Modelling liveweight performance in parasitised lambs under varying grazing rotation lengths

A dynamic model for nematode parasites in lambs which links their adult worm burden with decrease in liveweight gain has recently been developed. This model allows for individual lamb variability in response to parasite infection. We use this model to track the liveweight performance of a mob of weaned lambs under six different grazing rotation length scenarios. In all scenarios, lambs with a low worm burden were weaned onto parasite-free pasture. Post-weaning, lambs were either grazed in a 12-week, 8-week, 6-week, 4-week or 2-week grazing rotation, or set stocked. Scenarios were run for 24 weeks with no anthelmintic intervention. The 12-week and 8-week grazing rotation scenarios provided lower worm burdens, higher liveweight gains and lower variation between individual lambs than the other scenarios due to the delay in grazing self-contaminated pastures. For systems that use little or no anthelmintics, being able to provide 8 weeks or more of parasite-free pasture to weaned lambs is likely to provide substantial benefits in the form of higher liveweight gains with less variation between individuals.