Weedy Red Rice Control, 2018

 

Whitney Brim-DeForest, farm advisor, UCCE, Sutter, Yuba, Placer, and Sacramento counties

Since the beginning of the 2016 rice growing season, the UC Cooperative Extension rice team has been working with growers, pest control advisers, and county agricultural commissioners to identify weedy red rice infestations. By the end of that season, five distinct populations or biotypes had been identified on more than 10,000 acres of rice in every rice growing county except Sacramento County.

Immature panicle of weedy red rice type 6. In the field, the long, reddish awns are clearly visible before maturity.
Seed from weedy red rice type 6 at maturity. Photo by Timothy Blank, California Crop Improvement Association.

Surveys and specimen submissions continued, with good participation, in 2017. From 53 samples submitted for testing, 15 were confirmed to be weedy red rice. The California Crop Improvement Association found eight seed fields infested with weedy red rice—all were rejected for seed. Three of the fields were new medium grain seed fields that had never been in the seed certification program before. One field was a previously certified medium grain seed field. Four fields were specialty variety seed fields newly submitted to the quality assurance program.

In 2018, 25 samples were submitted. Five were confirmed to be weedy red rice at four sites. One was a new biotype. Its characteristics include black hull, red awns, and similar in height (tall) to four of the other biotypes. To date, six biotypes have been identified.

Soil sampling and grower interviews

Soil sampling and nine grower interviews were conducted in the fall of 2016 to learn more about the extent of weedy red rice in California. All but one biotype was represented in the fields sampled, and all but Yolo County was part of this work.

The number of seeds per soil volume varied considerably between fields, as did the percentage of soil cores containing seeds. In general, the longer the length of time that the field was infested, the more weedy red rice seeds were found in the soil. Growers who were taking steps to reduce the infestation tended to have lower seed counts. Some fields had increases in 2017, while others saw decreases.

Once 2018 samples are processed, trends should become more clear. The rice team will then be able to better advise growers on best practices to reduce weedy red rice seeds in the soil bank. Some general recommendations include:

  • Rogue weedy red rice plants early in the season.
  • Use less tillage or shallow tillage.
  • Rotate crops or leave fallow.
  • Genetic analysis

    Preliminary results from a genetic analysis of samples from 2016 and 2017 did not locate a molecular marker to distinguish biotypes from each other. However, a gene for red pericarp can be used to distinguish between red and brown pericarped biotypes. It does not distinguish between weedy and cultivated rice, so determining whether specimens are of the weedy type will require growing them to heading in the greenhouse to evaluate shattering and dormancy.

    Weedy red rice ecology

    A greenhouse experiment was conducted to determine the relative competitive ability of the weedy red rice biotypes against M-206. This research also examined the growth potential of each biotype.

    To test the effect of weedy rice competition on the yield of M-206, plants of M-206 and five weedy red rice biotypes were grown in pots. Weedy red rice was planted at increasing numbers of plants per pot, while the number of M-206 plants per pot remained the same. Tests were repeated, with four successive plantings two weeks apart in August and September 2017. Beginning a week after planting, the height and tiller number of each plant was measured weekly for 12 weeks. At maturity, final yield component measurements were taken for plant height, tiller number, panicle number, panicle weight, seed weight, and both fresh and dry biomass.

    A separate experiment was conducted to determine growth potential of the five weedy rice biotypes, including height, panicle number, tiller number, panicle weight, and yield (seed weight per plant).

    Preliminary results indicate that four of the weedy red rice biotypes were significantly taller than M-206. The other one was significantly shorter. Tillering rates were about the same as M-206 for one biotype and significantly higher in the other four. Two of these biotypes had more than twice as many tillers per plant as M-206. The difference in total panicle number was approximately the same as the differences in tiller numbers. Total panicle weight was significantly higher in one biotype, but all the others were similar to M-206. Yield per plant was significantly higher in only one biotype. All other biotypes had yields similar to M-206. Regardless of biotype, weedy red rice adversely affected M-206 yield components, including aboveground biomass, panicle weight and number, and yield per plant.

    Drone mapping

    Drone imagery of rice fields infested with weedy red rice was gathered in 2018. However, viewed from an overhead perspective, the stalks of both cultivated and weedy rice appeared very similar, blending together in the resulting imagery. Work on the drone concept will continue in 2019.