Crop Rotations in California Rice Systems, 2020

 

Cameron Pittelkow, assistant professor, Department of Plant Sciences, UC Davis

The overall goal of this new project is to understand the advantages and disadvantages of crop rotations and their potential role in advancing the long-term sustainability of the California rice industry, particularly from a weed management perspective.

Major research activities in 2020 were centered around grower interviews that took place in the summer. Focus groups could not be conducted because of COVID-19 restrictions. Research also included an analysis of land use maps and soil properties. Interviews were conducted with 43 growers throughout the Sacramento Valley to better understand the perceived benefits and challenges associated with crop rotations in rice systems. Roughly 47% of those interviewed considered themselves rice-only growers, while another 26% rotated crops using both conventional and organic methods. Another 26% had both continuous rice and rotated fields in their operations.

Weed control, soil health, economics

Interviewees indicated that rotations could provide numerous benefits to both growers and the greater rice industry. In particular, there was strong agreement about the potential benefits for weed control and reduced reliance on herbicides.

Growers who practice conventional rotations reported that they have reduced the number of spray applications needed to control weeds. These growers have diversified rotations that include crops like safflower, sunflower, tomato, corn, beans, and other field crops.

Some growers reported that using rotations helped reduce weed populations. The longer the rotation out of rice, the less intrusive were the weeds. Similarly, the longer a field was in rice, the more challenging the weeds become. After rotations, growers reported cleaner fields and a bump in yields.

Organic growers reported that weed control, although a motivating factor for crop rotation, was not proving effective. In fact, some of them said they weren’t sure whether alternating between fallow and rice would work in the long-term because weeds are getting worse. Both conventional and organic growers indicated that over the past five years, weeds have been increasing—something that requires further research.

Soil health was identified in the survey as another area of prime benefit by growers who rotated crops. Rotations improved soil tilth and decreased the need for fertilizer and pesticide inputs. It’s worth noting that soil health was mentioned nearly as much as weed control in the survey, signifying the high importance of and motivation for using rotations.

The other major benefit of rotations identified by some growers concerned profitability. Growers who rotate indicated that diversification helped increase market resilience. By having multiple options in their portfolio, they are better positioned to respond to market fluctuations. However, it’s important to note that no rice-only growers rated economic factors as a benefit, a contrast to growers who saw rotations as more profitable. Some other areas that came up during the survey include the benefits of rotations to conserve rice ground, water, and, if a winter crop is left into spring, wildlife habitat.

Appropriately, growers wanted more information on the economic advantages and disadvantages of rotations to help in their decision making. Similarly, growers felt there was a lack of basic information about the impacts of rotations on rice yields, herbicide use, and soil health.

Interview coverage by acreage within each county

County Average rice acreage
(2008-2019)
% of total Acreage covered
by interviews
% of total acreage
interviewed
Difference (acreage
interviewed vs. actual)
Colusa 140,630 28 14,156 20.6 -7.1
Butte 95,050 19 11,055 16.1 -2.6
Glenn 76,970 15 1,921 2.8 -12.4
Sutter 107,360 21 15,779 23.0 1.9
Yolo 33,760 7 13,948 20.3 13.7
Yuba 36,130 7 5,445 7.9 0.8
Placer 10,580 2 1,237 1.8 -0.3
Sacramento 3,950 1 496 0.7 -0.1
Other 3,615 1 4,600 6.7 -6.0
Total 508,045 68,637

Types of rotation vary

Vetch rotations were the most broadly used across different regions. Organic growers used a rice/vetch/fallow sequence. Sometimes these growers would harvest vetch for seed. Although vetch is a low-value market crop, growers said it adds value by increasing soil organic matter and nitrogen supply. Some growers combined vetch with oats to be used for dairy feed or as a second type of cover crop mixture. Vetch tolerates heavier soils better than other crops.

Growers who rotated with row crops used a flexible mixture of sunflowers, safflower, beans, vine seed, corn, tomato, and wheat, largely influenced by weather and markets. Sunflower or safflower was used as a tool following rice to “clean up the ground” and prepare it for a more profitable crop like tomatoes. Recently, growers have switched from safflower to sunflower because prices have increased for sunflower; however, safflower is more effective in weed control.

Alternatively, some growers in areas where rice and pasture dominate the landscape chose to incorporate cool season forages or hay into their rotations. To integrate cool season forage crops into a rotation, a summer crop or fallow would typically follow. One grower, for example, followed winter barley with beans in the summer or fallow, depending on the weather. Beans have the ability to be planted later in the season and require similar equipment as rice.

Growers reported that the type of crop in rotation wasn’t as important as just being out of rice to improve weed control, although further research is needed to confirm this. The major weeds affected by rotations are semi-aquatic weeds such as grasses and sedges.

Environmental limitations—specifically soil texture and the ability to grow other crops in wet conditions—were the dominant barriers preventing more growers from adopting rotations. However, lack of experience, availability of markets, and infrastructure limitations all play a crucial role in rotation feasibility.

Growers who rotate identified specific conditions for success. These included having lighter, loamy soils that support drainage; access to contractors, diverse markets, and flexible land payments or ownership; and having appropriate infrastructure such as equipment, land size, and access to information.

Tool developed for rotations, next steps

Despite the barriers, rice growers showed an interest in the prospect of using rotations to control weeds and reduce input costs. Relative to other topics, 80% of the growers ranked this type of research as either neutral or high.

More than any other concern, growers discussed the lack of viable options for alternative crops, especially for heavy clay, rice-only soils. Alternative crops also need to be easily marketable and economically competitive with rice.

To illustrate the strengths and weaknesses of different crops being rotated with rice, information from the interviews was used to create a crop rotation summary table. The table takes into account factors such as production costs, soil and equipment requirements, profitability, and benefits to the broader rotation. This preliminary tool can help growers narrow down the choice of crops that other growers have successfully implemented into their operations.

Further research is necessary to address the benefits of rotations and to further refine the list of most promising crops for different soils and production environments. Proposed research in 2021 will compare weed and soil health indicators across multiple fields with and without rotations, while also conducting a cost/benefit analysis on crop rotations. The goal of this research is to further investigate growers’ priorities and concerns related to crop rotations and to identify the most logistically feasible alternative crops to rotate with. Impacts to the following rice crop in terms of economics, herbicide use, and soil health also will be examined.