Validation of a weed:crop competition model

A model of crop:weed competition, based on parameters from the logistic biomass growth of both crop and weeds in monocultures, was validated under several sets of conditions. Model inputs were (i) dry matter per unit area at 50% of seedling emergence ( B_i ) for each species; (ii) maximum total biomass of each species in monocultures ( K ); (iii) intrinsic growth rate of each species in monocultures ( r ); (iv) the accumulated thermal time required for each species to reach 50% emergence; and (v) the total accumulated thermal time during the growing season considered in the simulation. The model adequately described the dynamics of the competition between species when the relative yield total of the mixture was close to one, i.e. the slope and intercept of the regression between observed and simulated values was not significantly different from one and zero respectively. Sensitivity analysis indicated that final crop biomass was particularly affected by changes in the relative growth rate of the species. The occurrence of a critical period for weed control was explored using the model. The simplicity of the model validated here would allow its use as a tool to predict the outcome of competition and species relative importance, particularly when parameters needed to run more complex models are not available.