Modelling glyphosate use in roundup ready soybean

Modelling glyphosate use in Roundup Ready soybean (199) IVAN SARTORATO1, ANTONIO BERTI2, GIUSEPPE ZANIN1,2 1Research Centre for Weed Biology and Control, Legnaro, Italy; sartorato@pdadr1.pd.cnr.it;2Dept. of Environmental Agronomy and Crop Production, Univ. of Padova, Italy Crop yield can be expressed as a function of the maximum yield of the crop kept weed-free, the weed competitive load and time of weed emergence and removal. These factors determine the variation over time of the financial advantage of a herbicide application. Knowing the weed population and its emergence pattern, an optimum application time that maximises net return can be identified. With herbicide resistant crops, this aspect is of particular interest because the focus shifts from herbicide choice to the identification of the best time window for spraying the crop. The relationships between Roundup Ready soybean yield and time of weed control have been studied at the Experimental Farm of the University of Padova (Italy) over three years. Natural weed populations were treated at 7 different times, from 2 to 49 days after soybean emergence, with glyphosate (2 L ha-1 of Roundup Bioflow). Each year the weed emergence evolution was assessed and yields measured. The three years were well differentiated both in terms of infestation levels and weed emergence patterns; the final weed densities in untreated plots were 247, 131 and 423 plants m-2 in 1997, 1998 and 1999 respectively, and the percentage of weeds emerged at soybean emergence were 61%, 17% and 75%. Crop yield loss as a function of time of weed emergence and removal, as well as of total weed density, was simulated with a bioeconomic model developed at our Centre. Despite some discrepancies, ascribable to differences in the weed flora composition in the three years, the model gave an acceptable description of the observed yield losses. The model indicates a good flexibility of glyphosate use; in spite of different weed emergence dynamics soybean yield is not affected if glyphosate is applied from 50-80 to 250-300 degree-days after crop emergence, corresponding to a time window of about 20 days.