Influence of carrier water pH and hardness on imazapic efficacy for sicklepod (Senna obtusifolia L.) control in peanut

Carrier water quality is an important consideration for herbicide efficacy. Field and greenhouse studies were conducted from 2021 to 2023 to evaluate the effect of carrier water pH and hardness on imazapic efficacy for sicklepod control in peanut. Imazapic was applied postemergence at 0.071 kg a.i ha-1 with carrier water pH levels (5, 6, 7, 8 or 9); and hardness levels: 0 (deionized water), 100, 200, 400 or 500 mg L−1 of CaCO3 equivalent in separate field experiments. In greenhouse experiments, imazapic was applied to either 10-cm, 15-cm, or 20-cm-tall sicklepod at similar carrier water pH levels and hardness 0, 100, 200, 400, or 800 mg L−1 of CaCO3. In the field study, sicklepod control, density and biomass reductions were lower with carrier water pH 5 or 9 compared with pH 7. In the greenhouse study, control was not different among carrier water pH levels when imazapic was applied to 10-cm sicklepod; however, when applied to 15-cm or 20-cm sicklepod, control was at least 25% greater with acidic (pH 5) compared with alkaline (pH 9) carrier water. Results from the field study showed that carrier water hardness ≤500 ppm did not reduce imazapic efficacy for sicklepod control. In the greenhouse study, regardless of sicklepod height, carrier water hardness 800 mg L-1 reduced sicklepod control by 15% and biomass reduction by 17% compared with deionized water (pH 7). The effects of carrier water pH and hardness on imazapic efficacy did not compromise peanut yield in the field study. However, this study indicates both acidic and alkaline carrier water pH and hardness (800 mg L-1 CaCO3 L-1) has the potential to reduce imazapic efficacy on sicklepod, and appropriate spray solution amendments maybe be needed to maintain optimum efficacy.