MODELLING THE FARM SCALE IMPACTS OF CLOVER ROOT WEEVIL HERBIVORY

Clover root weevil ( Sitona lepidus, CrW) is a major New Zealand pasture pest. This study used computer simulation and decision support modelling to simulate CrW herbivory, estimate the long-term consequences on clover abundance, pasture production and quality, and financial implications to a typical sheep and beef farmer. Three farm scenarios were explored, the absence of C rW and the presence of C rW with and without additional nitrogen (N). For a hypothetical 325 ha Waikato sheep and beef farm, C rW decreased mean clover abundance from 21 to 13%, pasture production from 9200 to 7900 kg dM/ha/year, pasture quality from 10.5 to 10.2 MJM e/kg dM and N fixation from 60 to 42 kg N/ha/year. This resulted in a 16% reduction in the annual gross margin. however, assuming current prices and costs, and that an N response could be consistently achieved, urea could be used to replace the reduction in N fixation without affecting profits.