- Author: Michael D Cahn
Introduction
Traditional winter cereal cover crops planted in the Salinas valley have many potential benefits including, scavenging nitrate in the soil profile, increasing organic matter in the soil, and protecting the soil from erosion during storm events. However, when grown for 3 to 4 months during the late fall and winter, cereal rye, triticale, or barley can accumulate 5 to 6 tons of dry matter biomass that must be incorporated into the soil before planting a spring vegetable crop. Tilling in a high amount of cover crop biomass can be disruptive to spring planting schedules. Consequently, only a small fraction of the vegetable ground in the Salinas valley is cover cropped each year.
Previous studies demonstrated alternative strategies can limit the biomass growth of these cereal cover crop species so that they can more easily be tilled into the soil, and therefore less disruptive to spring planting schedules. After fall land preparation, the cereal cover crops are seeded into listed beds and/or in the furrow bottoms. After they become established they can reduce runoff and protect the soil from erosion during early winter storm events. Before the cover crops grow too big, they are terminated with an herbicide to limit the amount of above ground biomass that needs to be incorporated in the spring. For organic systems, planting a mustard cover crop on listed beds or furrows which can be terminated mechanically by mowing is another strategy to limit biomass. A good target for these low biomass cover crops is between 0.5 to 1 ton of dry matter per acre by the date of termination. Once terminated, the biomass begins to decompose. However, the residue on the surface continues to protect the soil from erosion and can significantly increase infiltration from rain events. This helps to leach accumulated salts in the soil as well as recharge groundwater aquifers. The remaining decomposed residue can easily be incorporated into the soil during bed preparation in the spring.
One risk of this low biomass approach is accessing fields during the winter to terminate the cover crop. If soil conditions are too wet or if there is not enough available labor, it may be difficult to fit in a spray application or to run a flail mower. This termination step also increases the cost of managing the cover crop. A possible solution is to use species that grow slowly during the winter when temperatures are cold. Sudangrass and sorghum-sudangrass hybrid are warm season adapted species that could be used in this low biomass approach to managing winter cover crops.
Field trial with warm season adapted cover crop species
A field trial was conducted with sudangrass and sorghum-sudangrass in the 2023-2024 winter to evaluate biomass growth, and the effect on storm water runoff and soil erosion compared to bare-fallow plots. The site was located on an Arroyo Seco gravelly loam soil with a slope of more than 5%. Plots measuring 1050 ft in length by four 40-inch wide peaked beds were planted with either sudangrass, sorghum sudangrass hybrid, or left bare fallow. Treatments were replicated 4 times. The cover crops were seeded at 60 to 80 lbs/acre on October 4th and were subsequently sprinkle-irrigated several times. Total water applied for establishment was 2.6 inches. One application of the herbicide Bromoxnil (Maestro) was applied about 45 days after planting to kill emerged broadleaf weeds.
Results
Above ground biomass, N uptake, and carbon accumulation
Both cover crops had limited biomass growth, accumulating only 0.35 to 0.5 tons/acre of dry matter by early January and less than 1 ton/acre by mid March (Table 1). Growth was set back by cold conditions that occurred from mid November through early January, occasionally reaching freezing temperatures which caused damage to leaves (Fig. 1). However, the freezing temperatures lasted only a few hours and were not severe enough to kill the cover crops (Fig. 2). By March 13th the cover crops had taken up 45 to 55 lbs N/acre and had a carbon to nitrogen ratio of 15. The C:N ratio of 15 would suggest that after soil incorporation the residue would decompose rather quickly and release N for the following vegetable crop.
Runoff, rainfall infiltration, and control of soil erosion
Total rainfall measured at the trial site was 10.2 inches for the winter season. The most intense period of rainfall occurred in late January and early February which resulted in several runoff events (Fig. 3). During this period about 50% of the rainfall in the bare fallow plots was lost as runoff compared to 15% lost as runoff in the cover crop plots (Fig.4). Over the entire winter season, runoff was reduced by an average of 70% under the cover cropped plots compared to the bare fallow plots, and significantly more rainfall was infiltrated into the ground in the cover cropped plots. In addition, suspended sediment concentration was 90% and 77% less in the sudangrass and sorghum-sudangrass cover crop plots, respectively, compared to the bare plots. Turbidity, total P, and total N concentration in the runoff were also reduced under the cover crop plots compared to the bare fallow plots (Table 2).
Seasonal soil erosion losses could be calculated based on the volume of the runoff and sediment concentration in the runoff. The total loss of sediment averaged more than 3500 lbs per acre in the bare fallow plots during the winter, while erosion losses were reduced by 96% to 98% in the sorghum-sudangrass and sudangrass plots (Fig. 5). Total N losses were reduced by 83% to 86% in the cover crop plots compared to the fallow plots, and total P losses were reduce by 81% to 85% in the cover cropped plots compared to the bare fallow plots.
Conclusions
The use of warm season species such as sudangrass and sorghum-sudangrass hybrids as winter cover crops provides several advantages compared to planting cereal cover crops. The biomass growth through the winter is self-limiting due to the cold conditions that typically occur in the Salinas Valley. Because the final biomass would likely be less than 1 ton per acre, these species can be planted on listed beds in the fall rather than on flat ground. This means that in the spring, the remaining cover crop can be lillistoned into the peaked beds a few weeks before final bedshaping and planting. Cover crops planted on flat ground and have high amounts of biomass usually require many tillage passes to prepare ground for planting. Despite, having less biomass than traditional winter cereal species, sudangrass and sorghum-sudangrass hybrid cover crops provided excellent erosion control compared to leaving the ground bare, and increased infiltration of rainfall during storm events. Also these species may be able to scavenge significant amounts of nitrogen from the soil which can limit nitrate leaching during the winter months.
On the east-side of the Salinas Valley groundwater levels have been in the decline for several decades. Infiltrating as much rainfall as possible during the winter using strategies such as low biomass cover crops could potentially help recharge the aquifer in this region. We plan to conduct a second year of field trials with these warm season species to continue evaluating this approach to managing winter cover crops in vegetable systems.
Acknowledgments
This project was funded by the California Leafy Greens Research Board.
- Author: Jeffrey P Mitchell
July 5, 2024
CASI's Jeff Mitchell was a "three-peater" July 5th on Tom Willey's "Down on the farm" radio program on KFCF's 88.1 FM station on July 5, 2024. He shared information from the recently published article, No-tillage, surface residue retention, and cover crops improved San Joaquin Valley soil health in the long term, that was published in the May 2024 issue of California Agriculture. Willey was a co-author on the work and he talked with Mitchell about how the work came about, what its study goals were, how it was conducted, and what its findings were. Willey's program airs every first Friday of the month and has a loyal following that includes a great diversity of folks who greatly enjoy his 'on the front porch' conversations with his guests. A video of the radio interview is available at the You Tube link
https://www.youtube.com/watch?v=CXoyBLlImUo
- Author: Jeffrey P Mitchell
CASI's Mitchell on MyAgLife podcast June 28, 2024
July 1, 2024
Jeff Mitchell, CASI member and Professor and Cropping Systems Cooperative Extension Specialist in the Department of Plant Sciences at the University of California, Davis, appeared in a 39-minute podcast with host Taylor Chalstrom on June 28, 2024 to talk about conservation agriculture in general and the recently-published article in California Agriculture on the 20-year study in Five Points, CA. The podcast is available at
https://spotifyanchor-web.app.link/e/JuQ7qakGSKb
You may need the podcast platform, Spotify, to listen to it.
/span>- Author: Pamela S Kan-Rice
Additional guidance needed for groundwater management strategies
Cover crops are planted to protect and improve the soil between annual crops such as tomatoes or between rows of tree and vine crops, but growers may be concerned about the water use of these plants that don't generate income.
“Cover crops are one of the most popular practices we see farmers employ through our Healthy Soils Program,” said Karen Ross, secretary of the California Department of Food and Agriculture. “Cover crops supply a host of benefits, such as helping to protect against soil erosion, improving soil health, crowding out weeds, controlling pests and diseases, and increasing biodiversity; and they can bring increased profitability as the number of other inputs are reduced. They also provide water benefits such as improved infiltration and reduced runoff.”
These potential benefits are especially salient in the San Joaquin Valley, where groundwater challenges are more acute. A new report evaluates the water implications of cover cropping practices to lay the groundwork for their adoption in the context of the Sustainable Groundwater Management Act, or SGMA, which is intended to protect groundwater resources over the long-term.
“Yes, cover crops require a nominal amount of water to establish – and sometimes rainwater is sufficient – but the myriad co-benefits are worth it,” Ross said.
Growers, water resource planners and managers, crop consultants, irrigation practitioners and policymakers may find the cover crops report useful.
The report is the product of a convening process jointly developed by the California Association of Resource Conservation Districts, CDFA, Natural Resources Conservation Service of California, and University of California Agriculture and Natural Resources, and assembled by nonprofit Sustainable Conservation.
The multidisciplinary group of more than 30 individuals has published “Cover Cropping in the SGMA Era.” The literature review, policy analysis and recommendations pertain to the water impacts of cover crop practices in California's Central Valley under SGMA.
Cover crops and their potential
“Wintertime rain-fed cover cropping does not necessarily significantly increase water losses compared to bare ground in the winter months,” said co-author Daniele Zaccaria, associate professor in agricultural water management for Cooperative Extension at UC Davis. “Cover cropping can significantly improve soil-water dynamics, increasing soil water infiltration and storage and reducing surface runoff.”
To reap the benefits of cover crops using minimal water, Zaccaria said growers will need to know how the plants perform under different conditions.
“We need to develop and implement a coordinated research effort to increase understanding of net water impacts of cover crops under various meteorological conditions – dry, wet, average," he said.
Report findings and recommendations
To understand the potential of cover cropping under SGMA, the report's authors came together to answer the following questions:
- What are the impacts of cover crops on water cycles (both benefits and use)?
- How does SGMA management account for cover cropping and is it capturing cover crop benefits alongside their water use?
- How can we ensure that this practice remains available to growers where and when it makes sense?
This report synthesizes the learnings from the collaborative initiative including 100-plus multidisciplinary experts, a policy analysis, interviews with Groundwater Sustainability Agency (GSA) staff and consultants, and the expertise contributed by its 30-plus authors. In light of these findings, the report advances a series of recommendations aimed at bridging critical knowledge gaps, enhancing the integration of cover crops into policies and incentive programs, and bolstering data infrastructure and other mechanisms to support sustainable groundwater management initiatives.
One vital throughline is the need for additional guidance from the state to support local GSAs in facing the complex challenges of developing and implementing groundwater management strategies for their local watersheds. These measures aim to optimize cover crop integration within SGMA frameworks and promote sustainable water management practices crucial for the region's agricultural resilience and environmental health.
“This report is unique because the university collaborated closely with state agencies and private sector partners to ensure that the different perspectives provided both the best science available as well as viable policy options,” said Glenda Humiston, University of California vice president for agriculture and natural resources. “By taking a comprehensive view, we can advance recommendations for cover crop policy that help us meet multiple goals, manage our natural resources more effectively, and avoid unintended consequences.”
Sarah Light, UC Cooperative Extension agronomy farm advisor, is one of the UC ANR experts who provided science-based information during the convening sessions and co-authored the white paper.
“Cover crops are a valuable soil health practice that can help ensure the resilience of California farms to climate extremes,” said Light. “As we balance the complexities of water and soil management, it is important to understand the role that cover crops play in an annual water budget so that they are not disincentivized in certain parts of the state. This paper can provide guidance to GSAs and policymakers who are charged with implementing SGMA in their regions.”
The report “Cover Cropping in the SGMA Era” can be downloaded for free at https://suscon.org/wp-content/uploads/2024/05/SC-Cover-Crop-SGMA-Report.pdf.
Related research links:
Long-term reduced tillage and winter cover crops can improve soil quality without depleting moisture https://escholarship.org/uc/item/1cr6w7dp
Impacts of winter cover cropping on soil moisture and evapotranspiration in California's specialty crop fields may be minimal during winter months https://californiaagriculture.org/article/108637
Water-related impacts of cover cropping in California https://www.youtube.com/watch?app=desktop&v=mTNLx6LzEt0
/h3>- Author: Jeffrey P Mitchell
May 17, 2024
Findings from twenty years of soil health research in Five Points, CA have been published in the University of California's California Agriculture peer-reviewed journal's May 1, 2024 issue. https://doi.org/10.3733/001c.94714.
This work has been a large collaborative effort involving twenty-one UC and non-UC coauthors. It began in 1998 initially as an effort to evaluate the potential of reduced disturbance tillage systems to reduce dust emissions from annual cropping systems that are common in California's San Joaquin Valley. It long-term nature however, allowed it to become a unique site for also monitoring changes in soil properties and function under four experimental systems: conventional tillage with no cover crop, conventional tillage with cover crop, no-till with no cover crop, and no-till with cover crop. Crops rotated between tomato and cotton initially, but later during the study, the rotation was diversified to include melons, sorghum, and garbanzo beans.
The work involved the Soil Health Institute's Shannon Cappellazzi, who sampled at the site in 2019. That sampling event led to the site becoming part of a multiple-publication series of articles that reported on soil health impacts in 124 long-term study sites across North America.
https://soilhealthinstitute.org/news-events/a-minimum-suite-of-soil-health-indicators-for-north-american-agriculture/
/span>