Rice Response to Sub-lethal Rates of Paraquat, Metribuzin, Fomesafen, and Cloransulam-methyl at Different Application Timings
Abstract Paraquat mixtures with residual herbicides before planting is a common treatment in Mississippi, and rice in proximity is susceptible to off-target movement of these applications. Four concurrent studies were conducted in Stoneville, MS, to characterize rice performance following exposure to a sub-lethal rate of paraquat, metribuzin, fomesafen, and cloransulam-methyl at different application timings. Applications were made to rice at spiking to one-leaf (VEPOST), two- to three-leaf (EPOST), three- to four-leaf (MPOST), 7 d postflood (PFLD), and panicle differentiation (PD) growth stages. Regardless of application timing, rice injury following exposure to paraquat was ≥ 45%. Delays in maturity were increased by 0.3 d d-1 following paraquat from emergence through PD. Dry weight, rough rice yield, panicle density, and germination were reduced 18.7 g, 131.5 kg ha-1, 5.6 m-2, and 0.3%, respectively, d-1 from paraquat at emergence through PD. By 28 d after treatment (DAT), metribuzin injured rice 3 to 6%, and that injury did not translate into a yield reduction. Regardless of application timing, rice injury following fomesafen ranged from 2 to 5% 28 DAT. Rice exposed to cloransulam-methyl EPOST exhibited greatest root and foliar injury 21 and 28 DAT, respectively. Additionally, yield was reduced to 6,540 kg ha-1 compared with 7,850 kg ha-1 in the nontreated when rice was exposed to cloransulam-methyl EPOST. Rice yield was negatively affected following paraquat applied any time after rice emergence. However, applications of paraquat to rice in early reproductive growth reduced rough rice yield and seed germination the greatest. Application timing is crucial in determining severity of rice injury. Early-season injury to rice following paraquat had less effect on yield compared with injury at later stages. Additionally, fields devoted to seed rice production are at risk for reduced seed germination if exposed to paraquat during early reproductive growth stages.