Effect of herbicide application time on dose efficacy for weed control in maize (Zea mays)

A split-plot experiment was conducted during 2016 with 3 replications to determine the optimum time and dose of tank mixture of Nicosulfuron and Bromoxynil+MCPA for weed control in maize (Zea mays) in Karaj. Main plots were four application times (different maize growth stages: 2-3, 3-4, 4-6, and 6-8 leaves) and subplots were five doses of herbicide mixture (0, 25, 50, 75, and 100 % of the recommended dose). Doses of herbicides and weed biomass relations were described with the standard dose-response model at each application time. Parameter estimates suggested that delaying in herbicides application increased ED 50 parameter (herbicide dose required for 50 percent reduction in weed biomass) from 6 to 54. In general, parameter estimates showed that Chenopodium album was more sensitive to herbicide mixture compared to Solanum nigrum and Amaranthus blitoides. Relationship between the doses of herbicide mixture and maize yield and total dry matter were described with a linear model. Delaying in herbicides application decreased the effect of each percent of the recommended dose of herbicide mixture on increasing maize yield and total dry matter (parameter b 1 ). For example, in the case of maize forage yield, it was 0.45 at first application time and reached 0.15 at the last application time. In general delaying herbicide mixture application time to 6-8 leaf stage of maize reduced herbicide efficacy for weed control and there was a need for higher doses to achieve an acceptable yield.