Orchard nut growers often plant grasses, legumes and/or brassicas between rows of trees, also known as cover crops, to prevent rain from eroding orchard soils, suppress weeds, add nutrients, and promote soil biodiversity, all of which help build more resilient, healthier soils.
Cover crop planting and its turnover are often accomplished with the aid of a tractor. Putting small grazing animals, such as sheep, at the task of controlling cover crops biomass instead of tractors has the potential to reduce greenhouse gas emissions, time, and labor, while also providing forage for the animals and improving the health of farm soils. Organic forms of nutrients offered by cover crops, such as carbon and nitrogen, are broken down more rapidly in the manure of animals, enriching soils further.
Growers face a barrier in adopting sustainable grazing practices due to challenges integrating this with other agronomic requirements, as well as concerns about food safety risks designed to protect consumers from foodborne pathogens. While there is growing interest from tree nut growers to integrate cover crops and grazing, uncertainty about potential food safety risks, cost benefits, and other important considerations, such as a reduction in pests and overall improvements to soil health, contribute to keeping grazing practices at a standstill.
Seeking Answers to Food Safety Risk
To address the growing need to re-couple crop and livestock production systems and assist agriculture in meeting its sustainability and resilience goals, researchers in the fields of veterinary medicine, plant and soil sciences, entomology, and economics from the University of California, Davis and Riverside, were awarded a $2 million grant through the U.S. Department of Agriculture's Organic Agriculture Research and Extension Initiative Program. Current USDA National Organic Program guidelines require a 90–120 day waiting period after incorporating raw animal manure before harvest. Studies are needed to verify adequate waiting periods for incorporating manure before harvest for integrated livestock orchard systems, especially for sheep, which are known to harbor lower foodborne pathogens than cattle. This is especially problematic for almonds and walnuts, which make contact with the ground during harvesting as they are first shaken off of trees and dried on the soil surface. While off-ground harvest of pistachios reduces exposure to soilborne and foodborne pathogens, dust and wind can still move these organisms onto the crop.
What is it that the scientists hope to achieve?
For this four-year project, “Influence of Orchard Grazing on Soil Health and Pest Control While Mitigating Food Safety Risk,” the scientists will study the impacts of sheep grazing of cover crops in organic almond, pistachio, and walnut orchards in two distinct nut-growing regions in California – the Sacramento Valley and San Joaquin Valley. This project will provide tree nut growers, stakeholders and policymakers regulators guidance on the risks and benefits of livestock grazing of cover crops in orchards to remove barriers to the expansion of livestock integration in organic orchard cropping systems while maintaining food safety.
How does livestock integration impact soil health and food safety risks?
Through sampling of cover crop vegetation and soils at periods before and after grazing, scientists will provide insights on how grazing impacts the addition of nutrients and their availabiltiy during the tree growth period as well as foodborne pathogens environemnet dynamics, and shifts in soil microbes regulating both nutrient cycling and food safety outcomes. How cover crop and grazing practices impact soil health parameters of importance to growers will be assessed near tree roots and in the middle of orchard rows through measurements of salinity, compaction, and water infiltration levels, among other parameters. Additional incubations of soil samples will determine microorganism nutrient cycling through measurements of respired CO², N release and enzyme activity.
Can sheep grazing help manage common pests of tree nuts?
Populations of navel orange worm (NOW), a problematic pest in almonds, pistachios, and walnuts, will be measured through surveys of plots before and after cover crop grazing. Fewer remnant nuts or "mummies" should be left behind in these plots, as they are eaten and trampled by sheep during grazing events. Furthermore, previous studies have demonstrated that mortality of overwintering NOW tends to increase in the presence of cover crops. As such, grazing coupled with the addition of ground cover should lower the overwintering population of NOW, which may potentially reduce the overall need for pesticide applications to control this pest.
How economically feasible is it to incorporate livestock for grazing into orchard crops?
The team will build on established cost-return budgets for cover crops in the region to appraise expected costs and benefits of grazing cover crops in nut orchards. Cost and return studies and partial budget analysis will be performed to develop a decision-making tool to allow growers to decide if it is profitable to incorporate grazing practices into their orchards. How will researchers share the data and what they learn with growers? The findings from this data will be used to develop an extensive outreach program that will include field days, a decision-making tool, in-person presentations, and social media outreach to disseminate these findings to organic nut growers across multiple states in the U.S. The impact and application of this study will extend beyond California to Arizona, Hawaii, Texas, and other states in the U.S., where organic nuts are grown.
The trials for this four-year project commence in 2023, 2024, and 2025, and grazing events will take place in each subsequent spring.
Year 1 - 3: Determine impacts of planted cover crops and sheep grazing on nitrogen cycling, soil health, and navel orangeworm populations in organic almond, pistachio, and walnut orchards in the Central Valley.
Year 1 - 3 : Assess diversity of microbes, pathogens, and food risk from grazing, and identify underlying differences in environmental, pest management practices, and soil health factors associated with the presence of any pathogens in organic nut orchards.
Year 4: Determine the economic costs and benefits of grazing sheep in organic almond, pistachio, and walnut orchards as a production practice. Create an interactive decision tool to assist tree nut growers in deciding whether to incorporate grazing sheep.
--
The Organic Center is a 501(c)(3) non-profit research and education organization based out of Washington D.C. Our mission is to conduct and convene credible, evidence-based science on the environmental and health effects of organic food and farming and communicate the findings to the public.