UC Davis

A system of permanently flooded rice and a lack of diverse weed management techniques have selected for flood-tolerant weed populations in California water-seeded rice agroecosystems. As a result of limited available herbicides in California and a system of continuous rice monocropping, herbicide resistance has developed in several weeds found in California rice fields. The rise in herbicide resistance has increased the cost and difficulty of weed management, necessitating demand for novel herbicide development to delay resistance expansion and assist the management of current herbicide-resistant weed biotypes. Greenhouse and field experiments were conducted to 1) characterize pyraclonil activity on common California rice weeds alone and in combination with currently available herbicides and 2) evaluate the differential response of rice genotypes to POST application of metribuzin at various rates. Field experiments were conducted to evaluate pyraclonil, a novel protoporphyrinogen oxidase (protox) inhibiting active ingredient under development for California rice, applied alone and in combination with other herbicides to determine grass, sedge, and broadleaf control and crop safety. These experiments indicated that pyraclonil applied alone is insufficient for broad-spectrum weed control, but, when applied in combination with currently available herbicides, provides consistently greater control of watergrass species, bearded sprangletop, ricefield bulrush, smallflower umbrellasedge, ducksalad, and redstem. Pyraclonil demonstrated no lasting injury that affected rice yields and was effective as a base treatment herbicide for incorporation into water-seeded rice weed management programs. Greenhouse experiments were conducted to evaluate the differential response of 142 California rice cultivars to foliar-applied metribuzin when rice plants were at the three- to four-leaf stage. Dose-response experiments failed to confirm a differential response for specific rice cultivars but did indicate differential responses among the different types of rice available. At tested rates, short-grain rice cultivars were more susceptible injury including stunting and biomass reductions compared to long-grain or medium-grain cultivars at the same rates. These results suggest that further research is needed to establish metribuzin’s candidacy for development as a postemergence product in rice. These results are pertinent to the rice growers and pest control advisors who are concerned with the management of herbicide resistance in California rice and may contribute to the ongoing research concerning differential response of cultivars to novel chemicals.