Use of a Capsule Suspension Formulation of S-metolachlor in Fenclorim-treated Rice

Abstract

As herbicide resistance continues to render commonly used rice herbicides ineffective, alternative sites of action are paramount to maintaining yield and producer profitability. Combining a slow-release formulation and a fenclorim seed treatment might allow the safe use of S-metolachlor in rice. Experiments were initiated in 2022 and 2023 near Colt, AR, on a silt loam soil to evaluate crop safety using a capsule suspension (CS) formulation of S-metolachlor and a fenclorim seed treatment in rice. The first experiment assessed the tolerance of two cultivars (Diamond and DG263L) to three rates (0.42, 0.84, 1.68 kg ai ha^-1) of a CS S-metolachlor at a delayed preemergence (DPRE) application timing in conjunction with a fenclorim seed treatment. The second experiment evaluated a 1- to 2-leaf (EPOST) application of a CS S-metolachlor at 0.56 and 1.12 kg ai ha^-1 to fenclorim-treated rice. Fenclorim reduced injury and partially protected rice yield when S-metolachlor was applied DPRE at 1.68 kg ai ha^-1 in both years. However, in one year under adverse conditions, rice yields were only 65% and 66% of the nontreated control for fenclorim-treated Diamond and DG263L, respectively. An EPOST application of S-metolachlor at 1.12 kg ai ha^-1 resulted in 44 to 51% visible injury 35 d after treatment. Relative rice yields were 88% and 89% of the nontreated weed-free treatment in 2022 and 2023, respectively. Fenclorim provided enhanced crop safety at both the 0.84 and 1.68 kg ai ha^-1 rates of S-metolachlor. However, the potential for reduced yield can occur when unfavorable conditions occur soon after application. An EPOST application timing of CS S-metolachlor at 0.56 kg ai ha^-1 may be a viable option in rice, but 1.12 kg ai ha^-1 is too high on a silt loam soil, resulting in significant rice injury.