CALIFORNIA
Field Crops A SPECIAL GROWERS’ REPORT OF AG ALERT ®
Varied solutions eyed for chickweed in small grains By Vicky Boyd Researchers at the University of California and California State University, Fresno, say an integrated approach is needed to control common chickweed, inset, a widespread and invasive weed that impacts grain fields such as wheat or triticale. The challenge for growers in the southern San Joaquin Valley region is the species is resistant to popular herbicides.
This led Clark, fellow UCCE farm advisor José Dias and UCCE weed specialist Brad Hanson to conduct trials in a field with documented escapes and with two ALS herbicides. Two and four times the labeled rate of one herbicide only slightly reduced common chickweed populations, while two and four times the labeled rate of the other had virtually no effect on the weed compared to the untreated check. Clark enlisted the help of Anil Shrestha, a weed science professor at California State University, Fresno, to conduct formal greenhouse tests to confirm their suspicions. Joining them was Fresno State graduate student Jennifer Valdez-Herrera. They verified that the samples of common chickweed found in southern San Joaquin Valley small grain fields were resistant to three ALS herbicides. These were tribenuron methyl, the active ingredient in Express Herbicide with TotalSol soluble granules; py- roxsulam, the active ingredient in Simplicity CA; and mesosulfuron-methyl, the active ingredient in Osprey. Brian Gogue, a PCA with Helena Agri-Enterprises in Hanford, said he saw the green- house trial results, and they were “pretty cut and dry.” “That explained a lot,” said Gogue, who was one of the PCAs who had brought the issue to Clark’s attention.
Although researchers have confirmed that common chickweed in California is resistant to popular herbicides that target the enzyme acetolactate synthase, or ALS, they say that doesn’t necessarily mean eliminating use of them. Weed experts recommend growers and pest control advisors take an integrated ap- proach that includes identifying resident weed species and, if needed, applying a herbicide tank mix that includes at least two different effective modes of action for weed control for small grain crops such as wheat or triticale. “If you notice that ALS inhibitors are not controlling common chickweed, you need to look at other chemistries to control it,” said Nick Clark, University of California Cooperative Extension agronomy farm advisor for Kings, Tulare and Fresno counties. “If you have a whole ecosystem out there and you know ALS herbicides still smoke four of the five weeds, you keep them in the tank and add something you know will get the chickweed.” The issue appears to be localized in the southern San Joaquin Valley. Clark said he has talked to colleagues in the northern San Joaquin Valley and Sacramento Valley, and they say ALS herbicides continue to work well. The problem came to light in 2021 when a few pest control advisors alerted Clark about possible ALS failures to control common chickweed in wheat and triticale fields. They had ruled out potential application errors or poor environmental conditions.
See CHICKWEED, Page 11
10 Ag Alert July 10, 2024
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