- Author: Michelle Leinfelder-Miles
- Author: Brenna Aegerter
In 2020, we completed a three-year on-farm trial in the Delta to evaluate warm-season legume cover cropping between winter small grain forage crops. Cover cropping is a management practice identified in the Healthy Soils Program of the California Department of Food and Agriculture as having the potential to improve soil health, sequester carbon, and reduce greenhouse gas emissions. Our objectives were to evaluate summer cover cropping for its potential to improve soil tilth at a time of year when the soil would usually be fallowed and dry with no soil cover, and to better understand the agronomic practices that might make summer cover cropping more feasible for Delta farmers. This article summarizes select results from the trial. A detailed report is available on the Delta Crops website.
The trial took place over 4.5 acres of a commercial field, and we compared a cowpea (cultivar ‘Red Ripper') cover crop treatment (CC) to fallow soil (No CC). The cultural practices varied across years (Table 1). Irrigation was only applied to the cover crop plots. In 2020, we estimated that five inches of irrigation was applied to the cover crop, using surface water with moderately low salinity (seasonal ECw of 0.5 dS/m).
Table 1. Agronomic practices during the three-year study.
We soil sampled twice per year. The first sampling occurred following triticale harvest but prior to tillage and cover crop planting. The second occurred at the end of the cover crop season immediately prior to cover crop termination. Soil was sampled from 0-6, 6-12, 12-24, and 24-36 inch depths. We evaluated bulk density, salinity (electrical conductivity, ECe), pH, total nitrogen (N), and total carbon (C). Additionally, in-situ water infiltration was measured at the conclusion of the project (i.e. prior to 2020 cover crop termination). We hand-harvested cover crop biomass, separated it into cultivated cowpeas, volunteer small grains, and weeds and analyzed each component for total C and N. We hand-harvested triticale forage in 2019 and 2020.
Soil properties. After three years of cover cropping, we did not observe improvements in total N or bulk density from cover cropping, and our statistical analysis indicated that total C was impacted by plot location. This suggests that an inherent soil characteristic, like texture, was having more of an impact on total C than the cover crop treatment. We observed better water infiltration in the CC plots (Figure 1). Cover crop roots likely contributed to better soil structure and water conductance. We also observed lower salinity and higher (i.e. less acidic) pH in the CC plots. Root zone salinity (0-36 in) averaged 1.4 dS/m in the CC plots and 2.2 dS/m in the No CC plots. Root zone pH averaged 5.7 in the CC plots and 5.5 in the No CC plots. These results suggest that cover cropping can improve certain soil characteristics, particularly those related to soil-water status, on a relatively short time frame. Changes in nutrients and C storage, however, are less likely to be observed following short-term changes in management.
Figure 1. Three years of cover cropping improved water infiltration (P=0.0198) compared to the standard dry fallow. The error bars represent the standard errors. The photo illustrates how there were visible differences between treatments, even after triticale forage harvest and uniform tillage operations. No CC soil was a fine powder (bottom of the photo); whereas, CC soil was observed to have better aggregation. The grower observed differences in subsequently-planted small grains, with seedlings in the CC plots emerging about five days earlier than seedlings in the No CC plots.
Cover crop stand. Cover crop composition varied over the course of the study and was likely impacted by cultural practices, like planting and irrigation methods. While cowpea was the only seed planted, the stand was a mix of cowpea, volunteer wheat/triticale, and weeds. We observed that the 2020 practices and timing of operations resulted in the least amount of weed growth (Figure 2) and seed heading.
Figure 2. Proportion of cowpeas, small grains, and weeds in total cover crop biomass, and total C and N inputs from the cover crop.
Triticale forage yield. Despite certain soil health benefits, cover cropping did not improve triticale forage yield. The No CC treatment yielded higher than the CC treatment across both years (Figure 3). The CC plots yielded below the two-year field average of 5.5 tons per acre, and the No CC treatment yielded above the field average yield. Given the improved infiltration, pH, and salinity conditions in the CC treatment, the yield result is difficult to explain, but machine harvesting over a larger area might lessen the difference between treatments.
Figure 3. Triticale forage yield as tons of dry matter per acre. The No CC treatments yielded higher than the CC treatments across both years (2019-2020) (P=0.0059).
Summary. In our three-year study, cover cropping had no effect on total N, bulk density, and total C, but water infiltration, salinity, and pH were improved. Triticale forage (i.e. cash crop) yield did not improve as a result of cover cropping, however. Cowpea stand establishment and volunteer grain and weed competition were the biggest challenges to growing a summer cover crop at this site, but earlier planting and termination reduced the weed pressure. Despite these challenges, the grower observed better soil aggregation in areas of the field where the cover crop had grown. Overall, the potential benefits of cover-cropping may not be realized in the first few cover crop cycles, which could hinder long-term adoption. Results may also depend on the cover crop biomass obtained and other site-specific factors. While scientific studies have demonstrated soil health and cash crop yield improvements with cover cropping, more long-term studies are needed in California to demonstrate how these benefits can be realized.
Acknowledgments. This project was supported by the California Climate Investments program. We thank Dawit Zeleke, Morgan Johnson, and Jerred Dixon of Conservation Farms and Ranches for hosting the trial. We thank Tom Johnson of Kamprath Seed and Margaret Smither-Kopperl and Valerie Bullard of the NRCS PMC for information and advice on cover cropping.
We began the meeting with “lightning talks” from organizations working on cover cropping and climate-smart agriculture, including UC Cooperative Extension, Contra Costa Resource Conversation District, Community Alliance with Family Farmers, and USDA-NRCS. We then showcased the cover cropping trial that we established in cooperation with Conservation Farms and Ranches. A CDFA Healthy Soils Program grant supports the Delta trial, which is part of a larger effort that includes our farm advisor colleagues in the Sacramento and San Joaquin Valleys – Sarah Light, Amber Vinchesi, and Scott Stoddard – along with Jeff Mitchell and Will Horwath at UC Davis.
This is the second of a three-year on-farm trial to evaluate warm-season, annual legume cover cropping between winter small grain crops compared with a standard dry fallow. Cover cropping is a management practice identified in the Healthy Soils Program as having the potential to improve soil health, sequester carbon, and reduce greenhouse gas emissions. Cover cropping is not a typical practice in the annual crop rotations of the Delta region, however, and summer cover cropping is particularly rare. The Delta is a unique agricultural region with unique environmental challenges. Some soils in the region are subsided due to oxidation of organic matter, and some soils suffer from salinity, having limited ability to leach salts due to low permeability soils and shallow groundwater. Because surface waterways provide water for irrigation, summer cover cropping with a legume has the potential to improve soil tilth at a time of year when the soil would otherwise be fallow and dry with no soil cover.
The soil type across the experimental site is a Valdez silt loam. The trial is approximately 4.5 acres and compares three replicates of two treatments: an irrigated cover crop and a dry, fallow soil in between small grain crops. A cover crop of cowpea (Vigna unguiculata cv. ‘Red Ripper') has been planted in July of 2018 and 2019 after small grain harvest and tillage operations. Irrigation is provided to the cover crop plots only. The cover crop is terminated in the fall ahead of tillage and planting of small grains. Soil properties tested to date include bulk density, salinity (EC), pH, total nitrogen (N), and total carbon (C). We have also evaluated cover crop characteristics and 2019 triticale yield.
Among the soil properties, we have observed essentially no change in bulk density, total C, and total N from the July 2018 baseline condition. We are monitoring salinity and pH semi-annually because we have observed these properties to improve in the cover-cropped plots. After one cover cropping season, salinity increased in both treatments, but it increased more in the dry fallowed plots, averaging 1.22 dS/m from 0 to 12 inches, compared to 0.64 dS/m in the cover crop (CC) treatment. Rainfall during the 2018-19 winter season leached salts in both treatments, but the CC treatment started the 2019 cover cropping season with a lower average rootzone salinity (0-36 in) of 0.78 dS/m, compared to 1.13 dS/m in the dry fallow (No CC) treatment. Soil at this site is acidic, which is typical for the region, but pH was observably higher in the CC treatments.
We made changes to our planting and irrigation scheme in 2019 – changing from flood to sprinkler irrigation – and this has improved cowpea stand in 2019, compared to 2018. There has been a lot of competition from volunteer wheat (2018)/triticale (2019) and weeds, but we decided in both years not to manage these with tillage or herbicides. Both add biomass to the soil, which is an objective of the Healthy Soils Program. Competition, however, likely impedes cowpea growth and nitrogen fixation, and future study should investigate how these soil properties are affected by single-species and mixed cover crop stands. At the end of the first cover cropping season, biomass largely favored the volunteer wheat. Of the total C added to the soil from biomass, the wheat contributed 42-71%, compared to 15-24% from the cowpea, across the three replicate plots. Of the total N added from biomass, the wheat contributed 68-87%, and the cowpea contributed 9-15%. The triticale forage crop (winter 2018-19) yielded 5.4 tons per acre for the CC plots and 6.3 tons per acre for the No CC plots, but there was high variability among subsamples. The overall field averaged approximately 5.5 tons per acre. More detailed methods and results are available in our preliminary report.
In summary, cover cropping, particularly in the warm-season, is not a typical management practice in the annual crop rotations of the Delta region. After the first year of a three-year study, cover cropping had no observed effect on bulk density, Total N, and Total C. We observed better salinity and pH conditions in the cover-cropped plots. Cowpea stand establishment and volunteer grain and weed competition have been the biggest challenges to growing a summer cover crop at this site, and the cover crop was not observed to improve cash crop yield in the following season. We will continue to monitor soil and cover crop properties in 2019 and 2020, and additionally, we will reach conclusions about greenhouse gas (CH4, N2O) emissions, which are being evaluated by our UC Davis colleagues.
This project is financially supported by the California Climate Investments program. We thank Dawit Zeleke and Morgan Johnson of Conservation Farms and Ranches for hosting the trial. We thank Tom Johnson of Kamprath Seed and Margaret Smither-Kopperl and Valerie Bullard of the NRCS PMC for information and advice on cover cropping.
The meeting showcased the UC Davis wheat and triticale variety testing program for the Delta, and presentations were given by UC Cooperative Extension and USDA-NRCS scientists. UCCE Grains Specialist, Mark Lundy, demonstrated a soil nitrate quick test and how it can be used in small grains fertility programs. UCCE Cropping Systems Specialist, Jeff Mitchell, described tillage research taking place at the UC Westside Research and Extension Center and demonstrated how no-till plots had better soil aggregation and tilth than conventionally tilled plots. USDA-NRCS Director, Margaret Smither-Kopperl, described winter and summer cover cropping trials at the Plant Materials Center in Lockeford, CA.
Additionally, Brenna Aegerter and I described an upcoming cover cropping trial that we will conduct on Staten Island. We were awarded a CDFA Healthy Soils Program grant with our farm advisor colleagues in the Sacramento and San Joaquin Valleys – Sarah Light, Amber Vinchesi, and Scott Stoddard – along with Jeff Mitchell and Will Horwath at UC Davis. On Staten Island, we will trial legume cover cropping versus no cover cropping treatments for soil health properties, greenhouse gas emissions, and grain yield from 2018-2020.
The trial will take place in a field that is in small grains (wheat and triticale) rotations, with soil classification Valdez silt loam. Cover cropping will take place in the summer months following the small grains harvest. Initial soil sampling will take place after wheat harvest and subsequent tillage. We will take baseline soil samples, measuring bulk density, pH, salinity, total C and N, aggregate stability, infiltration, and active C (a measure of the carbon available as an energy source for soil microbial communities) in the top foot of soil. At deeper depths, we will also test bulk density and total C. We will soil sample each fall, at the end of the cover crop season, to evaluate changes in soil properties over the three years. Greenhouse gas (N2O and CH4) monitoring will allow comparative evaluations of cumulative emissions between the soil management systems. Small grains yields will also be determined.
We look forward to this trial and will share results as we have them. We want to thank Dawit Zeleke and Morgan Johnson at The Nature Conservancy's Staten Island, Margaret Smither-Kopperl and Valerie Bullard at the USDA-NRCS Plant Materials Center, and Tom Johnson at Kamprath Seed for their collaboration on this trial.
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