UC Davis

Rice (Oryza sativa L.) is a staple crop globally and California is the second-largest rice producer in the United States with more than 200,000 hectares of rice planted in the Sacramento Valley and the Sacramento-San Joaquin River Delta. Weeds have historically been one of the biggest challenges of California rice production systems, where herbicide-resistant weeds have increased the complexity of the weed management. Florpyrauxifen-benzyl (FPB) is a new auxin-mimic herbicide with a novel binding site of action for selective grass, sedge, and broadleaf weed control in rice. The objectives of this research were to 1) characterize the effects of FPB on rice crop safety and weed control when applied alone or in mixture with other partner herbicides; 2) determine optimum FPB application timing to control smallflower umbrella sedge; 3) elucidate the response of late FPB applications on rice flower sterility; 4) determine the effects of FPB on nontarget crops including almond, grape, peach, pistachio, plum, and walnut; and 5) compare the onset of foliar symptoms resulting from simulated FPB drift with residues in almond, pistachio, and walnut leaves at several timepoints after exposure.In the first study, FPB was applied at 1/2X, 1X, and 2X field use rates based on 30 g ai ha–1 alone as well as in mixture combinations with bispyribac-sodium, penoxsulam, and propanil in fields near Arbuckle, Biggs, and Willows, CA. FPB applied alone at 30 g ai ha–1 on 4-5-leaf rice stage controlled more than 80% of watergrasses, ricefield bulrush, and smallflower umbrella sedge (SMF) as well as more than 99% of all broadleaf weeds including ducksalad, redstem, waterhyssop, and arrowhead at 28 days after treatment (DAT). The highest rice yield was observed with FPB plus propanil in Arbuckle and Willows, CA. At Biggs, the highest yield (4,626 kg ha–1) was achieved with FPB applied alone at 60 g ai ha–1. In the second study, FPB at maximum use rate of 40 g ai ha–1 was applied to SMF at 1-leaf, 10-, 15-, 20-, and 25-cm tall growth stages. SMF was controlled by 95%, 86%, 89%, 87%, and 85% when FPB applied on 1-leaf, 10-, 15-, 20-, and 25-cm tall growth stage at 42 DAT, respectively. In the third study, FPB at 40 and 80 g ai ha–1 rates were applied at rice panicle initiation (PI) and 50% flowering (FL) growth stages, respectively. While the weed control was more than 90% at 42 DAT for all applications, the FL application caused 26% and 35% rice sterility at the 40 and 80 g ai ha–1 rates, respectively. In the first off-target drift study, fractional rates were 1/200X, 1/100X, 1/33X, and 1/10X of the FPB use rate of 29.4 g ai ha–1 used in 2020 and 2021 on almond, pistachio, and walnut trees treated early in the growing season. Herbicide treatments were applied directly to one side of the canopy of one- to two-year-old almond, pistachio, and walnut trees. The general symptoms were chlorosis, chlorotic spots, leaf curling, leaf narrowing, leaf distortion, leaf malformation, leaf crinkling, shoot curling, stem coloring, stunting, terminal bud death, and twisting. Most symptoms peaked between 14 through 28 DAT with the 1/10X FPB rate, maximum visible injury was 16%, 49%, and 79% on almond, walnut, and pistachio, respectively. The 1/10X FPB treated pistachio trees did not recover as fully as almond and walnut, and injury symptoms persisted for the remainder of the 2021-2022 growing seasons on pistachio. In the second drift study, FPB was applied to one side of the canopy of one- and two-year-old almond, pistachio, and walnut trees at 1/100X and 1/33X of the field use rate of 29.4 g ai ha–1 in 2020 and 2021. Leaf samples were randomly collected for residue analysis at 7, 14, and 28 DAT. Seven DAT with the 1/33X rate, almond, pistachio, and walnut leaves had FPB at 6.06, 5.95, and 13.12 ng g–1 (fresh weight; FW) leaf, respectively. By 28 DAT, all samples from all crops tested with the 1/33X drift rate had FPB at less than 0.25 ng g–1 FW leaf. This study showed that the ideal time frame to collect leaf tissues from trees should be within 14 days after exposure; chemical analysis after this time may underestimate actual exposure. In the third drift study with grapevine, peach, and plum, the fractional rates were 1/200X, 1/100X, 1/33X, and 1/10X of FPB based on 29.4 g ai ha–1. Herbicides treatments were applied on one- to two-year-old peach and plum trees as well as on established grapevines in 2020 and 2021. The general symptoms were chlorosis, chlorotic spots, leaf curling, leaf distortion, leaf malformation, leaf crinkling, necrosis, necrotic spots, and twisting on leaves. Most symptoms appeared at 1/10X FPB rate and peaked from 14 through 42 DAT depending on the species. At 1/10X rate, visible injury was 5%, 50%, and 71% for plum, peach, and grapevine, respectively, at 14 DAT. Some grapevine clusters showed deformation, asymmetrical growth, and fruit dropping. Foliage of all treated crops gradually recovered throughout the growing season regardless of the application rate. Because of low injury symptoms and rapid recovery from herbicide injury in almond, peach, plum, and walnut trees, the proper herbicide drift management, and application precautions are likely to reduce the risk of crop injury from florpyrauxifen-benzyl drift for these crops; however, extra precaution should be taken if there are nearby grapevine vineyards or pistachio orchards because of their greater sensitivity.