Two experimental runs were carried out, seeded on April 1, 2022 and May 5, 2022. Pendimethalin applications were made at the four-leaf stage rice on April 21, 2022 and May 18, 2022. At this stage, rice could tolerate the pendimethalin application as observed in preliminary field and greenhouse studies. Before the application, pots were maintained at moist soil to shallow flood to encourage seedling establishment. Pots were then flooded maintained to 5-cm or 10-cm water depth above the soil surface after seedling establishment and the target water depth continuously maintained throughout the study by adding water as needed every 24 h. These two flood depths were selected because 10 cm is the recommended flood depth and occasionally growers may lower water depths to decrease water use . The study was arranged in a factorial randomized complete block design with four replications at each experimental run. Pendimethalin was applied at 0, 2.3 and 3.4 kg ai ha-1 onto the respective pots. Pendimethalin was applied with a track-sprayer equipped with a single 8001EVS nozzle and calibrated to deliver 187 L ha-1 . A 16-hr photoperiod was provided and natural light was supplemented with metal halide lamps at 400 µ mol m-2 sec-1 photosynthetic photon flux when necessary. The greenhouse was maintained at 30 ± 2 /25 ± 2 C day/night temperature. Rice seedlings were sampled at 7, 14, and 21 DAT from each pot in the greenhouse study. Shoot length, weed dry rack root length and dry biomass of both shoots and roots were collected at each sampling date.
The studies were terminated three weeks after the herbicide treatment.Statistical analysis of the field data was carried out using R v.4.2.1 with mixed model regression analysis for the visual ratings and rice grain yield data . A generalized linear model with a gaussian function was implemented for weed and rice tiller count data . Mean separation with Tukey’s HSD at α=0.05 was implemented where appropriate. Greenhouse study data were subjected to mixed model regression analysis and mean separation with Tukey’s HSD at α=0.05, when appropriate using R v4.1.2 . Data transformations were performed as needed by visually assessing the models with quantile-quantile plots and plotting residuals.Pendimethalin applied alone at 1.1 kg ha-1 caused the lowest Echinochloa control at 14 DAT; however, increasing pendimethalin rates to 2.3 and 4.4 kg ha-1 did provide greater control levels . The grass control demonstrates that pendimethalin cannot be a standalone herbicide; however, when rates were greater than 2.3 kg ha-1 grass control was increased. The foliar active herbicides in the mixtures provided good grass control for Echinochloa control and no antagonistic effect was observed. An additive trend was observed with the pendimethalin rate of 4.4 kg ha-1 adding greater value to the overall control compared to the lower pendimethalin rates . Cyhalofop and bispyribac-sodium are excellent Echinochloa herbicides, while propanil has suppression activity on Echinochloa . Despite the differences in Echinochloa populations by year,Echinochloa counts in the pendimethalin mixture treatments were similar to the standard treatment of clomazone followed by propanil plus triclopyr in both years .
Bearded sprangletop populations in the field site are low and typically controlled with the continuous 10-cm to 15-cm flood level . The non-treated had four emerged sprangletop per plot and no treatment decreased the number of emerged sprangletop . Cyhalofop is the only post-emergence herbicide used in the study with activity on sprangletop and it is not surprising that cyhalofop treatments had excellent sprangletop control. It is difficult to conclude that there was any benefit from pendimethalin application for sprangletop control in this study. Sedge and Broadleaf control. Pendimethalin does not have activity on sedges observed in this study. The herbicide mixtures and follow-up treatment provided greater than 91% control of small flower umbrella sedge and rice field bulrush by 56 DAT . The sedge density in the pendimethalin alone treatments were 143 m-2 and similar to the non-treated which demonstrated a density of 99 m-2 . Ducksalad and water hyssop were the most dominant broad leaf species at this site. Ammannia spp. was present in the field but at low population presence with observed 95% control or greater over all treatments, most likely the Ammannia spp. were outcompeted by the crop and other weeds . Pendimethalin does not have activity on the broad leaves present in this study. Ducksalad control levels were greater than 38% control after application of pendimethalin plus bispyribac-sodium and pendimethalin plus propanil treatments early in the growing season .
After the follow-up treatment at the mid-tiller rice timing, broad leaf control increased to 89% in 2022 but not in 2023 . In 2023, the bispyribacsodium mixtures resulted in the greatest broad leaf weed control .Rice injury was minimal, only up to 8% visual injury was observed at 20 DAT . Rice root growth inhibition injury on the nodal root growing region was observed early in the growing season caused by pendimethalin; however, rice recovered from this injury and appeared normal by 40 DAT . The reduced tillers were most likely caused by increase of weed pressure during the early rice growth stage which was managed with the mid-tiller rice application later in the growing season and not caused by pendimethalin injury . Otherwise, tiller counts were similar across treatments and comparable to the standard treatment . There was no difference in plant height across treatments . Grain yields were similar across the treatments where pendimethalin was applied in herbicide mixtures resulting in 6,186 to 8,263 kg ha-1 . The pendimethalin alone treatments pendimethalin plus cyhalofop resulted in similar yields to the non-treated .Results from the greenhouse study demonstrated differences across experimental runs from the response levels measured . The second experimental run generally resulted in 1.4 times greater shoot length and is probably because at the time of the study solar radiation increases in the Northern Hemisphere and the seedlings may have received greater natural light than the previous run . Shoot length was generally similar across treatments and similarly increased throughout the sampling dates. Only the 3.4 kg ha-1 pendimethalin rate did cause 8% to 12% reduction in shoots when compared to the non-treated on both experimental runs by 14 and 21 DAT .Shoot biomass was not different among treatments at all sampling dates and both runs . These results demonstrate that pendimethalin applied to four-leaf stage rice can result in shoot reduction and the level of injury is rate dependent. Root length was reduced 15% only at the 7 DAT at the 10-cm flood depth compared to the 5-cm flood depth in the first run but not the second run . However, there were no differences in root length observed by 21 DAT. In general, root biomass was not affected by the two flood depths; however, only at 7 DAT at the first run root biomass was greater at the 5-cm than the 10-cm flood depth averaged over rates but no difference by 14 DAT . The 3.4 kg ha-1 pendimethalin application reduced root biomass, averaged over flood depths, by 54% in the first run at 14 DAT and no reduction by 21 DAT and in the second run .Pendimethalin applied alone did not provide adequate weed control. Therefore, herbicide mixtures would need to be incorporated into a successful weed management program. The results from the field study show no antagonistic effect from these particular herbicide mixtures with pendimethalin and the importance of herbicide combinations to manage the different weed species in the field is emphasized. Osterholt et al. also demonstrated pendimethalin to have no antagonistic effect on quizalofop control of emerged barnyardgrass when applied as a tank mix on dry-seeded rice. In this study, it was difficult to observe a grass control benefit from pendimethalin in the herbicide mixtures. However, drying rack weed the study demonstrated reduced rice injury from pendimethalin as a post-emergence application and similar grain yields to the standard treatment. Different fields have different weed populations .
Further work is needed to incorporate pendimethalin in water-seeded rice and understand uses and benefits for weed control in the water-seeded system across different sites. The pendimethalin alone treatment followed by cyhalofop plus florpyrauxifen-benzyl resulted in lower control of sedges because at the time of application the sedges may have been too large . In addition, an antagonistic effect resulting in mixing cyhalofop with florpyrauxifen-benzyl cannot be ignored . The broadleaves present in this study are typically easily controlled with bispyribac-sodium, propanil and florpyrauxifen-benzyl; however, differences in the population density each year can be a factor to the observed reduced control the second year in the field study . The yield decrease caused by pendimethalin plus cyhalofop may be due to lack of sedge and broadleaf weed control. The herbicides bispyribac-sodium and propanil have broad spectrum activity on Echinochloa, sedges and broadleaves . The yield decrease observed in the pendimethalin alone treatments most likely was caused because of the late application not controlling emerged grasses and other weeds . The grasses not controlled increased the interference time with the rice which would explain the reduction in tillers and grain yield . Pendimethalin is not highly water soluble, non-ionizable and not hydrolyzed in water; however, it has a high affinity for organic matter . Therefore, the flood depth may have minimal effect on the molecule’s activity. These characteristics can be the reason why no effect was observed from the different flood depths, since the pendimethalin molecule will tend to readily attract to the soil surface with no lateral or vertical movement . The greenhouse study results demonstrate a reduction in shoot length but not shootor root biomass or root length after a pendimethalin application in water-seeded rice at a four leaf stage application and no consistent effect from the two flood depths tested.The application of pendimethalin alone did not result in weed control greater than 63%; however, control was increased when pendimethalin was applied in herbicide mixtures. Pendimethalin should be used in conjunction with other herbicides. Pendimethalin did not cause substantial injury when applied at 4- to 5-leaf stage rice even at the 4.4 kg ha-1 rate application. The rice recovered from minor early-season injury and grain yields across treatments with pendimethalin in mixtures were comparable to the standard treatment. In general, the results suggest that flood depths are not likely to have an effect on the level of rice injury from a pendimethalin application. Therefore, pendimethalin can be incorporated with reduced injury to water-seeded rice as a post-emergence application.These studies were funded by the California Rice Research Board. The authors would like to acknowledge the California Rice Experiment Station for providing the field site, greenhouse and assisting with the field management. The authors acknowledge the various lab members and technicians that helped with carrying out the studies. The authors also acknowledge the D. Marlin Brandon Rice Research Fellowship by the California Rice Research Trust, the Horticulture and Agronomy Graduate Group scholarships including the Bert and Nell Krantz Fellowship and the Jack Pickett Agricultural Scholarship, the William G. and Kathleen Golden International Agricultural Fellowship, and the Department of Plant Sciences, UC Davis for the award of a GSR scholarship funded by endowments, particularly the James Monroe McDonald Endowment, administered by UCANR which supported the student.Rice is a major crop, valued for its nutritious components as a food crop and produced worldwide . In the US, rice production is centered in Arkansas, California, Louisiana, Texas and Mississippi producing nearly two million metric ton of grain for the export market in 2022 . Weed management is a major challenge to achieve economically viable production levels. Cultural practices to achieve an integrated weed management program in California rice include use of certified seed, proper land preparation, and water management . However, to reach the economically viable rice yields, herbicides are necessary to control weeds . The limited number of available herbicides and continuous rice cultivation year after year in California have selected for herbicide-resistant weeds and have caused a reduction in weed control from the available herbicides . The lack of crop rotations makes water management and herbicide use the most important tools to manage weeds . Therefore, new herbicide modes of action are needed to help manage herbicide resistant weed populations. California rice is uniquely different from the other US rice producing states because nearly 90% of the production is medium-grain rice and produced in a water-seeded system .