Effect of reduced herbicide rates on weed control, environmental impact and profitability of corn

A study was conducted over a 3-yr period (2003, 2004, and 2005) to evaluate the effect of reduced herbicide rates - 20, 40, 60, 80, and 100% of the manufacturer’s recommended rate (MRR) - on weed biomass reduction, environmental impact (EI), yield, and profitability of corn (Zea mays L.) in Ontario. The herbicide rate required to provide 95% biomass reduction of velvetleaf (Abutilon theophrasti Medic.), redroot pigweed (Amaranthus retroflexus L.), common ragweed (Ambrosia artemisiifolia L.), common lambsquarters (Chenopodium album L.), and annual grasses was 92, 30, 41, 28, and 83% of the MRR for isoxaflutole plus atrazine, >200, 119, 23, 23, and 117% of the MRR for dimethenamid plus dicamba/atrazine, 141, 72, 46, 45, and >200% of the MRR for glufosinate plus atrazine, and 81, 29, 18, 24, and 88% of the MRR for nicosulfuron/rimsulfuron plus dicamba/diflufenzopyr, respectively. The herbicide rate required to provide 95% of weed-free corn yield was 61, 22, 130, and 11% of the MRR for isoxaflutole plus atrazine, dimethenamid plus dicamba/atrazine, glufosinate plus atrazine, and nicosulfuron/rimsulfuron plus dicamba/diflufenzopyr, respectively. Nicosulfuron/rimsulfuron plus dicamba/diflufenzopyr had the lowest EI. The results of profitability analysis suggested that the MRR rates do not tend to maximize profit margins. In most cases, there were no significant differences in profit margins for treatments with 40, 60, 80, and 100% of the MRR. Key words: Atrazine, dicamba, diflufenzopyr, dimethenamid, glufosinate, nicosulfuron, rimsulfuron, Zea mays L.