Emerging Weed Issues in Rice, 2019


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

Within the past six years a number of new weed species have been identified, including winged water-primrose, multiple weedy rice biotypes, and possibly two new species (or subspecies) of watergrass. False pickerel weed in rice seed production is another concern. The range of some of these species is unknown. Since the last weed survey was conducted in the 1980s, this project was initiated to provide the rice industry with more current information.

Winged water-primrose is an emerging non-native weed of concern because of its potential impact to certified rice seed fields
In the fall of 2019, a comprehensive survey took place by soil sampling in fields from each major rice growing county. The fields were randomly selected and include 10 from Glenn, Butte, Sutter, and Colusa counties; five from Yolo and Yuba counties; and two from Sacramento and Placer counties.

The samples were taken after rice harvest in October and November. Samples were placed in freezers and were scheduled to be grown out in a greenhouse in early 2020. Each soil sample would be marked simply for the presence or absence of known weed species. Any unidentifiable or unknown species were to be grown to flowering. Specific field locations are being kept confidential by the UC weed science team, but a general map showing weed locations by county will be published online and made public for future reference. Final survey results were still pending at the time of this writing.

A second objective of this project was to screen the new watergrass weeds for herbicide resistance. Ten samples collected in 2018 were multiplied in the greenhouse in 2019 to produce enough seed for screening. Herbicides for screening include clomazone (Cerano®), thiobencarb (Abolish® and Bolero®), cyhalofop (Clincher®), bispyribac-sodium (Regiment®), and propanil (Stam® or SuperWham! ®). Highest recommended label rates were to be used.

Also, a field test with an experimental herbicide was conducted in a Yuba County grower’s field in 2019 for control of an unidentifiable watergrass population. Results were not promising, showing little to no control. However, the herbicide manufacturer is collaborating with the project leader on other locations for testing with additional chemicals in 2020.