By Phyllis Bongard and Dan Smith et.al
Cover crops are thirsty, but they also protect the soil surface and provide other benefits. Dan Smith, Nutrient and pest management outreach specialist at the University of Wisconsin, and Dr. Anna Cates, State soil health specialist, discussed the push-pull relationship of cover crops and soil moisture in the February 28 Strategic Farming: Let’s talk crops session.
Cover crops in dry years
Smith saw zero rain for two months between May 6 and July 4 in southern Wisconsin last year. By July 1, the precipitation deficit was already 6 inches below normal at the UW Arlington Research Station. Similar scenarios played out in much of the Upper Midwest during 2023, including significant portions of Minnesota. Under this kind of stress, how do cover crops impact the following row crops?
After a tough June, Smith dug into their cropping systems study for some answers. The study included full tillage, no-till, and “planting green” treatments – a system where corn was planted into 12-inch-tall cereal rye. In the full tillage treatment, the corn was 2 growth stages ahead of the other two treatments, but the roots and soil were powder dry. In the no-till treatment, roots were concentrated closer to the soil surface suggesting there was some moisture in the soil. Even so, it was still very dry. What surprised Smith the most was that there was soil moisture and good soil structure in the top 2-inches of the planting green treatment. The cereal rye hadn’t stolen all of the moisture and in fact helped retain more soil water than the other treatments.
An early adopter of regenerative ag practices in Dodge County, Wisconsin, has also seen these benefits. During the peak of the 2023 drought, his fields were able to retain soil moisture resulting in excellent crops. The various mixes of cover crops he employs year-round have helped increase soil organic matter, water retention, and other soil health factors, but maintaining soil moisture is a major component of his success.
Nutrient management study observations
Smith also conducted nitrogen (N) rate studies with corn planted into cereal rye residue. One might expect the corn to be limited where no fertilizer N had been applied; however, it was a healthy green, had good shoot and root growth, and yielded well. It’s likely that the level of precipitation wasn’t enough to flush the previous year’s nitrates through the soil profile and the cereal rye helped retain what soil moisture there was.
Termination challenges
Deciding when to terminate a cover crop can be challenging. If your goal is maximizing biomass, the competition from the cover crop could compromise both the cash crop and soil moisture in a dry year. Just how long is too long when it comes to leaving a cover crop in the spring? Studies conducted in both Wisconsin and Minnesota shed some light on this.
Wisconsin studies
Wisconsin studies were located at the Arlington and Lancaster farms on silt loam soils in both corn and soybeans. Conventional tillage, no-till, three rye termination timings and forage treatments were included. Rye was terminated 2 weeks before planting (early), at planting, and 2 weeks after planting or planting green (late).
One of their goals is to reach 4,000 pounds of biomass per acre to suppress weeds, so rye is seeded at a bushel per acre. Most of the time, they’ve been able to achieve that level of biomass when rye is terminated two weeks after planting. However, with the dry conditions in 2023, the rye smothered out the crop when it was left in the field and corn grain and silage yields were significantly reduced. There was even a yield drag when the rye was terminated at planting. However, when the rye was terminated two weeks before planting, corn performed as well as the conventional and no-till treatments. While Smith would like to see biomass maximized, in a dry year he recommends terminating the rye before planting to avoid competition.
Soybeans showed a slight yield drag at high levels of biomass in 2023. Even in this dry year, however, the termination timing didn’t matter. As a crop, soybeans seem to be impacted less by cover crops and soil moisture concerns than corn is.
Minnesota studies
Minnesota planting green studies are concentrated in western Minnesota and currently focus on soybean. Termination timings were similar to Wisconsin’s: 1-2 weeks before planting, at planting and 1-2 weeks after planting. Biomass yields ranged widely, from a max of 72 lb/acre in Polk county, and over 2400 lb/ac in Yellow Medicine County.
Soybean yields were significantly lower with late termination, but not in the field with the highest biomass. Cates suspects that it was due more to planting issues and other limiting factors than to the quantity of rye biomass. In five of six study sites, preplant termination protected soybean yield.
Planter issues
In heavy residue systems, planter issues can follow, particularly in a dry year. Getting the planting depth right in powder dry soils is critically important. Not only does uneven emergence make nitrogen management and herbicide decisions challenging, but the field doesn’t look very good either.
Do I need a new planter for cover crops?
Unless your planter is severely worn out, Smith’s answer is a resounding ‘No, you don’t need a new planter!’ Instead, he suggests making planter adjustments. For example, he upgraded the double disc openers and closing wheels for their silt loam soils, keeping one stock closing wheel and adding a spike closing wheel. Some are better suited for clay soils, so decisions should be made on an individual farm basis. Other tools, like hydraulic down pressure, air down pressure, variable air ride and row cleaning wheels also help. Check with your neighbors who are doing this and see what they’re using for attachments.
While you may not need a new planter, you will need to frequently check that the seeding depth is correct and that the rows are getting closed. Because the gauge wheels are riding on top of the residue, you’ll need to adjust the seeding depth to account for that. To make sure it is operating correctly, tie up the closing wheels with a heavy bungee cord or ratchet strap, run the planter for 10 to 15 feet, then examine the row for proper seed spacing and seeding depth.
Seeding date
Research out of the Southwest Research and Outreach Center (SWROC) in Lamberton has found that for optimal seeding success and biomass production, rye should be planted by mid-September. When seeding gets delayed into mid-October, biomass production declines in both the fall and subsequent spring.
Seeding rate
Seeding rate thresholds are currently being researched and results will depend on the system and crop. In Wisconsin, Smith seeds a bushel per acre to maximize biomass, but if the goal is to hold soil in place, seeding rye at 20 pounds/acre after crop harvest might be adequate.
Other work at the SWROC studied higher seeding rates of 60, 90, and 120 pounds/acre to try and maximize biomass and weed control. Seeding rates above 60 pounds/acre are not recommended in a conventional system and rates below 60 pounds are an active area of research interest.
Drilling the cover crop between crop rows is another option. This strategy minimizes competition with the crop while providing soil coverage benefits.
Cover crops and soil water
Cereal rye removes a significant amount of moisture, particularly as it gets closer to reproductive stages. However, it also protects the soil surface from evaporation where residue is in place. How does this push-pull play out through the soil profile?
Cates compared volumetric water content (VWC) between a cover crop and no cover crop in different tillage systems in 2021 and 2022 at three soil depths at Waseca: 10, 30, and 60 cm. There were no differences due to cover crops at the 10 cm depth in either year. At 30 cm, there was a slight increase in VWC with a cover crop treatment in strip till, but that effect reversed at 60 cm in 2021. In 2022, there were few differences at 30 cm, but like 2021, the conventional system may have had a slight edge over cover crops at 60 cm. At shallower depths, the competing effects of evaporation and cover crop uptake may be offsetting each other over the course of the season.
Soil’s response to rain
Can soil with good structure in reduced tillage and cover crop systems take in more water? To get at this question, Cates’ graduate student studied soil moisture, aggregation, and pore size before and after rain at on-farm sites with long-term conventional and soil health systems.
The soil health systems generally captured more water, although results were somewhat inconsistent. Again, competing factors of cover crop uptake and evaporation from the soil surface could be at play.
Water stable aggregates – those greater than 2mm – are the larger elements of soil structure. Intensive tillage can destroy these elements by physically breaking them apart. Large aggregates are a good indicator of reduced soil disturbance and a healthy soil system. In this study, rain promoted biological activity in the soil health system resulting in an increase of aggregation. In contrast, rain tended to dissolve the aggregates in the conventional system over time.
Finally, pore size is also an indicator of good soil structure. In the soil health systems, pore sizes skewed larger than in the soil health system.
Building drought resilience
A strong plan for managing soil structure, soil fertility, crop pests, and variety selection are all pieces of a puzzle that have large implications. Be strategic about the number of passes across the field and remember that they affect both soil moisture and structure. Several practices promote long-term resiliency under dry conditions. Consider incorporating cover crops, reducing soil disturbance, and adding other crops into the rotation to build soil health and resiliency.
Why cereal rye?
What makes cereal rye a popular choice as a cover crop? In Cates’ experience, it’s reliable in terms of both establishment and getting decent growth in the short corn-soybean rotation window. It’s also relatively inexpensive and usually readily available.
More diverse mixes would be more appropriate with a longer window, such as after corn silage or small grains, when the goal is to graze the cover crop, or when interseeding into corn at V4 toV5. A mix with a brassica, oats, or a legume planted in the fall has the added advantage of not needing to terminate a grass in the spring.
Management in 2024
If you have been working with cover crops for a while, Smith recommends staying the course with your management plan. If 2024 remains dry, terminate cover crops 10 days to two weeks before planting to reduce competition with the crop. However, the rye must be actively growing for adequate control. Temperatures should be in the 50s at night and 60 to 70 degrees during the day to ensure that the herbicide will be translocated.
Cates agrees. Terminating rye before planting is less risky when it’s dry and if you’re going to err, it might be wise to err on the early side.
Cover Crop Academy registration is open!
If you are a crop advisor, consultant or an ag advisor in a Soil and Water Conservation District (SWCD) or the Natural Resources Conservation Service (NRCS), the Minnesota Cover Crop Academy is for you. The Academy is a year-long, hybrid course to give you the tools to work with farmers who want to incorporate cover crops into their crop production systems. We’ll explore how to utilize soil health practices that are agronomically and economically successful, how to work with Minnesota's short growing season, and how to help address crop production challenges with cover crops. For more information, visit the Cover Crop Academy webpage.
Source : umn.edu