CALIFORNIA
Field Crops A SPECIAL GROWERS’ REPORT OF AG ALERT ®
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Researchers say this rice weed, smallflower umbrella sedge, is among a rising number of species that are developing resistance to registered herbicides. That presents new challenges for rice producers.
Stubborn weeds challenge rice researchers, growers By Vicky Boyd
most common grassy weed in California rice—were also resistant to five different modes of action. “We don’t have many herbicide options, and half belong to one mode of action,” Al- Khatib said. “And we can’t use drill-seeded rice, so it’s a serious problem with the herbi- cide-resistance situation.” Resistance develops when herbicides that have the same mode of action—or that target the same biological processes within plants—are applied repeatedly. Initially, a small por- tion of the population survives the herbicide because of diverse genetic makeup. Those survivors pass along the resistant gene to the next generation. Repeated applications of the same mode of action select for less-susceptible individuals, and the more tolerant population eventually takes over. J.R. Gallagher, a PCA with Butte County Rice Growers Association in Richvale, said he has used the free seed testing service extensively over the years and found it invaluable. For example, one of his growers had horrible watergrass problems in 2020 and 2021 after using a herbicide program that relied heavily on the ALS mode of action. “I pulled the seed sample at harvest in 2021 because I wanted to be proactive and not reactive with this watergrass,” Gallagher said. “I knew something was going on with it.”
University of California Cooperative Extension rice specialist Kassim Al-Khatib doesn’t liken himself to British statesman Francis Bacon or Founding Father Thomas Jefferson. But when it comes to battling stubborn weeds in rice, he embraces a platitude both were credited with making famous: “Knowledge is power.” Al-Khatib said he hopes to empower rice producers with updated information to help them get the upper hand in controlling some of the most resistant weed types. He is de- veloping a knowledge base by testing weed seed samples submitted by rice growers and pest control advisors, or PCAs, for herbicide resistance. “The message is if you have resistant or susceptible species in your field, we test them for you and give you results so you can make decisions and aren’t just shooting in the dark,” Al-Khatib said at a recent UCCE winter rice grower meeting in Woodland. In a worriesome trend for growers, a rising number of rice weed species are becoming increasingly resistant to registered herbicides. In some cases, weeds tolerate not just one mode of action but multiple ones. For example, 64% of the samples of smallflower umbrella sedge, an aquatic grass-like plant, were resistant to two herbicide modes of action and 0.5% were resistant to four. In the worst cases, 3% of samples of late watergrass, a grassy rice weed, were resistant to five different herbicide modes of action. And 1% of samples of barnyardgrass—the
See WEEDS, Page 8
February 15, 2023 Ag Alert 7
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