Awan et al. did observe a decrease in plant height from pendimethalin treated plots in a dry-seeded system with no recovery by the final evaluation.Pendimethalin is currently not available for water-seeded rice; however, these results support the introduction of pendimethalin in California water-seeded rice. These results indicate rice injury is reduced with a post-emergence application after the 3- to 4-leaf stage rice in a water-seeded system compared to an application at 1- to 2-leaf stage. Pendimethalin is not a stand-alone herbicide and will need to be accompanied with other available herbicides to achieve season long weed control. In general, most rice cultivars tested were relatively tolerant to pendimethalin when treated after the 3-leaf stage rice; furthermore, cultivars with lower seedling vigor scores may become more injured from a pendimethalin post-emergence application. The results provide supporting data for registration of pendimethalin in water-seeded rice and provide a base knowledge from which further work should be conducted to enhance its use in this system.The authors would like to acknowledge the California Rice Research Board and BASF for providing funding for this project, the California Rice Experiment Station for their support in field management, and the various past and present lab members who assisted with this project, in particular Saul Estrada and Dr. Alex R. Ceseski. 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, and the Department of Plant Sciences, UC Davis for the award of a GSR scholarship funded by endowments, indoor weed growing accessories particularly the James Monroe McDonald Endowment, administered by UCANR, which supported the student.
Rice is a staple crop produced worldwide of cultural and economic value . The export exchange of rice has become a prominent market for many countries worldwide . In the US, the export value rice production was nearly 1.7 billion USD in 2022 . Therefore, worldwide rice production must be upheld to current or superior standards to continuously fulfill the global rice demands. There are various common rice production systems used worldwide like transplanted paddies, dry-seeded seasonally flooded and continuously flooded systems . Water-seeded rice is not common worldwide but is the primary method in some geographical areas such as the Sacramento Valley of California . Waterseeded rice is the practice of seeding pregerminated seeds onto fields with a 7- to 15-cm flood, then, typically continuously flooded for the remaining of the season. The water-seeded rice production is popular in areas with ample water for irrigation or where early flood occurrence and poor drainage lead to continuously flooded fields . The flood in water seeded rice helps to control weedy rice, weedy grasses and non-aquatic weed species . However, flood-adapted and herbicide-resistant weeds have further intensified the weed management challenges in many rice fields . Historically, there has been a limited number of herbicide modes of action available for water-seeded rice . Continuous rice cultivation is common in many growing regions because of soil types and economic limitations . Overuse of the same herbicides and continuous rice cultivation have selected for herbicide-resistant weeds which reduce weed control with the currently available herbicides. To support herbicide resistance management, additional herbicides would be beneficial for growers to practice herbicide mode of action rotations . Pendimethalin is a mitotic inhibiting pre-emergence herbicide from the dinitroaniline chemistry that halts seedling growth shortly after germination . In previous surveys and preliminary greenhouse work, pendimethalin has been successful in controlling herbicide-resistant grass populations . Therefore, pendimethalin was evaluated for rice response in water-seeded rice to understand its applicability in this system .
Results from Becerra-Alvarez and Al-Khatib, in chapter 2 of this dissertation, demonstrated rice injury from pendimethalin was reduced in a post-emergence application at the 4-leaf stage rice and in a capsule suspension formulation within 1.1 to 3.4 kg ai ha-1 . However, at the suggested rice stage timing, many grasses have already emerged and control with pendimethalin is reduced. Therefore, if applied post-emergence in herbicide mixtures to control the emerged grasses, then, greater season-long weed control can be achieved. Additionally, herbicide mode of action mixtures are important strategies for herbicide resistance management which help delay resistance development and can control herbicide-resistant populations . It is hypothesized that the residual pendimethalin soil activity when applied postemergence at 4-leaf stage water-seeded rice could assist in control of late-emerging grasses. Economically important late-emerging grasses in California rice include bearded sprangletop [Leptochloa fusca Kunth ssp. fascicularis N. Snow] and watergrass populations. Bearded sprangletop is characterized as a late-emerging grass weed when compared to barnyardgrass [E. crus-galli Beauv] . While the majority of watergrass will emerge early in the season, there are subpopulations that can emerge later and are characterized as prolonged emergence throughout the season . Populations of multiple-resistant late watergrass [E. phyllopogon Koss] have demonstrated evidence of biphasic emergence with the majority emerging early in the season followed by late-emerging cohorts within the population . There is potential benefit from a pendimethalin post-emergence application for control of late-emerging grasses in water-seeded rice. Preliminary field studies evaluating water-seeded rice response were conducted on a continuous 10-cm flood with application onto the water and demonstrated timing after the 3- to 4-leaf stage reduced injury . However, some growers lower the flood depth to encourage rice seedling establishment, or when irrigation water is limited that year.
Decreasing the flood depths can influence pre-emergence herbicide rice injury in water-seeded rice as observed with available herbicides . Therefore, knowledge of rice response as affected by pendimethalin applications at different flood depths in water-seeded rice is important to develop appropriate application methods and recommendations. The objective of the field study was to evaluate the weed control and rice response of a post-emergence application of pendimethalin alone and in mixtures with currently available herbicides. The objective of the greenhouse study aimed to characterize rice response from pendimethalin applications at two flood depths.The study was conducted at the Rice Experiment Station in Biggs, CA in 2022 and 2023. The field soil is characterized as an Esquon-Neerdobe , silty clay, made up of 27% sand, 39% silt, and 34% clay, with a pH of 5.1, and 2.8% organic matter. During the off-season months, the field stubble was burned in spring 2022 prior to a pass with a single offset stubble disc. Field preparation for both years consisted of one pass with a chisel plow to dry the upper soil surface and then two passes with a single offset disc, followed by a land plane to smooth the soil surface. A granule fertilizer starter mixture application of ammonium sulfate and potassium sulfate was applied at 336 kg ha-1 . Then, a corrugated roller was used to pack the soil and eliminate large clods on the soil surface. Individual 3-m wide by 6-m long plots surrounded by 2.2-m wide shared levees were made after fertilizing and prior to flooding to prevent contamination from adjacent treatments in a replication. Seeds of the rice cultivar ‘M-209’ were pregerminated in water. For disease control, a 5% sodium hypochlorite solution was used for the first hour, then drained and refilled with only water for the remaining 24 hours. The seed was then drained until dry up to 12 hours, and seeded at 170 kg ha-1 both years onto the field with a 10-cm standing flood. The flood was maintained the whole season with the exception of a temporary lowering for the post-emergence herbicide treatments but was reflooded back to 10 cm 48 hours after the application. Copper sulfate crystals were applied by plane at 17 kg ha-1 three days after seeding for control of algae. Standard agronomic and pest management practices were followed based on the University of California rice production guidelines . Seeding dates were May 23, 2022 and May 31, 2023. The herbicides and adjuvants used in the field study are outlined in Table 1. Pendimethalin with 0.4 kg L-1 of active ingredient, vertical grow rack system was applied alone and in mixture with foliar active herbicides at the four-leaf stage rice. The pendimethalin application rates were 1.1, 2.3 and 4.6 kg ai ha-1 . The selection of these rates was based on preliminary studies on pendimethalin rates and timings, where 1.1 and 2.3 kg ha-1 were most appropriate rates for water-seeded rice as a post-emergence application . The 4.4 kg ha-1 rate was included in this study to provide rice response data at 2X of the proposed rate for waterseeded rice. The treatment herbicide mixtures with each pendimethalin rate were propanil, cyhalofop-butyl , and bispyribac-sodium . The applications were carried out with a CO2 backpack sprayer calibrated to deliver 187 L ha-1 at 206 kPa traveling at 4.8 km h-1 . The sprayer boom was 3-m wide equipped with six flat-fan 8003VS tips . At time of herbicide applications, the flood water was lowered 24 hours before treatment and reflooded back to 10 cm 48 hours after the treatment. A non-treated control and a grower standard treatment of clomazone applied at day of rice seeding were included for comparison . The treatments were arranged in a randomized complete block design with four replications both years. A follow-up herbicide application of propanil plus triclopyr was applied for sedge and broadleaf control at the midway of full tiller formation rice stage on all treatments except the non-treated . The treatments with pendimethalin alone had a follow-up treatment of cyhalofop plus florpyrauxifen-benzyl at the mid-tiller stage to control all remaining weeds after the initial assessment date . The metabolites have not been labeled of environmental concern and for the most part the pendimethalin parent molecule remains intact when bound to organic matter . In plants, metabolites are also not common and the majority remain as pendimethalin parent molecule when absorbed . The metabolites are also not documented as of concern to the environment by the US EPA ; however, quantifying metabolites helps in understanding the partitioning behavior of an herbicide in an agricultural or environmental system.Visual weed control was recorded for Echinochloa spp., bearded sprangletop, ricefield bulrush [Schoenoplectus. mucronatus Palla], smallflower umbrella sedge , ducksalad , water hyssop and redstem on 14, 24 and 56 days after pendimethalin treatment , on a scale of 0 to 100, where 0=no control and 100=complete control. Weed density counts for Echinochloa spp., sedges and broadleaves were conducted 30 DAT by sampling twice in each plot with a 30-cm by 30-cm quadrat and data scaled to a meter squared area for presentation. Bearded sprangletop counts were conducted for the whole plot after heading of the grass due to a relatively low population density in the field. Visual rice injury assessments were conducted at 20 DAT and 40 DAT by observing present symptomology, which included chlorosis and stunting on a scale of 0 to 100, where 0=no injury and 100=plant death. Rice tiller counts were conducted at 75 days after seeding by sampling twice in each plot with a 30-cm by 30-cm quadrat and data scaled to a meter squared area for presentation. Plant height was recorded at 100 DAS. Rice grain yield was collected both years and adjusted to 14% moisture. The rice grain was harvested from a 2-m by 6-m area in the plots with a small-plot combine on November 2, 2022 and October 30, 2023 .A greenhouse study was conducted at the Rice Experiment Station in Biggs, CA to characterize rice growth as affected by two flood depths after a pendimethalin application. The greenhouse study allowed more accurate management of flood depths than feasible in the field study and direct side by side treatment comparison. Plastic containers with 34-cm by 20-cm by 12-cm dimensions, with openings for drainage were filled with soil from the field study and placed inside larger 58-cm by 41-cm by 31-cm plastic containers, with no drain holes. ‘M-206’ rice seeds were pregerminated by placing the seeds inside cloth bags, and submerging in five gallon buckets for 24 hr. Then, the seeds were air-dried and ten seeds were placed on the soil surface of each smaller container, which would later be thinned to five evenly spaced plants per plot.