AN ECOPHYSIOLOGICAL MODEL FOR INTERSPECIFIC COMPETITION, APPLIED TO THE INFLUENCE OF CHENOPODIUM-ALBUM L ON SUGAR-BEET .2. MODEL EVALUATION

A simulation model for crop-weed competition, parameterized for sugar beet and Chenopodium album L., was validated with experimental data from five field experiments. Between the experiments, 98% of the variation in yield loss, ranging from -6 to 96%, was explained by the simulation model. After validation, the model was used to analyse the backgrounds of the distinct differences in yield loss between the experiments. The contribution of differences in water shortage appeared to be negligible. The number of days between crop and weed emergence, which ranged from 0-31 days, appeared to be the main factor responsible for the differences in yield loss between the experiments (96%). Further analysis showed that water shortage only influences competitive strength of the weeds when overtopped by the crop. Temperature in the period between crop and weed emergence appeared to be an important factor determining the competitive relationships. It is concluded that the period between crop and weed emergence should be expressed as a developmental measure (i.e. degree days) instead of days. Morphological characteristics such as the relative growth rate of leaf area in early growth phases, specific leaf area and height increase largely determined the competitive strength of a species. The effect of physiological characteristics, such as maximum rate of leaf photosynthesis, was less significant.