The study site varied in weed population composition each year. Based on non-treated plots, watergrass species infestation in 2019, averaged 10% abundance compared with 28% in 2021 . Sedge and broadleaf pressure were similar in both years. Similar variations have been noted in this same field location and attributed to annual differences in temperature and management . In 2021, the study was seeded 12 days earlier than in 2019; the weather differences between the two years may have contributed to the difference in weed population .Pyraclonil applied alone or with other herbicides provided 76-96% control of watergrass species at 14 DAT . The efficacy of pyraclonil alone decreased as the season progressed, reaching 54% watergrass control at 42 DAT, which was less than pyraclonil applied with other herbicides. All tested herbicide combination treatments provided excellent seasonlong control of watergrass species. There were no differences among the pyraclonil combination treatments regarding their control of watergrass species at 14 DAT or 42 DAT; watergrass control ranged from 88-100% in treated plots by 42 DAT. Pyraclonil applied alone did not achieve season-long watergrass control and should be combined with other herbicides that express season-long activity on watergrass species. There was no difference in sprangletop control among pyraclonil treatments at 14 DAT; however, there was a non-significant trend of 100% control of sprangletop achieved with pyraclonil followed by propanil, pyraclonil followed by thiobencarb followed by propanil, grow room pyraclonil followed by bispyribac-sodium followed by propanil, pyraclonil followed by penoxsulam followed by propanil, and pyraclonil followed by florpyrauxifen-benzyl followed by propanil .
At 42 DAT, however, the herbicide programs of pyraclonil followed by propanil and pyraclonil followed by penoxsulam followed by propanil provided similarly low control of bearded sprangletop, ranging from 61-68% control. The decline in weed control from 14 DAT to 42 DAT may be attributed to the late emergence of bearded sprangletop that escaped the pyraclonil treatments . Bearded sprangletop requires 215 growing degree days to achieve 90% emergence, contributing to a later emergence compared to other common weeds such as watergrass species, which require only 124 GDD for 90% emergence . GDDs for June 2019 and June 2021 were estimated and used to establish 90% bearded sprangletop emergence in thermal time degree days . However, there was no difference in the amount of GDDs between the years that the field study was conducted. The field location received 220 GDDs by 21 days after seeding in 2019, enough to reach 90% emergence of bearded sprangletop in 2019. In 2021, the field location reached 229 GDD for 90% bearded sprangletop germination 20 days after the rice was planted. The combination of pyraclonil followed by benzobicyclon plus halosulfuron followed by propanil gave 50% control of bearded sprangletop at 14 DAT; however, at 42 DAT, no sprangletop was found in the treated plots. Benzobicyclon plus halosulfuron is applied at 1.5 rice leaf stage but exhibits long-lasting weed control through both foliar and root uptake that may account for the control of later cohorts of bearded sprangletop . Pyraclonil applied alone did not control ricefield bulrush substantially differently from the untreated plots and was insufficient for effective ricefield bulrush control; however, the pyraclonil applications followed by propanil, benzobicyclon plus halosulfuron and propanil, clomazone and propanil, thiobencarb and propanil, bispyribac-sodium and propanil, or penoxsulam and propanil achieved similar ricefield bulrush control ranging from 61-88% at 14 DAT .
Differences in ricefield bulrush control among the different herbicide combinations began to emerge at 42 DAT. The treatment of pyraclonil followed by benzobicyclon plus halosulfuron and propanil and pyraclonil followed by florpyrauxifen-benzyl and propanil controlled 97% of ricefield bulrush. These two treatments provided greater control over ricefield bulrush than pyraclonil followed by propanil, 48% control, and pyraclonil alone, 23% control. Benzobicyclon plus halosulfuron is a standard treatment to control ricefield bulrush in California, which explains the higher level of control resulting from this program . Later-season application of florpyrauxifen-benzyl at early-tiller stage may be timed toeliminate ricefield bulrush that are not controlled by the combination of pyraclonil and propanil, which may account for the high efficacy of this herbicide combination. Pyraclonil applied alone and all herbicide combinations achieved greater control of smallflower umbrellasedge at 14 DAT . Pyraclonil applied alone provided 65% control of smallflower umbrellasedge at 14 DAT. This level of control at 14 DAT was less than that of pyraclonil applied in combination with benzobicyclon plus halosulfuron followed by propanil and thiobencarb followed by propanil, ranged from 95 to 97% control, respectively. The following pyraclonil combinations provided greater smallflower umbrellasedge control compared to pyraclonil alone, which provided 48% control at 42 DAT: pyraclonil followed by benzobicyclon plus halosulfuron and propanil, pyraclonil followed by thiobencarb and propanil, pyraclonil followed by propanil and bispyribac-sodium, pyraclonil followed by penoxsulam and propanil, and pyraclonil followed by propanil and florpyrauxifen-benzyl.
All treatments provided significantly higher levels of control than the untreated plots at 42 DAT. Pyraclonil alone was insufficient for effective season-long control of small flower umbrellas edge; however, when partnered with other herbicides labelled for control or suppression of this weed, the level of control was excellent. Plots treated solely with pyraclonil maintained the lowest levels of ducksalad control of the treatments tested, ranging from 85 to 86% control throughout the season . All pyraclonil combinations had excellent control of ducksalad that ranged between 92 to 100% at 14 DAT. Ducksalad control remained high and ranged between 86% to 100% at 42 DAT for all herbicide combinations with no differences among treatments. Pyraclonil applied alone provided 86% control of redstem at 14 DAT . All pyraclonil combination treatments provided excellent control of redstem at 14 DAT.Some redstem appeared at 42 DAT in plots treated with the combinations of pyraclonil followed by propanil and pyraclonil followed by clomazone followed by propanil but there was no difference between these treatments and the other treatments tested. All herbicide treatments provided effective season-long control of redstem.Rice injury was observed as chlorosis and stunting across all treatments in both years from 7 DAT to 42 DAT. The rice injury data revealed significant year-by-treatment interaction, so the data were analyzed separately by year for both chlorosis and stunting ratings. Several treatments caused chlorosis at 14 DAT in 2019: pyraclonil , pyraclonil followed by clomazone and propanil , pyraclonil followed by thiobencarb and propanil , and pyraclonil followed by propanil and bispyribac-sodium . All other treatments had chlorosis ranging from 41 to 61%, with the exception of the herbicide combination of pyraclonil followed by propanil and florpyrauxifen-benzyl, which displayed 25% chlorosis. However, by 42 DAT in 2019, drying cannabis only pyraclonil alone exhibited chlorosis, at 4% in the treated plots . No rice chlorosis was observed in any treated plot in 2021 at 14 DAT . At 42 DAT, the herbicide combination of pyraclonil followed by thiobencarb and propanil caused 19% chlorosis, which was significantly higher than other treatments. Chlorosis gradually disappeared in the treated plots. Hakim et al. also found slight rice injury including chlorosis from herbicide applications consisting of thiobencarb and propanil in non-saline soils in Malaysia. The chlorosis ratings between 2019 and 2021 were diverse. All treated plots presented some chlorosis at 14 DAT in 2019 but recovered by 28 DAT and demonstrated negligiblephytotoxicity after 42 DAT. No chlorosis was observed in any treatment until after 21 DAT in 2021. Only the combination of pyraclonil followed by thiobencarb and propanil provided any sign of chlorosis at 19% at 42 DAT in 2021. Applying thiobencarb slightly earlier than recommended on the manufacturer’s label may have caused the early chlorosis, but the rice was able to recover from the early phytotoxicity. No stunting was observed for any treatments in 2019 at 14 DAT. The combination of pyraclonil followed by thiobencarb followed by propanil caused 24% stunting by 42 DAT, which was significantly different from the five other treatments .
Pyraclonil alone, pyraclonil followed by propanil, pyraclonil followed by benzobicyclon plus halosulfuron and propanil, pyraclonil followed by clomazone and propanil, and pyraclonil followed by propanil and florpyrauxifen-benzyl caused rates of stunting indistinguishable from the untreated plots. There was no significant stunting from any other treatments besides the combination treatment containing thiobencarb. The combination of pyraclonil followed by benzobicyclon plus halosulfuron followed by propanil caused 4% stunting in 2021 at 14 DAT, which was slightly more stunting than the other treatments, but otherwise no severe stunting was observed at that early date for any treatment . Pyraclonil applied alone and pyraclonil followed by benzobicyclon plus halosulfuron followed by propanil caused 7 and 8% stunting, respectively, at 42 DAT. These results agree with earlier research that noted that pyraclonil at rates ranging from 25 to 200 g ai ha-1 caused ≤ 8% shoot biomass reduction when applied to a commonly used rice variety in China . The herbicide program containing thiobencarb resulted in significantly greater stunting at 23% at 42 DAT compared to all other herbicide treatments . Baltazar and Smith Jr. found 30% stunting in rice treated with propanil and thiobencarband noted that yields were unaffected by this early season stunting. A possible explanation for the phytotoxicity from the herbicide combination containing thiobencarb may result from the application timing. Thiobencarb was applied at 1.5 rice leaf stage in order to coincide with 2 leaf stage of the watergrass species in the field. This application timing is slightly earlier than the recommended application timing of 2 rice leaf stage . The interaction of yield by years was significant; therefore, these data were presented separately. There were no significant differences in yield among treatments in either study year. In 2019 rice yields averaged 8,796 kg ha-1 , whereas in 2021, yields from the herbicide programs averaged 11,294 kg ha-1 . The difference between the two years’ yield may be due to the difference in planting date and weather patterns between the years this study was conducted. The difference in average yield in our study coincided with average rice yield for California. In 2019, the average yield for California rice was 8,536 kg ha-1 , whereas in 2021, California rice averaged 10,144 kg ha-1 .Rice is one of the most commonly grown agricultural commodities in the world and contributes significantly to sources of human energy across the globe . California is the second largest rice-growing state in the USA, with approximately 200,000 ha of rice acreage in California, much of which is concentrated in the Sacramento Valley. The majority of California’s rice production consists of short- and medium grain japonica varieties and a few long-grain indica varieties, including cultivars developed for both the local climate and a continuously-flooded cropping system, where rice is pre-germinated and seeded by airplane onto fields with a 10-15 cm standing flood . Decades of using this practice to suppress grass, sedge, and broadleaf weeds that would otherwise decrease yields, in addition to no crop rotation, have selected for weed species that exhibit ecological requirements and growing patterns that are similar to rice and can compete with rice resources . The flooded conditions in which most California rice is grown favor weedy grasses that are well-adapted to flooded conditions which include watergrass species Beauv. spp., bearded sprangletop [Leptochloa fusca Kunth ssp. fascicularis N. Snow] and weedy rice . Crop yields and harvest quality face the highest biological constraints due to weed infestations, and farmer inputs towards weed management are expected to increase as herbicide resistances spreads worldwide . Certain weeds and weed groups cause more yield loss than others, even at lower infestation densities . In rice systems, grasses are considered the most difficult weeds to control due to the narrow selectivity between the crop and the grass weeds . Rice yield losses can amount to79% after season-long interference from barnyardgrass [Echinochloa crus-galli Beauv.] and have been recorded as high as 59% due to season-long competition with late watergrass [Echinochloa phyllopogon . Koss] . Weedy rice is an increasingly problematic weed in rice-growing regions around the world causing yield loss and contamination due to the critical weedy traits of seed shattering and seed dormancy, which builds up a large soil seed reservoir for future years . The weedy rice infestation threshold stands at one to three plants m-2 in the USA, with higher ratios causing significant yield loss; weedy rice densities of 30 to 40 plants m-2 can reduce rice yields by 60-90%, depending on the height of the cultivar .