U of I study provides guidance for using cover crops in dryland wheat rotations

MOSCOW, Idaho — A recent University of Idaho-led study conducted in collaboration with regional dryland wheat farmers provides guidance on when to terminate a spring-planted cover crop.

Cover crops, which are grown primarily to benefit soil health and fertility rather than for commercial sale, are becoming increasingly common on northern Idaho and eastern Washington farms. However, little research has explored how they perform in local growing conditions and cropping systems.

Producers and researchers involved in U of I’s three-year study, called the Pacific Northwest Cover Crop Decision Aid System (PaNDAS), evaluated how cover crops with varying species diversity affected soil moisture, as well as nitrogen and carbon levels, when killed at three different growth stages.

The farmers and scientists agreed that terminating cover crops in early to mid-bloom stages — which generally occur from late June through the first week of July depending on water availability — struck the right balance between maximizing soil-health benefits and minimizing water consumption.

The early season termination date in low-precipitation areas, which occurred in early June, provided insufficient time for cover crops to make an impact. The late-season date, which occurred when seed pods started filling, around mid-July, used too much water to justify the additional cover crop growth in the study’s driest growing areas.

In growing areas with heavier precipitation totals, the ideal termination date was closer to full bloom. By the late-season termination date, farmers in wetter growing areas were concerned that some cover crop seeds had reached maturity and posed a threat of emerging as volunteers in subsequent commercial crops.

“It’s a tremendous amount of new information we obtained in partnership with eight producers, who were co-principal investigators,” said Sanford Eigenbrode, a distinguished professor emeritus in the Department of Entomology, Plant Pathology and Nematology, who was part of the PaNDAS research team. “All of our producers want to keep going with this research. They want to learn more because they really enjoyed this experiment, they enjoyed the experience of collaborating with us and they learned things.”

Farmers operating in three different rainfall zones in Genesee and Troy in Idaho, as well as Cloverland, St. John, Uniontown and Palouse in Washington — receiving annual precipitation of 10-12 inches, 14-18 inches or 18-22 inches — devoted 6 acres each toward replicated cover crop trials.

Trials, which were conducted in 2023 and 2024, compared a low-diversity mix containing a brassica, a grass and a legume against a high-diversity mix with three species from each genus. They also compared cover crop performance when terminating them in the early, mid and late season.

“We measured how much biomass cover crops produced and how much nitrogen and carbon were stored in that plant material,” said Kendall Kahl, a PaNDAS team member who is an assistant professor in the Department of Soil and Water Systems (SWS). “We also measured soil nitrogen during the cover crop year and then again every six months for the next year and a half to see if we could measure some of that nitrogen from cover crops in the soil.”

Cover crops replaced garbanzo beans in the crop rotation on farms in the wetter regions and were planted in place of summer fallow in the dry areas. Farmers planted winter wheat following cover crops.

The researchers found cover crops in all scenarios were effective at recapturing nitrogen that would have otherwise leached from fields, bringing it to the surface to slowly decompose over six months to two years, depending on conditions, and eventually become available for use by commercial crops.

“In our high-rainfall areas, those cover crops are holding about 70 pounds of nitrogen per acre in plant material that will become available at some point,” Kahl said. “In our drier areas, we are seeing 45 to 50 pounds per acre of nitrogen in that cover crop biomass. The question is, when will it break down and show up in the soil?”

The second termination date also resulted in the ideal carbon-to-nitrogen balance in the cover crop biomass for soil microbes to convert organic nitrogen from cover crop foliage into inorganic forms accessible by commercial crops.

Species diversity seemed to have no effect on nutrient cycling and soil water availability, though high-diversity mixes attracted a much greater diversity of insects.

“If you’re concerned about the cost of doing cover crops, I think this research shows that choosing a simple, less expensive mix will still give you a similar benefit in terms of nitrogen,” Kahl said.

The researchers saw no yield gains following cover crops in any of their management approaches, but they believe that winter wheat yields could increase with a longer study.

“The cover crops use a little bit more water than what you are going to lose in fallow, but you get a significant amount of carbon and nitrogen and potentially other soil benefits from growing the cover crop,” Kahl said. “We hope to continue this study to look more closely at how our cover crop management strategies impact soil health — in particular our soil arthropod and microbial populations, which the growers we’re partnering with are interested to learn more about.”

Other PaNDAS researchers included Erin Brooks, a professor in SWS; Cami Ditton, an SWS master’s student; Brianna Slothower, an SWS laboratory technician; Doug Finkelnberg, an area Extension educator of cropping systems; Patrick Hatzenbuehler, an assistant professor and Extension specialist in the Department of Agricultural Economics and Rural Sociology; Ryan Boylan, with the Palouse Conservation District; and Subodh Adhikari, an assistant professor with Utah State University.

PaNDAS was funded with a three-year, $350,000 grant from U.S. Department of Agriculture Western Sustainable Agriculture Research and Education, of which 100% was the federal share, under award number SW22-940.

Media contact  

Kendall Kahl
Assistant professor and Extension specialist, Department of Soil and Water Systems
208-885-7505
kkahl@uidaho.edu