Location: Crop Improvement and Protection Research
2024 Annual Report
Objectives
Objective 1: Develop cropping system management strategies for organic and conventional high-value vegetable and strawberry production systems that increase climate resilience, improve nitrogen cycling and water quality, enhance soil quality, and improve weed management.
Sub-objective 1.A: Evaluate the effects of cover cropping frequency and compost on weed biomass during cover cropping and on weed density and weed management costs in subsequent lettuce and broccoli crops.
Sub-objective 1.B: Evaluate the effects of a legume-rye cover crop mixture versus non-legume cover crops at two seeding rates on weed suppression during cover cropping, and on weed densities and weed management costs in subsequent lettuce and broccoli crops.
Sub-objective 1.C: Evaluate the effects of cover cropping frequency and compost on subsequent lettuce and broccoli marketable yields.
Sub-objective 1.D: Evaluate the effects of a legume-rye cover crop mixture versus non-legume cover crops at two seeding rates on subsequent lettuce and broccoli yields.
Sub-objective 1.E: Evaluate the effects of a legume-rye cover crop mixture versus non-legume cover crops at two seeding rates on soil water storage during winter cover cropping.
Sub-objective 1.F: Use deep soil samples, column experiments, and on-farm samples to model previously overlooked pathways of nitrogen loss from compost.
Sub-objective 1.G: Develop farmer-friendly, field-based methods to predict the biomass and carbon to nitrogen (C:N) ratio of non-legume cover crops.
Approach
The sub-objectives of the project are divided into the following two categories:
1. Long-term Salinas Organic Cropping Systems (SOCS) experiment. Sub-objectives 1.A to 1.F focus on vegetable yields, weeds, soil water storage and modelling of nitrogen loss pathways.
2. Ag Order 4.0 regulation cover crop nitrogen scavenging credits. Sub-objective 1.G continues and expands on-going short-term field studies to help growers get cover crop nitrogen scavenging credits in the Ag. Order 4.0 regulation.
The first six Sub-objectives (1.A to 1.F) will be addressed using unpublished data collected during the first 10 years of the SOCS experiment. This experiment began in 2003 and is the longest running trial in the U.S. focused on high-value, tillage intensive organic production systems. It includes 8 systems that for the first 8 years differed in cover cropping frequency, cover crop type, cover crop seeding rate, and compost rate. The experiment has two phases: the “intensive phase” (years 1 to 8) when management differed between systems, and the subsequent, and current “legacy phase” that monitors residual effects from the intensive phase. Commercial scale vegetable production was the focus during the intensive phase, whereas cover crops and occasional vegetables and strawberries are grown during the legacy phase. Data is collected on weeds, soil health, cash crop yields, cover crop services, and economics. The results of analysis from the first 20 years of the trial may indicate that there would be value in collecting additional field data during the remainder of the legacy phase or conducting additional analyses of archived soil samples. If this occurs additional data may be collected with existing or new collaborators.
During the next five years, the SOCS experiment will continue with a low intensity rotation of the ‘legacy phase’ that is an alternative to terminating long-term experiments that likely have some future value. During the project period at least six additional peer-reviewed papers from the existing unpublished data from the first 10 years will be published. The order in which the various components of the data will be published has been planned such that the papers build on each other and eventually provide a complete picture that addresses Objective 1.
Data to address Sub-objective 1.G will come from on-going and future replicated winter cover crop planting date trials at the USDA-ARS and on-farm plantings. In replicated trials, summer and fall cover crops will be planted with a grain drill and be irrigated as needed. Biomass samples will be harvested approximately every 30 days, oven-dried, and analyzed for total carbon and nitrogen. At each harvest, randomly chosen plants will be uprooted to measure the main stem length, and the Feekes growth stage. A similar approach will be used to evaluate the relationship between mustard stem length and biomass, and C:N ratio and growth stage.
Progress Report
This report documents progress for project 2038-21620-001-000D, "Strategies and Tools to Improve Soil Resources, Pest Management, and Climate Resilience on Organic and Conventional Vegetable and Strawberry Farms", which started October 2023 and continues research from project 2038-21620-014-000D, "Strategies to Improve Soil and Pest Management in Organic Vegetable and Strawberry Production Systems".
In support of Objective 1, an intensive field experiment was conducted to evaluate the potential ability of late summer/fall terminated non-legume cover crops (i.e., cereal rye) to scavenge nitrogen and reduce winter leaching to ground water from high-value, high-input vegetable production systems. The study was inspired by input and suggestions from local farms that need information to help them better manage nitrogen in fields that they plan to leave fallow over the winter but are able to cover crop in the late summer and fall. The first year of the two-year study was conducted with collaborators from University of California Cooperative Extension and Stanford University and is partially funded by the California Leafy Greens Research Board. Four treatments were compared including (1) a winter fallow control, (2) over wintered rye (considered the best management practice), (3) a 60-day fall terminated rye cover crop, and (4) a 90-day fall terminated rye cover crop. The study occurred on a field with large amounts of residual nitrogen that was at risk of leaching during the winter. Data were collected on cover crop nitrogen uptake, biomass production, soil moisture dynamics, greenhouse gas emissions, nitrate leaching, and yield of romaine lettuce in the subsequent spring. This research relates to the new California regulation (known as Ag Order 4.0) that is focused on protecting ground and surface water from nitrogen pollution from irrigated farms. This on-going study builds on previous research from this project that have increased nitrogen scavenging credits available to farmers who use cover crops on over 500,000 acres of land. This work relates to Sub-objective 1.G of the project that is focused on to developing farmer-friendly, field-based methods to improve nitrogen management using non-legume cover crops.
In additional support of Objective 1, a long-term, organic systems study was conducted within the Salinas Organic Cropping Systems (SOCS) experiment that is currently in its 20th year on ARS certified organic land, and is the longest running systems experiment in the United States focused on high-value, high-input organic crop production. The study addresses the needs of small, medium, and large-scale organic and conventional farms that are shifting to more sustainable soil management practices. The long-term study continues to investigate crop rotations, soil fertility management, weed management, and the effects of organic matter inputs from cover crops and compost in high-value organic vegetable and strawberry systems. This fiscal year was focused primarily on Sub-objective 1F. to use deep soil samples, column experiments, and on-farm samples to model previously overlooked pathways of nitrogen loss from compost. This was done with collaborators from the University of California, Santa Cruz and Berkeley.
Accomplishments
1. Identified the correct sampling methods for cover crop biomass. Cover crops provide numerous benefits in cropping systems such as soil erosion control, soil improvement, and weed suppression. But a review of 92 cover crop articles in top peer-reviewed journals from 2017 to 2021 revealed problems with how cover crop biomass is often estimated. These problems can lead researchers to under- or overestimate cover crop biomass. In a paper and YouTube video an ARS researcher in Salinas, California, described easy-to-make mistakes and best practices when using a quadrat to sample cover crops.
Review Publications
Brennan, E.B. 2024. “I” versus “the author”: The power of first-person voice when writing about science. Proceedings of the National Academy of Sciences (PNAS). 121(22). Article e2316966121. https://doi.org/10.1073/pnas.2316966121.
Brennan, E.B. 2023. Quadrat misuse: Confessions of a cover crop researcher's biomass sampling journey. Agronomy Journal. 115(5):2275-2285. https://doi.org/10.1002/agj2.21411.
Young, S.L., Anderson, J.V., Baerson, S.R., Bajsa Hirschel, J.N., Blumenthal, D.M., Boyd, C.S., Boyette, C.D., Brennan, E.B., Cantrell, C.L., Chao, W.S., Chee Sanford, J.C., Clements, D.D., Dray Jr, F.A., Duke, S.O., Porter, K.M., Fletcher, R.S., Fulcher, M.R., Gaskin, J., Grewell, B.J., Hamerlynck, E.P., Hoagland, R.E., Horvath, D.P., Law, E.P., Madsen, J., Martin, D.E., Mattox, C.M., Mirsky, S.B., Molin, W.T., Moran, P.J., Mueller, R.C., Nandula, V.K., Newingham, B.A., Pan, Z., Porensky, L.M., Pratt, P.D., Price, A.J., Rector, B.G., Reddy, K.N., Sheley, R.L., Smith, L., Smith, M., Snyder, K.A., Tancos, M.A., West, N.M., Wheeler, G.S., Williams, M., Wolf, J.E., Wonkka, C.L., Wright, A.A., Xi, J., Ziska, L.H. 2023. Agricultural Research Service weed science research: past, present, and future. Weed Science. 71(4):312-327. https://doi.org/10.1017/wsc.2023.31.