Research Highlights

Research Highlights
Fine-Tuning Recommendations for Cover Crops and Phosphorus in North Dakota

In this article, you’ll find details on:

  • Two research projects in North Dakota can assist farmers in increasing their return-on-investment for cover crops and phosphorus fertilizer applications.
  • A project explores winter rye cover crop fall seeding dates and rates specific to the state. This data will help North Dakota farmers be more assured when planting a cover crop.
  • A second project measured soybean response to a phosphorus fertilizer annually applied in high pH soils compared to application ahead of corn with residual phosphorus for next season’s soybeans.

Phosphorus fertilizer application timing trial at Carrington, North Dakota, July 16, 2022. Photo: Greg Endres

By Carol Brown

To achieve the best yields and return on investment, farmers should follow recommendations specific for their state on crop seeding rates and varieties as well as proper fertilizer and herbicide amounts. Greg Endres, an extension cropping systems specialist with North Dakota State University, recognized that recommendations for cover crop planting and phosphorus application for soybeans were lacking for North Dakota farmers.

“We conducted research that tries to answer questions we’ve received from soybean growers,” explains Endres, who is based at the Carrington Research Extension Center. “Farmers are increasing their interest and usage of cover crops, but the recommendations for winter rye seeding amounts and planting dates were not clear and certainly not associated with precision planting. We’ve also had questions about phosphorus application for soybeans, and we were using recommendations from other states.”

Through funding support from the North Dakota Soybean Council, Endres conducted two research projects to establish solid data for farmers in the state. One project looked at winter rye cover crop seeding dates and rates, and the other focused on soybean response to prior-year phosphorus fertilizer application. The cover crop research took place over four years at Carrington, and the phosphorus research was a three-year project in Carrington and Minot.

Cover Crop Confidence

Winter rye is the most common cover crop used in North Dakota, but data on when and how much to plant was not consistent. Endres wanted to fine-tune these management aspects so farmers had greater confidence when using cover crops. He and his team planted winter rye on two dates in the fall at seeding rates of 25, 50, and 75 pounds per acre. 

“Depending on a farmer’s goals, there is a range of winter rye seeding rates,” Endres says. “Their goals may be to have some ground cover to reduce soil erosion, weed suppression, or maybe they want plants for grazing or haying. When we know what their goals are, we can provide them a better recommendation than what we’ve had in the past.”

Over the four-year trial, the first planting dates were mid-September through early October, and the second planting dates ranged from early October through early November. He wanted to include a later cover crop planting date to document its growth performance in this scenario. Farmers may plant cover crops later in the fall due to late growing seasons and delayed harvest of small grains or other crops.

To find out how well the cover crop performed, Endres and his team measured plant density by counting winter rye plants within a square foot and converting that to plants per acre. They also measured aboveground biomass to calculate ground cover percentage. A higher percentage of cover crop ground cover helps with weed suppression and soil protection (see Table 1).

The actual rye stand varied from year to year, but when averaged over the four years, the plots performed best with the highest seeding rate and the earlier planting period. They evaluated cover crop ground cover in May prior to rye termination and soybean planting.

“When we compared the same seeding rates across the two planting dates, in some years we had the same stand count, but the cover crop was less developed with the later planting time,” he says.

The study included measuring soybean yield in each of the cover crop plots. They terminated the winter rye with glyphosate shortly before planting soybeans at the end of May. 

“The soybeans responded similarly across the years regardless of what type of cover crop stand we had,” he says. “Soybeans averaged more than 50 bushels an acre across the four years of the study – they performed well for the project.”

Soybean Response to Phosphorus

It is standard procedure for north-central farmers to apply high enough rates of phosphorus fertilizer prior to planting corn to leave a residual for the next year’s soybean crop, Endres says, but this is not typical in North Dakota. 

“We generally have high pH soils, which means more phosphorus is tied up. Applying enough phosphorus and relying on the residual for a second year may not work as well,” Endres explains. “We wanted to explore soybean response to phosphorus application in these soils.”

Endres and his team had three treatments on very low or low phosphorus soil, with a pH between 7.5 and 8. The applied treatment rates were selected according to NDSU’s recommendations and included:

  • Pre-plant applied and incorporated (PPI) phosphorus for a corn-soybean rotation, applied the year of corn production (0-46-0 triple superphosphate)
  • PPI phosphorus applied each year for respective crops, and
  • an untreated check where no phosphorus was applied.

“When we averaged the individual trials across the years, soybean yield was similar between the two phosphorus application methods and was greater, compared to yield with the untreated check,” he says. “Results indicate one phosphorus application for both crops provides similar yield response as yearly applications. Benefits of the single phosphorus fertilizer application include reductions in trips across the field, which reduce input costs. And it provides more flexibility in purchase price and obtaining fertilizer supplies.”

A new, expanded research project on phosphorus statewide application rates is underway and is also supported by the North Dakota Soybean Council.

Additional Resources:

North Dakota Soybean Council 2023 Research Report

Meet the principal investigator on this project: Greg Endres

Published: May 27, 2024

The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.