Herbicide resistance is a serious problem in California rice. However, not every control failure can be attributed to herbicide resistance.

Other factors can cause control failures. Among the most common are weather, incorrect rate, poor coverage or application timing, skips, and spray equipment malfunction.

When weed control fails, it is important to determine the cause. When the cause is herbicide resistance, herbicide programs should 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.

The UCCE Rice Weeds Program conducts herbicide resistance testing for the major rice herbicides used in California at the Rice Experiment Station (RES) in Biggs. The results of these tests help growers improve their weed control programs and also help the rice industry keep track of resistance issues.

Weed testing form

If you suspect herbicide resistance, collect seeds of the target weed, and fill out the resistant weed seed testing form available online at http://cecolusa.ucanr.edu/newsletters/Rice_Briefs_Newsletter58741.pdf. Bring these to your local UC rice farm advisor, or send or drop off at the RES for testing.

Weed seed collection

Five key weed seed collection guidelines:

1 - 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.

2 - Collect mature seeds that dislodge easily from the seedhead. In general, sprangletop matures the earliest, between rice panicle initiation and heading. Early watergrass, barnyardgrass, smallflower umbrellasedge, and ricefield 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.

3 - 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

4 - 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.

5 - 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 including sprangletop, smallflower umbrellasedge or ricefield bulrush, collect seeds from at least 20 mature seedheads at each location. For barnyardgrass, early and late watergrass, collect from at least 30 mature seedheads. 

Note: This article is from the University of California Cooperative Extension Colusa County's August Rice Briefs newsletter and is reprinted with permission from Luis Espino.

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