Weedy Rice Control, 2020

 

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

Weedy red rice, or simply weedy rice, was rediscovered in California in 2016. Since then, UC Cooperative Extension advisors and specialists have been working with the rice industry on a multifaceted research program covering genetics, phenotyping, surveying, and management advice.

The past four seasons of work has yielded important information about the nature, extent, and distribution of weedy rice in California. More than 13,000 acres have been infested with seven different biotypes, and possibly one or two new biotypes identified in 2020. Information on dormancy and shattering is crucial to provide management recommendations. Two years of field studies examining population dynamics and emergence have been completed. An overwintering experiment also is yielding important information.

2020 sampling results

In 2020, six out of 31 samples were confirmed to be weedy rice, including three new sites in Sutter, Yuba, and San Joaquin counties. Three from current locations had additional types—one in Yuba County and two in Butte County.

Four or five samples need further analysis of shattering and dormancy in the lab. One type found in both Yuba and Sutter counties was straw-hulled with awns, white-pericarped and short grained. Another sample found in Yuba and Sacramento counties was straw-hulled with no awns, red pericarp and long grained. Both shattered in the greenhouse. If the white-pericarped type turns out to be weedy (shattering and dormancy), it would be the first weedy rice type lacking the red pericarp in California.

A number of seed fields were also found contaminated with weedy rice (primarily type 5). It is unknown how weedy rice spreads, but shared equipment (including airplanes) is strongly suspected.

All previously infested weedy rice fields found since 2016 were resurveyed in 2020. Survey results will be reported in 2021.

In-season field trial

Work continued on an in-season field trial at UC Davis comparing conventional flooded rice, stale seedbed (flushed) management, and crop rotation. Weedy rice types 1,2,3, and 5 were seeded into the plots. Soil samples were taken after tillage and before planting. Soil seedbank analysis from 2019 has been completed. Analysis of the 2020 data is in progress.

Some differences were noted between treatments and biotypes. Variability within plots reflects the spread of plants across a field from seed sources, as well as spring tillage.

There was a slight difference in weedy rice emergence between the conventional and stale seedbed. Also noted were differences between the number of plants that emerged in the two treatments. In the flushed field, Type 3 had as many as five plants per square foot, compared to approximately two plants per square foot in the flooded field.

The number of emerged plants in the flushed treatment was almost 10 times higher in 2020 than in 2019 across most of the biotypes. The number of plants emerging in the flooded treatment decreased from 2019 to 2020. The data indicate that continuous flooding without draining has the potential to suppress weedy rice emergence if repeated over time.

In the 2019 stale seedbed treatment, glyphosate application controlled weedy rice in varying amounts—25% of biotype 1, 75% of biotype 2, 58% of biotype 3, and zero control of biotype 5. Although the herbicide application was made at the same time in 2020 (11 days after flooding), emergence patterns were quite different—40% control of biotype 1, 26% control of biotype 2, 40% control of biotype 3, and 40% control of biotype 5. Differences may be the result of varying temperature and soil moisture. This data will be further analyzed in 2021.

A few trends emerged from the harvest data. M-105 is more competitive under flooded conditions than under flushed conditions in terms of biomass and panicle weight. Type 3 is also more competitive under flooded conditions than under flushed conditions. Type 5 appears to do best under dry-seeded conditions. No clear trends emerged for the other biotypes.

Overwintering experiment

Overwintering research is taking place outdoors at a site in Colusa County. The first phase of the experiment began in the fall of 2019 and finished in spring 2020. It is being repeated for the winter of 2020 through spring 2021.

Weedy rice biotypes 1,2,3, and 5 were buried in mesh bags at two depths—near the soil surface and 6 inches deep. M-206 was utilized as a control. Bags were removed in December 2019, January 2020, and February 2020.

Seed was tested for dormancy and viability. As expected, M-206 had almost no dormancy, regardless of treatment or time in the soil. Similarly, the germination rate decreased over time and the proportion of dead seeds increased with time. (M-206 being a domesticated rice variety has little to no dormancy.) Type 5 showed similar behavior to M-206. Type 1 showed increased dormancy when buried in the soil under both flooded and ambient conditions. Type 3 showed an increase in dormancy over time only under ambient conditions when buried. Types 1, 3, and 5 showed increased mortality over time when buried under ambient conditions. Type 2 showed similar mortality under all field conditions over time.

At 90 days after burial, all types had higher mortality under ambient conditions than flooded conditions. However, there was some variation among them. Dormancy was higher for buried seeds of Types 1, 2, and 3. Type 5 had no dormancy regardless of treatment.