed, tie up the closing wheels, run the planter for 10 to 15 feet, then examine the row for proper seed spacing and twoinch corn seeding depth.
Cereal rye should be planted by mid-September in Minnesota for optimal seeding success and biomass production. Once into October, successful establishment and biomass production declines.
Seeding rate studies are ongoing. In Wisconsin, Smith seeds a bushel of rye to maximize biomass, but if the goal is to simply protect soil, 20 pounds of rye per acre after crop harvest may be adequate. Minnesota studies suggest that seeding rates above 60 pounds per acre are not needed.
Why cereal rye? “It’s reliable for both establishment and getting decent growth in the short corn-soybean rotation window,” explains Cates. It’s also relatively inexpensive and usually readily available.
More diverse mixes are more appropriate with a longer window, such as after corn silage or small grains, or when interseeding into corn. A mix with a brassica, oats or a legume has the added advantage of not needing to terminate a grass in the spring.
Develop a strong plan to manage soil structure, soil fertility, crop pests and variety selection to increase long-term resilience under dry conditions. Consider incorporating cover crops, reducing soil disturbance and adding other crops into the rotation to build soil health and resilience.
If 2024 remains dry, Smith recommends terminating cover crops 10 to 14 days before planting to reduce competition with the crop. Cates agrees, “If you’re going to err, it might be wise to err on the early side.”
For more information from University of Minnesota Extension, visit extension.umn.edu/cropproduction.
Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.
For more news from U of M Extension, visit https://www.extension. umn.edu/news or contact Extension Communications at extdigest@umn.edu. University of Minnesota Extension is an equal opportunity educator and employer.
