In California, six bio-types of weedy rice have thus far been identified . Infestations of weedy rice cause harvest quality problems, increased production costs, and reduced yield, so an effective method of control is needed . As a result of the continually flooded conditions under which most California rice is produced, the majority of growers rely solely on herbicides and deep-water flooding for weed management . Permanently-flooded rice agroecosystems are limited, however, to few available herbicides in California, largely due to ecotoxicity and strict regulatory structure . To date, there are 13 registered active ingredients across nine modes of action available for use in California flooded rice, which creates few opportunities for herbicide rotation to inhibit herbicide resistance development . Current herbicides in use in California rice systems include acetolactate synthase – inhibitors, protoporphyrinogen oxidase inhibitors, carotenoid biosynthesis inhibitors, acetyl CoA carboxylase inhibitors, tubulin inhibitors, photosystem II inhibitors,very long chain fatty acid inhibitors, auxin-mimics, 4-hydroxyphenylpyruvate dioxygenase inhibitors, and 1-deoxy-D-xylulose 5-phosphate inhibitors . Most California rice herbicides are limited in the spectrum of weeds controlled, bud curing requiring proper selection in combination and sequence to provide adequate weed control. Early season grass control applications commonly consist of field rates of carotenoid biosynthesis inhibitors, HPPD inhibitors, ALS inhibitors, or VLCFA inhibitors .
Late season cleanup applications often use PSII inhibitors, ALS inhibitors, or ACCase inhibitors in order to control later-emerging grasses . The continuous use of herbicides with similar modes of action has contributed to herbicide resistance development in several weeds found in California rice systems. California arrowhead and small flower umbrella sedge were the first confirmed cases of rice weeds with resistance to bensulfuron-methyl, an ALS-inhibitor, in 1993 . Since then, eight other rice weed species have been identified, some with resistance to more than one mode of action . The rise in herbicide resistance has increased the cost and difficulty of weed management, necessitating demand for novel herbicide development to postpone resistance expansion and assist the management of current herbicide-resistant weed biotypes . Metribuzin is a triazinone PSII inhibitor that binds to the QB binding site on the D1 protein of the Photosystem II complex in the chloroplast thylakoid membranes. Once the chemical binds to the site, electron transport from QA to QB is blocked and CO2 fixation and ATP and NADPH2 production is stopped, halting necessary resources for plant growth . Foliar-applied metribuzin is absorbed into the plant at moderate rates with apoplastic translocation. To date, metribuzin is labelled for use in alfalfa, asparagus, cereals, field corn, garbanzo beans, lentils, peas, potatoes, sainfoin, soybeans, sugarcane, and tomatoes.
There is no label for metribuzin for use in rice in California. Although information regarding the effect of metribuzin application rates and timing on weed control in rice is scant, recent studies from Mississippi have indicated that metribuzin applied post-rice-emergence at 42 g ai ha-1 caused 3-6% injury by 28 days after treatment . The same study found no correlation between rice injury from metribuzin and yield reduction, dry weight reduction, maturity delays, or seed germination . Mahajan and Chauhan evaluated metribuzin at rates 72 and 144 g ai ha-1 and were able to reduce Echinochloa colona biomass by 70 to 100%, respectively, compared to the untreated control. Crop tolerance to herbicides may result from the ability of a crop to metabolize the chemical . Selectivity differences among cultivars depends on accumulation of a critical amount of the active ingredient at the target site of action and a sufficient differential in chemical uptake, in-plant movement, and arrival of the chemical at the correct location in the active form . Although there may be several factors involved in selectivity, the most imperative function is that of resistant plants metabolizing and detoxifying herbicides rapidly and susceptible plants having reduced or no ability to do so . Differential tolerance responses of soybean cultivars to foliar-applied metribuzin have been noted . In rice, cultivar-specific responses to herbicide treatments have been previously identified and used to develop herbicide-resistant rice lines, such as Clearfield or FullPage and Provisia or Max-Ace rice, which confer resistance to imidazolinones and quizalofop, respectively. Differing levels of sensitivity to triclopyr and florpyrauxifen-benzyl , synthetic auxin herbicides, have also been observed in various rice cultivars.
The inherent genetic variability in rice cultivars may provide a resource for crop improvement through breeding. There is a need for additional and alternative herbicide programs to complement sustainable chemical weed control in rice systems. Investigation of differential responses to a chemical can reveal susceptible and tolerant crop lines that may prove useful in breeding programs. With limited knowledge of the response of rice cultivars to metribuzin, the objectives of this research were to evaluate the response of various rice genotypes to post-rice-emergence applied metribuzin and to determine if early-season injury symptoms from POST application of metribuzin are correlated with reduced shoot biomass.Experiments were conducted during 2021-2022 in greenhouses at the Rice Experiment Station in Biggs, CA, USA. Plastic flats measuring 28- by 54- by 6-cm were prefilled with a Esquon-Neerdobe silty clay with a pH of 5.11, and 2.8% organic matter that was sieved through a 2-cm mesh. One hundred forty two rice lines sourced from the Rice Experiment Station representing long-grain, medium-grain, and short-grain rice were selected and fifteen seeds of each cultivar were sown in rows in the flats, with eight rice lines per flat and each row serving as a single experimental unit . Flats were placed in large basins which were filled with 5 cm of standing water . Plants were grown in greenhouse conditions with average day/night temperatures of 32/18 °Cand 16-hour photoperiod with supplemental light intensity of 250 mmol m2 per second photosynthetic photon flux density.California produces 99% of the nation’s almonds, walnuts, and pistachios which contributed over $9.3 billion to the United States agricultural economy in 2019, these nuts are in the top 10 most valuable commodities in the state . Tree nut exports contribute significantly to the California Gross Domestic Product, almond exports alone generated $4.9 billion in 2019 . Tree nut orchards are planted on nearly 750,000 hectares in California; almonds account for 650,000 hectares . Weed control in orchards is an essential part of tree nut production because weeds can compete for resources with young trees, serve as a habitat for other orchard pests, interfere with irrigation, and hinder harvest operations . There are various ways to control weeds in orchards such as mowing, flaming, and weeder animals like ducks and goats, but by far the most common form of weed control in commercial orchards is chemical control using herbicides . In 2018, nearly 3 million total hectares of California tree nuts were treated with herbicides, this means most orchards are treated with herbicide multiple times a year . The frequent use of herbicides in orchard systems creates a space for questions surrounding food, crop, and environmental safety. Typically, tree nut orchards receive an application in the winter while the trees are dormant, in the spring, and an application before harvest . Orchard herbicides can be preemergent, meaning they are applied ideally before seed germination, or postemergent, meaning they are applied after the seed has germinated and become a seedling . Preemergent herbicides have soil activity, therefore, it is important to consider application timing and irrigation events so the herbicide can move into the weed germination zone but not further down into the crop root zone. For the most effective control, curing weed postemergent herbicides are applied when the weed is small and actively growing . In the California Central Valley, irrigation water comes from combination of groundwater and surface water .
In years of drought, surface water supplies are reduced and there is a greater reliance on groundwater resources for orchard irrigation to maintain crop health and productivity. Groundwater quality is dependent upon the region which it is drawn from and where it is drawn within the water table . Irrigation water quality parameters such as pH, salinity, absorption ration , and ion toxicity have a great effect on crop production by limiting water availability at the root zone, reducing infiltration, and causing nutrient imbalance . Information on how water quality affects the crop is abundant but there is space to explore how water quality influences herbicide partitioning into soil solution, particularly ionizable herbicides. The purpose of the preharvest burn down herbicide treatment is to remove as much vegetation as possible to allow the harvest equipment to move cleanly and efficiently through the orchard, usually the treatment is done with one or more broad-spectrum herbicides relatively close to the planned harvest date . As harvest equipment navigates through the orchard the top layer of soil on the orchard floor is disturbed by the equipment, generating dust. Every year there are detections of herbicide residues above maximum residue limits in almonds and it is hypothesized that interactions between freshly harvested almonds and recently treated soil may be contributing to these residues. Orchards are a complex system and there are moving parts to make the system work effectively and efficiently. Because herbicides are applied over the entire course of the growing season, herbicide use intersects with irrigation events and harvest operations. This body of work focused on the influence of these orchard management practices on herbicide residues in soil and almonds.California produces 99% of the nation’s almonds, pistachios, and walnuts which contributed over $9.3 billion to the United States agricultural economy in 2019. Most tree nuts in the state are grown in the Central Valley, a region with a Mediterranean climate receiving 12.7 to 50.8 cm of annual precipitation, primarily during winter. Almond, pistachio, and walnut crops have an average water requirement of 0.49-, 0.41-, and 0.41-hectare meters of water per year, respectively which is mostly met with irrigation during the growing season. Irrigation water in the Central Valley comes from a combination of surface water distributed from reservoirs that impound winter runoff and snowmelt and from groundwater sources. In years of drought, surface water allocations may be curtailed which, in turn, leads to greater reliance on groundwater for orchard irrigation to maintain short-term crop productivity and long-term orchard health. Groundwater quality can vary among regions of the Central Valley and often is dependent upon where it is drawn from in the water table. In particular, some areas have substantial depth-related differences in salinity and pH6 . Irrigation water quality plays an important role in crop safety and crop yield. Factors such as salinity, sodium absorption ratio , ion toxicity, and pH can drastically affect crop production by limiting water availability at the root zone, reducing water infiltration, damaging the crop, and causing nutritional imbalance. The Food and Agriculture Organization of the United Nations have set guidelines for these parameters; a modified summary of salinity and pH guidelines is presented in Table 1.1. Weed control in orchard systems is an essential part of pest management as weeds can compete for resources, interfere with irrigation, serve as a habitat for pests, and disrupt harvest operations. Chemical control with herbicides is the most common form of weed control in commercial tree nut production systems and applications of herbicide often occur multiple times throughout the growing season. Saflufenacil, indaziflam, and penoxsulam are three weakly acidic herbicides commonly used in orchard herbicide programs. In 2018, a cumulative total of 485,000 tree nut orchard hectares were treated with saflufenacil, indaziflam, or penoxsulam. Weak acid herbicides are partially ionized within the normal range of soil pH, and this affects their reactivity and partitioning between the soil surface and soil solution. The pKa of saflufenacil, indaziflam, and penoxsulam is 4.41, 3.5, and 5.1 respectively. As the pH of the soil or soil solution nears the pKa of the herbicide, more of the herbicide will be in its neutral form which could promote binding to the negatively charged soil surface. Irrigation delivery via sprinklers, microsprinklers, or drip lines typically overlap with areas of the field that have been treated with herbicides. Water with high salinity or pH could lead to transient changes in herbicide-soil interactions in surface soil causing concerns about herbicide performance and environmental fate of the herbicides.