Foramsulfuron has recently been registered for weed control in corn in Ontario, but there is very little information on the rate of foramsulfuron required to obtain at least 90% weed control. Our objective was to determine the foramsulfuron rates giving at least 90% weed control while maintaining crop yield loss due to weed interference and injury at less than 5%. Ten field trials were conducted at five Ontario locations (Exeter, Harrow, Ridgetown, Woodslee, and Woodstock) in 2001 and 2002 to evaluate the effectiveness of foramsulfuron at rates ranging from 8.75 to 140 g ai/ha. To obtain a reduction in biomass of 90% (I₉₀) at 78 d after treatment (DAT), foramsulfuron must be applied to common lambsquarters at 68 g/ha and to common ragweed at 86 g/ha, respectively. For green foxtail a foramsulfuron rate of 25 g/ha was required to achieve 90% control. The application of foramsulfuron caused injury to corn at 7 DAT at Ridgetown and Woodstock only, but did not exceed a rating of 10%; by 14 and 28 DAT no corn injury was recorded at any location. Corn yield of at least 95% of a weed-free check was obtained at Woodstock when foramsulfuron was applied at 70 g/ha. At Exeter and Woodslee yield was 90% of the weed-free check at a foramsulfuron rate of 35 g/ha. Finally, at Harrow and Ridgetown, corn yield was lowered at all foramsulfuron rates because of broadleaved weed interference. Tank-mixing foramsulfuron with dicamba plus prosulfuron improved common lambsquarters and common ragweed control and final corn yield was improved by more than 20% when compared with an application of foramsulfuron alone. Thus, these results show that weed control with foramsulfuron is species specific and that tank mixtures with a broadleaf herbicide may be required for broad-spectrum weed control and to protect the full yield potential of corn.

Nomenclature: Foramsulfuron; common ragweed, Ambrosia artemisiifolia L. AMBEL; common lambsquarters, Chenopodium album L. CHEAL; green foxtail, Setaria viridis (L.) Beauv. SETVI; corn, Zea mays L.