UC Davis

Rice lands are important agroecosystems for providing food for over half the world's population while also mimicking wetland environments which support wildlife habitat and are known for carbon sequestration. At the same time, rice lands are facing a number of challenges related to climate change, increased herbicide resistance, labor shortages, and market shift. In response, diversification of rice systems of historically rice dominated landscapes is becoming more common. Crop rotations are being proposed as a practice for California’s rice sector to mitigate challenges related to herbicide resistance and water scarcity in the Sacramento Valley region. However, while crop rotations have been shown to support aerobic cropping systems with regards to weed control, soil health, and yields, switching between a flooded environment to an upland environment can be complex, and may compromise certain soil health related properties related to storing carbon and nitrogen. Furthermore, profitability of crop rotations with rice remains uncertain. Limited data exists on the benefits and tradeoffs of crop rotations with rice systems and their overall feasibility in the Sacramento Valley region. With the help of multiple collaborators, I assess the implications of crop rotations with rice systems by evaluating growers’ perceptions and experiences on barriers to adoption, compare long term profitability of crop rotations with rice compared to continuous rice, and evaluate differences in soil health and agronomic factors between these systems.

In chapter one, I use semi structured interviews to gather a breadth of information from growers regarding the types of rotations that exist in California with rice, the benefits, and barriers to adoption growers’ experiences with rotations, and the requirements for rotations to be successful. Crop rotations ranged in complexity and incorporated many upland crops such as tomato, sunflower, beans, safflower, vine seed, cool season forages, and more. I learned that multiple factors limit growers’ ability to rotate including environmental limitations, lack of available contracts and markets for other crops, financial barriers, and limited experience and knowledge of other viable crops. Growers who rotated agreed that weed control and reduced reliance on herbicides were benefits of rotation, as well as soil health and economic benefits. In chapter two, I investigate the economics of crop rotations with rice systems by evaluating long-term profitability of crop rotations, one with tomato and sunflower, and one with safflower and beans, over a 15-year time frame, and show how rotational benefits and water scarcity events impact these outcomes. I used a Monte Carlo to randomly select variable costs based on data collected from growers and published work to account for economic uncertainty. I found that there is a high likelihood that crop rotations can be as or more profitable than continuous rice, under the assumptions of rotations providing increased rice yields and reduced herbicide inputs or under scenarios where fallowing becomes more common with continuous rice systems. However, there could be a substantial investment period for farmers depending on number of factors. In chapter three, I sample soil from 46 farmers’ fields to assess differences in soil health and agronomic indicators between rotated and continuous rice fields, under both organic and conventional management. Furthermore, I evaluate tradeoffs between systems using a multifunctionality lens to assess system performance and effects on broader ecosystem service categories. This study found that crop rotations had reduced organic carbon and nitrogen pools, as well as less weed abundance and increased yields and minor elements. Organic rotated systems showed less soil health differences and had no significant changes with weed abundance. Conventional rotated fields had higher agronomic efficiencies with lower environmental and regulating services, compared to continuous rice systems.

Together this dissertation understands the role crop rotations can play for California rice sector, as well as the expected challenges and tradeoffs. Overall, this research confirms multiple benefits for crop rotations with rice systems and concludes that rotations should be considered as a tool for farmers to adapt to if challenges continue to persist. However, rotations are unlikely to become a common practice due to low feasibility. Outcomes of this research can be used for developing extension programing, supporting farmers decision making, and supporting policy goals.