SELECTIVITY AND EFFICACY OF POST EMERGENCE HERBICIDE ON FORAGE SORGHUM

The objective of this study was to determine the selectivity of Primestra SC(200 g L-1 of atrazine + 300 g L-1 of metolachlor) on forage sorghum and the efficacy to controlweeds at different rates. The treatments were five rates of Primestra SC (1,5; 3,0; 6,0; and 12,0 L ha-1) with or without Assist (mineral oil) at 1,5 L ha-1, Gesaprim 500 (500 g L-1 ofatrazine) at 4,8 L ha-1, Primóleo (400 g L-1 of atrazine + 300 g L-1 of mineral oil) at 6,0 L ha-1,weedy check plot, and check plot without weed. The herbicides were sprayed 11 days aftersorghum (AG 2002) emergence when broadleaf weeds had two to four leaves and grassesone tiller. Symptoms recovery of herbicide toxicity were detected even when higher rates wereapplied. Fresh weight reduction of 24% was observed in the weedy check plot compared tothe check plot without weed, however dry matter did not show significantly reduction. PrimestraSC at rates higher than 6,0 L ha-1 provided excellent broadleaf weed control (98%). PrimestraSC controled efficiently grasses (94%) at 12 L ha-1. Gesaprim 500 and Primóleo reduceddensity and biomass of broadleaf weeds although no significant differences were detectedbetween the herbicides. Primóleo obtained better grass weed control than Gesaprim 500.Grass weed and total weed control did not differ significantly between Primestra SC andPrimóleo. The addition of Assist to Primestra SC reduced the efficacy of broadleaf weedcontrol indicating a possible antagonism. None of the evaluated herbicides showed efficientgrass weed control except for Primestra SC at 12,0 L ha-1 (94%).