Weedy rice is rice with undesirable characteristics, like seed shattering and dormancy. Because it is closely related to cultivated rice, herbicide programs used in rice systems will not control weedy rice. It is sometimes called “red rice” because many biotypes have a red pericarp. In California, several biotypes of weedy rice have been identified and have a range of characteristics, including different hull colors (e.g. straw, bronze, black) and having awns versus being awnless.
Early in the season (before heading), weedy rice is often mistaken for watergrass because it grows taller than the cultivated rice. I have observed, however, that weedy rice vegetation is a lime green color in full light compared to the dark green color of cultivated varieties. I have also observed that watergrass (and barnyardgrass) will head sooner than weedy rice. To determine whether a suspicious plant is weedy rice, sample a stem and observe whether there is a ligule and auricles at the collar. If those features are present, then the plant is rice (either cultivated or weedy). There are photos and a video on the CA Weedy Rice website that can help with identification, or call me if you would like another set of eyes to help with identification.
We have identified weedy rice on a few farms in the Delta, so we need to stay vigilant in our efforts to prevent the spread and manage weedy rice. Only plant pest-free, certified seed, and rogue weedy plants before heading. There is an organic herbicide (Suppress) that is registered for spot spraying, and this can be a viable practice for small patches before plants have headed. If plants have already headed, cut off the panicles into a bag and remove them from the field. Seeds left by the side of the field have the potential to recontaminate the field. Harvest fields with weedy rice last, if possible, and thoroughly clean combines at the end of the season. Remember, weedy rice seeds have high dormancy (seeds may stay dormant for years), so seed that is not cleaned from the combine has the potential to contaminate fields in the following year. Post-harvest field management should include straw chopping but no tillage, which would bury the seed. Flood the field for as long as possible to help decompose any seed that shattered to the soil surface before harvest. Since weedy rice seeds may have high dormancy, high infestations may require field fallowing and/or crop rotation so that different herbicide modes of action may be used.
Weedy rice is not a quarantine pest; however, research from southern states indicates that potential yield declines can reach 60 percent under severe infestations. Additionally, if weedy rice contaminates cultivated rice in drying and storage, extra sorting may be required, which increases cost and decreases quality.
UCCE hosted a weedy rice meeting in early August to provide information about weedy rice distribution, identification, and management. Presentations from that meeting are available from the Delta Crops Resource Management website.