Final report for ONC23-132
Project Information
Among grain growers in the Midwest, interest is growing in cultivating winter small grains followed by a legume cover crop to take advantage of the growing window after small grain harvest in July. While frost seeding red clover into small grains was historically utilized and nitrogen (N) credits have been published in the past, the system has been understudied using modern approaches. Moreover, research on the soil health benefits and nitrogen credits from newer practices like planting diverse mixes after small grains are harvested in summer, lags behind grower adoption of diverse cover crop adoption. Our project recruited 7 growers in southeast Wisconsin who employ a small grain-cover crop phase in rotation with corn, and we measured soil health indicators, fall and spring cover crop biomass accrual, and preplant and presidedress soil nitrate tests to compare summer cover cropping after small grains with a small grain-summer fallow control. We also deployed a nitrogen rate response study to quantify the N credit to corn by measuring corn yields on each farm. On one farm with a history of frost seeding red clover into rye, we observed a N credit of 212 lb N/acre in corn; on this farm; corn yields following rye-red clover without additional N fertilizer were more than 20 bu/ac greater than the highest N fertilizer rate (160 lb N/ac) applied to the no-cover control. On the second farm frost seeding red clover into wheat, we did not observe a significant N credit; however, corn leaf N content was greater following cover crops vs. the control. On farms employing diverse cover crop mixes, we did not observe significant N credits. Our results suggest that cover crop mixes will likely need to contain a high percentage of legumes to result in significant N effects, and N credits, in corn the following year. However, we did observe benefits of diverse cover crop mixes on corn uptake of phosphorus and sulfur, suggesting that diverse cover crops may enhance non-N nutrient cycling processes in soils. We are continuing research with a larger grower cohort to build a larger dataset to gain more insight into the benefits of diverse cover crop mixes, and to identify which species constituents exert an effect on corn nutrient uptake and yield benefits.
- Quantify spring and fall biomass production of several legume-based cover mixes by collecting data on 6 collaborating farms.
- Identify nitrogen credits of legume-based mixes to the following corn crops by measuring corn yield in nitrogen fertilizer response field trials on each collaborating farm.
- Investigate diverse cover crop mix effects on nutrient cycling in the following corn crop for nutrients other than nitrogen, including phosphorus, potassium, and sulfur, significant fertility factors for corn.
- Present results and disseminate published products alongside collaborating growers at 2 on-farm field days and at 3 annual grower conferences.
Cooperators
- (Researcher)
Research
We conducted planning meetings with the seven collaborating growers in southeast WI (farms were located in Dane, Dodge, and Waukesha counties) in early spring of 2023 to select fields for trialing, and to design trial layouts. Each grower had used a wheat-cover crop phase in their rotations prior to the study, so most fields had a history of this practice. Fields planted to winter wheat or rye the previous fall of 2022 were selected, and the two organically managed sites using frost-seeded red clover broadcast seeded at a rate of 10-12 lb/acre in early March 2023. Wheat or rye grain was harvested on each farm in mid July of 2023, and straw was retained on the field (not baled and removed). The five conventional farmers terminated post-harvest weed regrowth with glyphosate prior to planting summer cover crops. The cover crop (CC) and no cover crop (NCC) treatments were deployed in a randomized complete block design with 4 replicates, with plots 30 or 36 ft wide and lengths of 400 to 1000 ft long, depending on the farm. Prior to cover crop planting, soil samples in 0-6" and 6-12" depth increments were collected in each whole plot and analyzed for soil health indicators. Each collaborating farmer who was planting CC after wheat harvest selected their diverse cover crop mix; the mixes ranged in diversity from 4 to 14 cover crop species included. Fall CC biomass sampling was performed by hand cutting three 4-ft2 quadrats per CC plot; subsamples were combined to form one sample per plot, dried, and weighed. In some NCC plots on some farms, volunteer wheat regrowth was observed because post-harvest glyphosate applications were not effective during a rainy period of the summer; in these plots, biomass sampling was conducted in the same way as previously described for CC plots.
CC overwintered from 2023 to 2024; non cold-hardy species in the mixes winter-killed, whereas cold hardy species and the red clover regrew in spring of 2024. Spring CC biomass was collected using the same methods as previously described for fall CC biomass, and dry weights were recorded. CC were terminated with two disk plus finisher passes on the two organically managed farms in late May 2024 whereas the five conventional farms terminated CC with glyphosate in early April. Hybrid corn for grain, the predominant cash crop of the region, was planted across the entire trial area in early May on the conventional farms, and early June on the organic farms. Just prior to planting corn, preplant soil nitrate measurement was conducted by soil sampling and soil analysis using the Wisconsin Preplant Soil Nitrate Test (methods described in Bundy et al. 1994). In early June, soil sampling was again conducted to perform the presidedress nitrate test, where five 1-ft depth soil cores were collected per plot, composited, and fresh soil was analyzed for nitrate content (methods described in the UW-Wisconsin Extension's Nutrient and Pest Management publication here: https://ipcm.wisc.edu/wp-content/uploads/sites/54/2022/11/UWSoilNitrateTests_final.pdf).
In mid-June, the split-plot treatments of nitrogen (N) fertilizer treatments were deployed to create the N fertilizer response series, to enable N credit determination. Each whole-plot strip of CC or NCC was divided into 5 equal-size split plots, and N fertilizer treatment assigned randomly, using an N fertilizer series of 0, 40, 80, 120, and 160 lb N/acre. Fertilizer treatments were broadcasted on the two organic farms using a NatureSafe bloodmeal-based product (13-0-0); conventional farms used standard liquid urea injection methods to apply N treatments. Corn ear leaf samples were collected in late July when corn was at growth stages V6-V8 by harvesting 15 ear leaves per split plot, drying, and analyzing for N, P, K, and S content. Corn yield was measured in each split plot by calibrating yield monitors and cutting one combine strip down the center of each plot (header width was 15-ft wide, harvested the length of each split plot); yield monitor yields over each split plot harvest strip were recorded. In one site we had access to a weigh wagon, so each split plot was harvested using the methods described above, but the combine dumped each harvest strip's grain into the weigh wagon, and the weight was recorded.
Fall CC biomass was significantly higher for diverse summer cover mixes than for frost-seeded red clover at 3 of the 4 sites where diverse mixes were used. Diverse mixes generated 1500 to 3400 lb DM/acre biomass across the 4 sites where they were used. Frost-seeded red clover produced 2300-24oo lb DM/acre at the 2 farms where used. Significant volunteer wheat regrowth of 1000-2000 lb DM/acre was observed at 2 of the sites. However, in the spring, red clover biomass was generally greater than the diverse mixes where the same range of dry matter production was observed as was measured in the fall. Most species constituents of the diverse mixes winter-killed; diverse CC dry matter production in the spring was lower than 1200 lb DM/acre at all 4 sites where tested, and 3 of the 4 sites produced less than 1000 lb DM/acre cover crop dry matter in the spring. Soil preplant nitrate levels were quite low at 6 of the 7 sites measured (<5 ppm nitrate); however, the site with a history of frost seeding red clover had soil nitrate levels of 30 ppm. Fall CC biomass production was positively correlated with preplant soil nitrate, particularly in the 1-2 ft depth increment (p=0.004, R2=0.64). Despite the relationship observed between CC biomass and preplant nitrate, CC N content was not correlated with preplant soil nitrate content. Spring CC biomass was negatively correlated with preplant nitrate, suggesting that fall but not spring CC biomass is playing a measurable role in early-season N cycling.
CC effects on presidedress nitrate were not clear; no difference in presidedress nitrate levels were observed between CC and NCC strips on 5 of the sites; at the site using the 14-way CC species mix, presidedress nitrate was significantly higher in the NCC plots than the CC plots; at the site with a history of frost seeding red clover, similar to preplant nitrate, presidedress nitrate was significantly higher in the CC plots.
Surprisingly, the only site where we were able to calculate an N credit from cover cropping was the site with a history of frost seeding red clover, where we observed an N credit of 212 lb N/acre to corn from frost-seeded red clover. Corn yields at this site displayed a classic response to the N fertilizer series; corn yields in the NCC plots ranged from 120 bu/acre at the 0 N fertilizer rate, to 170 bu/acre at the 160 N fertilizer rate, with a classic quadratic response across the N series. As expected, the N fertilizer response in the CC plots at this site was less steep, with corn yields of 190 bu/acre at the 0 N rate following frost seeded red clover, and yields of 200 bu/acre at the 160 N rate. This value is similar to old values reported for frost seeded red clover from agronomic trials in the 1990s. Corn leaf testing supported these trends; corn leaf N responded with a similar model to the N fertilizer series in the CC and NCC plots as for corn yields, where leaf N was significantly higher for all N fertilizer rates following CC than following NCC. These higher N uptake values likely contributed to the greater yields. However, we also observed significantly higher leaf K and leaf S following CC compared to NCC, suggesting that frost seeded red clover may be playing a role in nutrient cycling in other important plant nutrition factors, besides just N.
Of the diverse summer cover mixes (which contained warm-season grasses, legumes, broadleaves, and often, pea), no significant differences were observed in corn yields across the N fertilizer response treatment series, compared to the NCC. At the Dane1 site where clover species dominated the mix, while we did not detect a significant yield difference, we did observe significantly higher corn leaf N in the corn following CC compared to NCC. Due to the lack of difference in corn yields following CC vs. NCC, we were not able to calculate N credits for these mixes, as we had originally hypothesized. The N content and possible facilitation of soil dynamics that do not mobilize mineralizable N in the summer CC likely creates these conditions that do not demonstrate a rotation effect. However, there were some indications, especially at the Dodge County sites for potassium (K), that summer diverse CC mixes were resulting in increased availability of other non-N plant nutrition factors; the differences we detected at some of the sites between the CC and NCC for some macronutrients warrants further investigation. Again, the Dane1 site where clover species dominated the mix showed a marked increase in corn uptake of S, which can be an important nutrient for grasses and increasing grain yield. While diverse warm-season-dominated summer CC mixes did not result in detectable N credits, they likely have other beneficial impacts on the crop rotation, but these other factors need to be quantified.
This research motivated many more questions and follow-up studies; see below for more information on next steps.
Educational & Outreach Activities
Participation Summary:
Original collaborators Tautges and Fulwider gave many presentations about this project to grower groups over the 2024-25 winter season. The project findings were presented to 40 growers at a December 2024 meeting of producer-led watershed groups that gathered 5 different groups spanning most of southern WI. Findings were also presented to the Dodge County producer-led watershed group, "Dodge County Farmers for Healthy Soil & Water," from which 3 of the group members served as trial hosts. Other farmers viewed the findings and reported potentially making changes to their CC mix composition, to attempt to add more legumes and increasing their ability to cycle N from their summer CC phase. Findings were also presented at the Illinois Organic Small Grains Conference in February 2025, and at the Marbleseed Conference of 2025, cumulatively resulting in another 100+ presentation viewers. Participants reported leaving the presentations with a better understanding of the practice of frost seeding red clover, and what the rotational and economic benefits of red clover cropping could be when rotated between small grains and corn. These topics were discussed at other field days in 2o24, including the UW Small Grains Field Day in June 2024, and at the Hazzard Free Farm Field Day in September 2024 in Pecatonica, IL.
This research project is still active, with more farms and events, through funding from WI-DATCP, and outreach is ongoing (see the Project Outcomes section).
Learning Outcomes
nitrogen crediting; nutrient management
cover cropping
nutrient cycling
Project Outcomes
Original collaborators Tautges and Fulwider gave many presentations about this project to grower groups over the 2024-25 winter season. The project findings were presented to 40 growers at a December 2024 meeting of producer-led watershed groups that gathered 5 different groups spanning most of southern WI. Growers were impressed by the potential N fertilizer savings of the frost-seeded red clover CC practice that resulted in more than 200 lb N/ac credits; workshop participants expressed that the red clover cover crop was likely worth up to $200/acre in cost savings and benefits, if not more. Findings were also presented to the Dodge County producer-led watershed group, "Dodge County Farmers for Healthy Soil & Water," from which 3 of the group members served as trial hosts. Other farmers viewed the findings and reported potentially making changes to their CC mix composition, to attempt to add more legumes and increasing their ability to cycle N from their summer CC phase. Findings were also presented at the Illinois Organic Small Grains Conference in February 2025, and at the Marbleseed Conference of 2025, cumulatively resulting in another 100+ presentation viewers. Participants reported leaving the presentations with a better understanding of the practice of frost seeding red clover, and what the rotational and economic benefits of red clover cropping could be when rotated between small grains and corn.
The project inspired another proposal to the Nitrogen Optimization Pilot Program (NOPP) in 2023, which was successful, and which helped fund more sites to make the on-farm location number larger; we were able to include more farmers in the project. With this funding, we were able to repeat the data gathering another year, through 2025. In addition to the NOPP 2023, we joined with more University of Wisconsin Extension Specialists and educators to develop a proposal with similar methods but with 10 on-farm sites per year, for the NOPP 2025 call, and the proposal was awarded. Through this SARE project, we learned that legumes are an important constituent of CC mixes if N crediting is the goal; therefore, all growers in our new project have pledged to include a minimum of 80% of legumes in their seeding mixes. We also learned that quantifying, through biomass sorting, the species composition (on a dry weight basis) of the summer CC mixes is important for data analysis and outcome interpretation, and have added those methods to this new project.
At the end of this year we will have accumulated 20 site*years of data, supported by the SARE and WI-DATCP NOPP programs, and it all started with this SARE award. We will create a large dataset that will accrue on-farm-tested summer cover cropping management decision-support resources, helping growers to choose mixes and really understand what the mixes are doing for their economics and nutrient management programs.
"I have been frost seeding red clover into rye for the past 5 years, and over that time have learned I can completely eliminate additional fertilizer inputs to my corn the following year. We ran some non-replicated test strips over the years, but this project with the Michael Fields Agricultural Institute and SARE has really put some numbers behind what my eyes tell me, and now I have data to show to other farmers and to my customers about how my practices really work, and how they set our farm apart for regenerative practices."