The water-seeded production system in California is a common method to suppress weedy grasses and non-aquatic weed species. In California, pregerminated rice seed is air-seeded onto fields with a 10- to 15-cm standing flood and the fields are typically maintained continuously flooded throughout the growing season . The California rice cropping system is again unique because of its presence near growing urban communities and a variety of neighboring high value crops. Surface water used for rice production is mainly derived from reservoirs that capture water in the Cascade Mountain Range and Sierra Nevada from the Sacramento River and the Feather River, respectively . There is potential for contamination of drinking water and water for wildlife by herbicide use in California rice fields, which has historically been documented with the rice herbicides thiobencarb and molinate . Production lands further away from the water sources will also use drainage water downstream as irrigation . Many neighboring crops can be susceptible to pesticide residues at low concentrations and this can be of concern if herbicide residues are present in the irrigation water . Historically, regulatory agencies and the California rice industry have collaborated to implement successful programs to manage and reduce off-target pesticide effects by mandating report of pesticide use, monitoring water quality, indoor grow table and water-holding periods after chemical applications . Pesticide use reporting and monitoring encourage stewardship of chemical use among agencies and applicators .
Water-holding periods prevent the pesticide active ingredient from becoming runoff in the tail water and contaminating non-target areas and organisms. The water-holding period can differ among pesticides based on their physico-chemical properties and degradation pathways . Therefore, it is important to understand the behavior of herbicide active ingredients in the water-seeded system to successfully characterize them in support of sustainable stewardship and efficacious use of chemicals. Herbicide products can be developed in various formulations to assist with weed control, for instance, to achieve longer soil residual activity, reduce crop injury, affect dissipation or forapplicator safety . Formulation is also suggested to influence the potential of the active ingredient to contaminate surface waters . Pendimethalin is a mitotic inhibiting herbicide from the dinitroaniline chemistry, it is a selective pre-emergent that ceases seedling growth shortly after germination of susceptible plants . Physico-chemical properties of pendimethalin are presented in Table 1. Pendimethalin has been proposed for use in water-seeded rice, since it controlled herbicide-resistant grass populations and if labeled would provide an additional tool for management over herbicide-resistant grasses in California rice. However, there has been no work characterizing pendimethalin’s behavior in water from a water-seeded rice field. It is hypothesized, based on the physico-chemical properties, that pendimethalin will not persist in surface water, however, product formulation could affect dissipation in water. Therefore, the objectives of this study were to evaluate the dissipation behavior of pendimethalin across three formulations in rice flood water after an application in a water-seeded rice field.
A field study was carried out at the Rice Experiment Station in Biggs, CA . Because of scrupulous quality assurance for each experimental unit to meet regulatory standards, which led to extensive costs associated with the analysis and labor, the study was only conducted in 2021 with three replications. Individual plots were arranged in a randomized complete block design across the field. Soils at the site are characterized as EsquonNeerdobe , silty clay, made up of 27% sand, 39% silt, and 34% clay, with a pH of 5.1, and 2.8% organic matter. Irrigation waters at the research site on average have a pH of 7.81 and electrical conductivity of 0.12 ds/m. 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. Water temperature, when delivered from the irrigation canal, can average as low as 13°C, and in the field, it is recommended for the water to not be below 18°C for appropriate rice growth and development . Irrigation water was first delivered on June 2, 2021 into a warming field basin, where it circulated before traveling to the field basin with the plots. To move water inside each individual plot, 5-cm diameter by 1.5-cm length single bend aluminum siphon irrigation tubes were placed over the 2.2-m wide levees. The plots were flooded to 4-inch by June 4, 2021 and maintained at that depth for the duration of the study. ‘M-206’ rice was air-seeded at a rate of 170 kg ha-1 onto the field with a standing flood on June 5, 2021.Rice flood water was sampled at 1, 3, 5, 10 and 15 days after treatment application for each plot and replication separately. At each individual plot, a composite water sample was collected with a glass beaker from four areas in each plot near the center and quickly homogenized in a ~1-L plastic container . Then, 3 oz were poured in a 4-oz tight seal jar and placed in storage at 0°C immediately until delivered inside the lab within four hours. For each individual plot, new containers were used to sample each time. In the lab, water samples were cleaned and 50 mL were allocated from the filtered sample and placed in storage at -20°C until analysis. Daily temperature, relative humidity and solar radiation data were obtained from the California Irrigation Management Information System , Biggs, CA weather station number 244 .
Liquid-liquid extraction methods were modified from USEPA . High pressure liquid chromatography tandem mass spectrometry was employed to analyze for residue in water samples. A standard for pendimethalin, were obtained as a reference to quantify residue in samples. The recovery in water samples was on average 79%. See supplementary material for details on method. Data analysis were performed using R v4.1.2 . Linear regression analysis and analysis of variance was used to determine associations on the concentrations across formulations, rates and sampling time with LMERTEST R package . Means separation with Tukey’s honestly significant difference at α=0.05 was then used where appropriate with EMMEANS R package . The data was log transformed to fulfill homogeneityand linearity requirements for a linear regression .There were differences in concentrations recovered from water samples across rates , sampling time , and formulation by sampling time . At 1 DAT sampling, the EC had the highest concentrations at 73.0 parts per billion averaged over rates . The CS and EC formulations maintained similar concentrations throughout sampling times after the 1 DAT . The GR maintained the greatest concentrations at 10 and 15 DAT compared to the CS and EC . The differences in dissipation across formulations could be attributed to the formulation properties. The EC is constructed of an oil-water-emulsion with organic solvents, drying rack cannabis while the CS encapsulates the active ingredient in layers of water-soluble polymers . As an oilbased formulation, the EC would make pendimethalin persist in suspension on the water at higher concentrations early on because of the inactive carriers being not water soluble. The encapsulating polymers in the CS would allow the compound to be water soluble and extend the amount of time the compound is suspended in water . These characteristics can explain the higher concentrations early on from the EC formulation compared to the other two formulations. GR herbicide formulations tend to have the active ingredient adsorbed to inert material, allowing slow and continuous release of the active ingredient . This characteristic of the GR formulation may help explain the increases of concentration in water three days after the application of the 3.4 kg ha-1 rate . The delayed increase in concentration was rate dependent, however. Similarly, Ngim and Crosby observed formulation affected dissipation of the insecticide fipronil in water-seeded rice, with the granule formulation being most persistent. A GR pendimethalin application onto a water-seeded rice field may need a longer waterholding period than the liquid formulations. Dissipation generally followed first-order kinetics . The GR demonstrated halflives up to 6.9 days. The CS had half-lives three to four days less than GR and the EC had halflives nearly seven days less . The average daily temperature for the duration of the study was 25°C with a low of 16°C and high of 34°C. Daily solar radiation averaged 346 Watts m2 with a low of 341 Watts m2 and high of 366 Watts m2 .
Relative humidity averaged at 50% with a low of 30% and high of 80%. These are the typical conditions during the early rice growing season in California and are important to note as factors that can affect the pendimethalin degradation. Half-lives of pendimethalin in water were reduced in this study probably due to greater degradation occurring in a field environment stimulated by microorganisms, photolysis degradation and partitioning onto organic sediments from the soil . Pendimethalin residue half-lives in water have been previously reported at 12.7 and 13.7 days afteran application of an EC pendimethalin formulation at 0.5 parts per million and 1.0 ppm , respectively, onto irrigation canal water . Degradation pathways can be inferred based on the physico-chemical properties of pendimethalin. The pendimethalin molecule is not high water soluble, non-ionizable and not hydrolyzed in water and possesses a high affinity for organic matter ; therefore, sediment partition is most likely the significant degradation pathway. Partitioning of pendimethalin onto sediment in water/sediment investigations in dark demonstrated to be within 0.4 to 1.6 days for 50% allocation onto sediments . Pendimethalin is moderately volatile and volatilization is an important dissipation pathway in dry and moist soil, however, as soil moisture increases over soil field capacity, volatilization decreases due to lower movement of the vapor phase in wetter soils . Solar radiation was high in the study area and can be a significant degradation pathway. Both photolysis and sediment partitioning are most likely the important pathways of pendimethalin degradation. While this study negates the pendimethalin metabolites, it is important to note there are three metabolites that can form in water . Nevertheless, the pendimethalin residues in the water indicate the importance of holding flood water in the field after an application to allow the herbicide molecule to settle on the soil surface when applied onto a flooded rice field.The US EPA has recorded an observed maximum level of pendimethalin in surface water at 17.6 ppb, probably contaminated by spray drift, and expressed the risk of pendimethalin contaminating surface waters to be less than 2% . While there is no water quality criteria level for pendimethalin, residues of pendimethalin have been observed in surface water tributaries near agricultural regions with concentrations up to 0.02 ppb . Additionally, pendimethalin residues as low as 30.0 ppb in soil have shown to cause injury on tomato , a common crop grown near California rice fields . Despite observed concentrations above these levels from the EC and CS formulations early on, pendimethalin dissipated quickly below levels of concern . Apart from preventing potential herbicide runoff, water-holding periods can be useful for increasing herbicide efficacy. Some pesticides currently used need the water for activation or to evenly distribute in the field and holding water in the field is common practice for California growers when using granule pesticides in rice . The concentrations observed from this study also suggest pendimethalin could benefit from a water-holding period to increase the efficacy when applied onto the flood. However, an increase in efficacy can also develop greater rice crop injury and should be balanced through application rates and timings. The rates used in this study were the typical use rates in dry-seeded rice, which are known to provide adequate weed control. This study did not focus on weed control but ongoing work is examining this aspect to enable efficacious and safe use of pendimethalin for water-seeded rice. Pendimethalin did not persist to levels of concern in the surface-water of a water-seeded rice field and was detected at very low concentrations, in general. The results from this study can assist regulatory agencies and registrants in articulating a water-holding period for pendimethalin in water-seeded rice, which can help prevent potential contamination to municipal drinking waters, prevent damage to downstream high value crops and ensure efficacious use, therefore, promoting responsible stewardship of chemical use in California rice.In the lab, water samples were cleaned from debris by periodically pouring the 90 mL sample through a funnel with filter paper of 11 µm Whatman 1 of 90 mm diameter outlining the inside the funnel’s wall.