Sugarcane tolerance and Italian ryegrass (Lolium multiflorum) control with paraquat

Paraquat applied from mid-February through early April over 2 yr was evaluated for sugarcane tolerance and Italian ryegrass control. Sugarcane 31 cm tall at application was injured 16 to 25% and 8 to 14% 28 and 56 d, respectively, after mid-March application of paraquat at 0.35 or 0.70 kg ai/ha. Early-April application to 61-cm-tall sugarcane caused 13 to 25% injury. The observed injury was not reflected in reduced sugarcane shoot population or height or sugarcane or sugar yield when compared with diuron, the standard herbicide treatment. Italian ryegrass control 28 d after the mid-February application of paraquat alone at 0.53 or 0.70 kg/ha was variable, ranging from 80% in 1994 to no more than 66% in 1995. For each year, diuron at 3.2 kg/ha in combination with both rates of paraquat increased Italian ryegrass control 28 d after the mid-February application 11 to 17 percentage points. At 56 d after the mid-February application, addition of diuron proved beneficial only in 1994 when the paraquat and diuron combinations controlled ryegrass 93% compared with no more than 62% for paraquat applied alone. In contrast, Italian ryegrass was controlled the second year no more than 80% 56 d after the mid-February application of paraquat alone or with diuron. Paraquat applied at 0.70 kg/ha with diuron in mid-March controlled Italian ryegrass 80 and 86% 28 d after treatment in 1994 and 1995, respectively. For the standard herbicides metribuzin, terbacil, and diuron applied in mid-March, weed control was no greater than 38%. Although differences in Italian ryegrass control among herbicide treatments were observed, efficacy was sufficient to reduce weed competition such that sugarcane growth and yield were not negatively affected.