Imazosulfuron is a sulfonylurea herbicide recently labeled in U.S. rice at a maximum rate of 336 g ai ha⁻¹. Soybean is prone to drift of herbicides from rice fields in the southern United States because these crops are often grown in close proximity. Field trials were conducted to determine the effect of low rates of imazosulfuron applied to nonsulfonylurea-resistant soybean at different growth stages. Soybean was treated at the vegetative cotyledonary (VC); vegetative second trifoliate (V2); vegetative sixth trifoliate (V6); and reproductive full bloom (R2) growth stages with 1/256 (1.3 g ha⁻¹) to 1/4 (84.1 g ha⁻¹) times (X) the maximum labeled rate of imazosulfuron. Soybean was injured regardless of application rate or timing. At 2 wk after treatment (WAT), imazosulfuron injured soybean 23 to 79, 44 to 76, 32 to 68, and 14 to 50% when applied at the VC, V2, V6, and R2 growth stages, respectively, where the highest injury was caused by the highest imazosulfuron rate (1/4X). However, by 20 wk after planting (WAP), soybean treated with imazosulfuron at the VC and V2 growth stages had only 0 to 17% and 8 to 53% injury, respectively. At higher rates [1/8 (42 g ha⁻¹) and 1/4X] of imazosulfuron, soybean treated at the VC growth stage recovered more from injury than did soybean treated at the V2 growth stage. Soybean treated with imazosulfuron at the V6 and R2 growth stages had better recovery from the injury at the lower two rates [1/256 and 1/128X (2.6 g ha⁻¹)] than at the higher rates [1/64 (5.3 g ha⁻¹) to 1/4X]. Imazosulfuron, at all rates tested, delayed soybean maturity by 1 to 4, 2 to 6, 1 to 12, and 3 to 16 d for the VC, V2, V6, and R2 growth stages, respectively. Yield loss was greater when imazosulfuron was applied at V6 and R2 compared to applications at VC and V2. Results from this research indicate that imazosulfuron can severely injure soybean regardless of the growth stage at which drift occurs; however, soybean injured by imazosulfuron at early growth stages (VC and V2) has a better chance of recovery over time compared to drift at later growth stages (V6 and R2).

Nomenclature: Imazosulfuron; rice; Oryza sativa L.; soybean; Glycine max (L.) Merr.

Imazosulfuron es un herbicida del grupo sulfonylurea que fue recientemente registrado para uso en arroz en Estados Unidos a una dosis máxima de 336 g ai ha⁻¹. La soya tiende a estar expuesta a deriva de herbicidas provenientes de campos de arroz en el sur de los Estados Unidos porque estos cultivos se encuentran generalmente cerca. Se realizaron experimentos de campo para determinar el efecto de dosis bajas de imazosulfuron aplicado a soya no-resistente a sulfonylurea en diferentes estadios de desarrollo. La soya fue tratada en los estadios de desarrollo vegetativo-cotiledonario (VC), vegetativo-segunda hoja trifoliada (V2), vegetativo-sexta hoja trifoliada (V6), y reproductivo-floración completa (R2) con 1/256 (1.3 g ha⁻¹) a 1/4 (84.1 g ha⁻¹) veces (X) la dosis máxima de imazosulfuron según la etiqueta. La soya sufrió daño sin importar la dosis o el momento de aplicación. A 2 semanas después del tratamiento (WAT), imazosulfuron dañó la soya 23 a 79, 44 a 76, 32 a 68, y 14 a 50% cuando se aplicó en los estadios VC, V2, V6, y R2, respectivamente, y en donde el mayor daño fue causado por la dosis mayor (1/4X) de imazosulfuron. Sin embargo, a 20 semanas después de la siembra (WAP), la soya tratada con imazosulfuron en los estadios VC y V2 tuvieron solamente 0 a 17% y 8 a 53% de daño, respectivamente. Con las dosis más altas de imazosulfuron [1/8 (42 g ha⁻¹) and 1/4X], la soya tratada en el estadio VC se recuperó más del daño que la soya tratada en el estadio V2. La soya tratada con imazosulfuron en los estadios V6 y