COMPARISON OF EMPIRICAL-MODELS DEPICTING DENSITY OF AMARANTHUS-RETROFLEXUS L AND RELATIVE LEAF-AREA AS PREDICTORS OF YIELD LOSS IN MAIZE (ZEA-MAYS L)

The outcome of crop-weed competition should be predicted as early as p ossible in order to allow time for weed control measures. Maize grain yield losses caused by interference from Amaranthus retroflexus L. (re droot pigweed) were determined in 1991 and 1992. The performance of th ree empirical models of crop-weed competition were evaluated. Damage f unctions were calculated based on the weed density or relative leaf ar ea of the weed. In the yield loss-weed density model, values of I (per centage yield loss at low weed density) were relatively stable for sim ilar emergence dates of A. retroflexus across years and locations. Est imated maximum yield loss (A) was more variable between locations and may reflect environmental variation and its effect on crop-weed compet ition, at least in 1991. The two-parameter yield loss-relative leaf ar ea model, based on m (maximum yield loss caused by weeds) and q (the r elative damage coefficient) gave a better fit than the single-paramete r version of the model (which includes only q). In both relative leaf area models, the values of q varied between years and locations. Attem pts to stabilize the value of q by using the relative growth rate of t he leaves of the crop and weed were successful; however, the practical application of such relative leaf area models may still be limited ow ing to the lack of a method to estimate leaf area index quickly and ac curately.