Prediction of corn (Zea mays) yield loss from early observations of the relative leaf area and the relative leaf cover of weeds

The relative leaf area of weeds is a good predictor of the outcome of weed- crop competition. However, this variable has not been used in decision-maki ng tools for integrated weed management because leaf area cannot be measure d quickly. A powerful image analysis system for measuring leaf cover (the v ertical projection of plant canopy on the ground) has been developed and va lidated. This research was conducted to compare the efficiency of weed rela tive leaf area and relative leaf cover in predicting corn yield loss. Field studies were conducted in 1996 and 1997 using varying densities of common lambsquarters, barnyardgrass, common lambsquarters plus barnyardgrass, and a natural weed community. Corn grain yield and biomass loss varied with wee d infestation type and year. Values of the relative damage coefficient of w eeds (q) were smaller in 1997 compared with 1996. For both years, the relat ive leaf area of weeds was an adequate predictor of corn yield loss (r(2) v aried from 0.61 to 0.92). The precision of the predictions was not influenc ed by the leaf area sampling period (four- or eight-leaf stage of corn). In general, smaller values of q and m (predicted maximum yield loss) were obt ained as a consequence of using the relative leaf cover of weeds in model f itting. However, percentages of variation explained by the model (from 0.67 to 0.90) were similar to values obtained with the relative leaf area. On t he basis of the residual mean squares, neither of the variables could be de clared superior to the other in yield loss prediction. The development of w eed control decision-making tools using the relative leaf cover of weeds ma y require improvements prior to being used in weed management systems. Such improvements would include use of appropriate sampling and image-processin g techniques, development and validation of empirical models specific to in dividual situations, and proper identification of the crop growth stage at which leaf cover must be assessed.