Additionally, downstream water quality affected by use of herbicidesin water-seeded rice is of paramount concern because of the proximity to other high value crops which may use the water for irrigation and proximity to urban settlements which may use the water for consumption . The US Environmental Protection Agency recorded pendimethalin risk of contaminating surface waters in agricultural use to be less than 2% . There are no water quality criteria for pendimethalin; however, pendimethalin residues in surface water tributaries near agricultural regions have been documented up to 0.02 parts per billion. The US EPA documented 17.6 ppb to be the maximum level of observed pendimethalin residue in surface water, most likely contaminated by spray drift . Pendimethalin metabolites are formed in various soil, water and plant environments In a water-seeded system, rice is pre-germinated in water for 24-36 hours and air-seeded onto flooded fields with 7-12 cm of standing water, creating an anaerobic environment. The excessive soil moisture immediately after a pendimethalin application in dry-seeded rice sowing significantly increases rice injury . Since water-seeded rice systems have a high-water saturation, then this makes rice seedlings prone to injury from pendimethalin. In drill-seeded rice, clone trays rice is seeded to a depth of about 3.2 cm into the soil and the application of pendimethalin occurs on the soil surface about 1 to 3 days after seeding.
The depth of the seed allows for germination and early growth of the seedling to occur before it comes into contact with the herbicide on the soil surface . Pendimethalin has low volatility, low solubility and strongly attaches to the soil, and will only stay on the top surface layer of the soil . The placement of the seed provides crop safety to pendimethalin in a drill-seeded system . Conversely, in water-seeded rice, rice seed is placed on the soil surface and the initial seedling roots can have direct contact with the herbicide applied on the soil surface. Therefore, an application of pendimethalin later in the season after the seedling is developed and more deeply-rooted may reduce the risk of rice injury. The herbicide formulation may also influence the risk of injury in water-seeded rice. Hatzinikolaou et al. demonstrated that greater injury to oat roots from pendimethalin occurred from the emulsifiable concentrate formulation compared to the granular and the capsule suspension formulations. Hatzinikolaou et al. suggest that the granular and the capsule suspension formulations influence the release rate of the active ingredient resulting in lower risk of crop injury. If increased soil moisture results in greater pendimethalin effectiveness and the emulsifiable concentrate pendimethalin formulation causes greater injury to grass species, then,the use of a slow-release formulation of pendimethalin will reduce rice injury and result in greater crop safety on water-seeded rice. There is no previous research evaluating the partitioning behavior of pendimethalin in water-seeded rice.
The knowledge of pendimethalin behavior in water-seeded rice will help establish proper use of the herbicide to increase herbicide efficacy and decrease off-target contamination potential. Therefore, the objectives of this research discussed in the following dissertation chapters were to evaluate pendimethalin use in water-seeded rice, optimize pendimethalin use for the water-seeded rice system and characterize pendimethalin behavior in flood water of a water-seeded rice field.Rice is an important staple food in many countries and produced worldwide . Water-seeded rice is a common production system in California , Europe, Australia and some Asian countries . The water seeded system is useful for managing grasses, weedy rice, and other non-aquatic weeds . In California water-seeded rice, pregerminated rice seed are air-seeded onto fields with a standing flood of 7-cm to 10-cm, the field will typically be continuously flooded throughout the growing season . Weeds are a major management challenge encountered in rice production . Weedy grasses in the California water-seeded rice agroecosystem include barnyardgrass [Echinochloa crus-galli Beauv], early watergrass , late watergrass [E. phyllopogon Koss], and bearded sprangletop [Leptochloa fusca Kunth ssp. fascicularis N. Snow]. There is potential for up to 70% rice yield loss from season-long barnyardgrass competition and up to 36% rice yield loss from competition with bearded sprangletop . Therefore, weedy grasses are the most economically important weeds in rice production . In California, herbicides continue to be an important tool for weed management in waterseeded rice, but herbicide-resistant weeds have exacerbated the issue leading to poor weed control. A high observed incidence of resistant weed populations is common . The prevalence of resistance has developed due to the limited number of effective herbicide sites of action available and continuous rice production year after year . Multiple herbicide resistance in Echinochloa spp. has made control in rice production a significant challenge. Therefore, there is need for new tools to help implement herbicide resistance management like herbicide mode of action mixtures and herbicide mode of action rotations .
Pendimethalin is a mitotic inhibiting herbicide from the dinitroaniline chemistry, its use is a selective pre-emergent that ceases the seedling growth shortly after germination . Pendimethalin has activity on Echinochloa spp. and bearded sprangletop . Currently, there is no recorded resistance to pendimethalin in California rice; therefore, it has potential to be a new herbicide for waterseeded rice . Pendimethalin is registered for use in drill-seeded rice as a preemergence or as an early post-emergence , however, it is not available in water-seeded rice because of significant crop injury potential . In drill-seeded rice, pendimethalin application is suggested to be at three to seven days after planting and rice should be seeded at depths of 3.2 cm or greater to reduce injury . A deeper planting depth allows the seedlings to grow before contacting pendimethalin on the soil surface . In water-seeded rice, rice seed is sown on the surface of the soil in high moisture levels, therefore, a post-emergence application may reduce injury by allowing seedlings to establish before a pendimethalin application. The 1.1 kg ha-1 rate is the typical label rate used in drill-seeded rice for watergrass control . Pendimethalin degrades faster in anaerobic conditions than in aerobic conditions . Using higher rates may still provide adequate activity in an anaerobic condition. Therefore, the 2X and 3X of the labeled rate were selected to evaluate for rice response and weed control. Herbicide formulation and application timing can be significant factors to reduce the rice injury to acceptable levels in a water-seeded system. Hatzinikolaou et al. recorded theemulsifiable concentrate of pendimethalin had greater soil activity, cannabis drying room but the water dispersible granule and capsule suspension formulation remained active in the soil longer, producing an extended soil residual activity. Hatzinikolaou et al. observed that the EC formulation resulted in a greater reduction in root length than GR and CS formulations, however, the GR and CS formulations also resulted in root length reduction in various plant species tested. Tolerance to herbicides can also vary among rice cultivars. Koger et al. observed differential response to pendimethalin among three long grain rice cultivars, with the ‘Wells’ cultivar demonstrated greater susceptibility to pendimethalin when compared to ‘Cocodrie’ and ‘Lemont’ cultivars in a conventional tillage, dry-seeded system at different seeding depths. Bond et al. observed no differences with minimal to no rice injury, among the same three long grain cultivars in a stale seedbed dry-seeded field study. Because of differences in cultivars and production practices, it is important to examine the response from common California rice cultivars to pendimethalin to understand the practicability and limitations of its use in the waterseeded system. Field and greenhouse studies were conducted to examine the response of water-seeded rice to a pendimethalin application. In the field study, we evaluate rice plant response to three pendimethalin formulations, GR, EC and CS, at three different application timings and three pendimethalin rates. The greenhouse study evaluated the response of five common California rice cultivars after a GR and CS pendimethalin application in a simulated water-seeded condition. The objectives of these studies were to characterize the response of water-seeded rice after a pendimethalin application and evaluate its potential use for water-seeded rice.The field study was conducted in 2020 and 2021 at the Rice Experiment Station in Biggs, CA. Soils at the study site are characterized as 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. Following rice cultivation during the off-season winter months, the field was flooded to 10 cm above the soil after a pass with a single offset stubble disc and then drained in early spring of the following year. Field preparation in spring consisted of one pass with a chisel plow and two passes with a single offset disc, followed by a land plane to smooth the soil surface. A corrugated roller was used to pack the soil and eliminate large clods on the soil surface prior to planting. A granule fertilizer starter mixture of ammonium sulfate and potassium sulfate was applied by plane at 336 kg ha-1 prior to the corrugated roller pass. Seeds of the rice cultivar ‘M-206’ were pregerminated in steel bins filled with water until all the seeds were completely covered. For disease control, a 5% sodium hypochlorite solution was added in the water for the first hour, then drained and refilled with only water for the remaining 24 h.
The seed was then drained until dry for 12 h, and seeded by aircraft at 140 kg ha- 1 seeding rate in 2020 and 170 kg ha-1 seeding rate in 2021 onto the field with a 10-cm standing flood. Individual 3-m wide by 6-m long plots surrounded by 2.2-m wide shared levees were made to prevent contamination from adjacent treatments in a replication. The flood was maintained the whole season and other than being temporarily lowered for application of foliar herbicides for sedge and broadleaf control. Standard agronomic and pest management practiceswere followed based on the University of California rice production guidelines . Seeding dates were May 23, 2020 and June 5, 2021. The study design was in a factorial arrangement of the treatments under a randomized complete block design with four replications. The treatment factors were three formulations, three application timings, and three application rates. The pendimethalin EC formulation was BAS 455 39H with 0.4 kg L-1 of active ingredient, the CS formulation was BAS 455 48H with 0.5 kg L-1 of active ingredient, and the GR was BAS 455 20H with 2% of active ingredient per weight. Application timings were 5, 10, and 15 days after seeding , corresponding to 1-, 2- to 3- and 3- to 4-leaf stage rice, respectively. The application rates were 1.1, 2.3 and 3.4 kg ai ha-1 . A non-treated control plot with no pendimethalin applied was randomly placed within each replication to serve as a reference for the assessments. The CS and EC formulations were applied with a CO2 pressurized backpack sprayer calibrated at 206 kPa to deliver 187 L ha-1 . The sprayer boom was 3-m wide equipped with six flat-fan 8003VS tips traveling at 4.8 km h-1 and spraying onto the water surface. The GR formulation was spread by hand in each respective plot. Additional herbicides were applied for control of emerged grasses in 2020 and for control of other weed species not controlled by pendimethalin both years. Due to a high population of grasses surviving the pendimethalin treatment in 2020, an additional post-emergence rescue treatment cyhalofop-butyl at 0.3 kg ai ha-1 and propanil at 1.7 kg ai ha-1 were applied at 21 DAS was applied which likely influenced the yield and weed control data to some degree. Copper sulfate crystals were applied byplane at 17 kg ha-1 three DAS for control of algae. In 2020, a mixture of carfentrazone-ethyl at 0.1 kg ai ha-1 and triclopyr at 0.3 kg ai ha-1 was also applied at 52 DAS for sedge and broadleaf control. In 2021, only carfentrazone-ethyl at 0.1 kg ai ha-1 and triclopyr at 0.3 kg ai ha-1 were applied for sedge and broadleaf control at 32 DAS. Visual weed control of the of Echinochloa spp. and bearded sprangletop were recorded on 14 and 56 days after pendimethalin treatment , on a scale of 0 to 100, where 0=no control and 100=complete control. Echinochloa spp. counts in the non-treated were conducted 30 DAS by sampling twice in the plots within a 30-cm by 30-cm quadrat.