Determining The Herbicide-Resistance In Grower Fields Is A Key For Successful Weed Control In Rice

Herbicide resistance is a serious problem in California rice. However, not every control failure can be attributed to herbicide resistance. Other factors can be the cause of control failures. Among the most common reason for failure include weather, incorrect rate, poor coverage or application timing, skips, and spray equipment malfunction.

When weed control fails, it is important to determine the cause and when the cause is herbicide resistance, herbicide programs need to be adjusted. Resistance occurs after the same herbicides have been used repeatedly at the same site for several years. You will notice a gradual decline in the efficacy of the herbicide to control weeds that were once susceptible. When herbicide resistance is the problem, you will find healthy plants alongside dead ones of the same species after treatment; surviving weeds form discrete patches that consistently survive the herbicide treatment.

Rice (photo credit: Brad Hooker)

The University of California Cooperative Extension (UCCE) Rice Weed Management Program conducts herbicide resistance testing for the major rice herbicides used in California at the Rice Experiment Station (RES) in Biggs. The testing is free of charge and funded by the Rice Research Board. The testing results help growers improve their weed control programs and also help the rice industry keep track of resistance issues. If you suspect herbicide resistance, collect seeds of the target weed, fill out the Resistant Weed Seed Testing form (included in this newsletter), and bring them to your local Farm Advisor, or send or drop off at the RES to be tested. These samples will be tested in the greenhouse at RES. To collect seeds for testing, follow these guidelines:

• Don't wait until harvest to collect the seed. By then, most weeds have shattered their seeds. If you collect after harvest, you may collect seeds from weeds that have emerged late and thus have not been exposed to the herbicide. The objective is to collect seed from plants that have survived the herbicide action.
• Collect seeds when they are mature and dislodge easily from the seedhead. In general, sprangle top matures the earliest, between rice panicle initiation and heading. Early watergrass, barnyard grass, small flower umbrellas edge, and rice field bulrush usually follow, maturing sometime before rice heading until maturity. Late watergrass matures last, at about the same time early rice varieties (M-205, M-206) mature.
• Collect seeds, not seedheads. Gently shake the seedhead inside a paper bag. Seeds that shatter are mature and will readily germinate. If seedheads are collected, seeds might not be mature or might have shattered already. It is good practice to keep the paper bag open for couple days to allow further seed drying.
• Collect seeds from areas of the field where you are certain the herbicide application in question was appropriate. Avoid field borders, tractor tire tracks, skips or areas where you suspect the herbicide was not sprayed correctly or not sprayed at all.
• Make sure to collect enough seed. In order to have conclusive results, several replications of herbicide resistance testing are needed. When not enough seed is provided, replications may not be possible. For small sized seed weed species such as sprangle top, small flower umbrellas edge or rice field bulrush, collect seeds from at least 20 mature seedheads at each location. For barnyard grass, early and late watergrass, collect from at least 30 mature seedheads.
• We will test every weed against all herbicides labeled to control that weed and you will be receiving a detailed report before the rice season start.