Modelling the effect of nematode parasites on the liveweight gain of lambs grazing alternately with cattle

A dynamic model for nematode parasites in lambs which links their adult parasite burden with liveweight gain was extended to investigate alternate grazing with cattle to provide relatively larval-free pasture for finishing lambs. Liveweight performance of weaned lambs was tracked fortnightly under rotational grazing using three cattle:sheep ratios, with three initial parasite burdens and two initial infective larval densities on pasture. Lambs were removed from grazing at 37 kg or if they failed to gain weight during the preceding fortnight. An 80%:20% cattle:sheep ratio reduced larval pasture contamination by 75% but did not affect parasitic burden and total lamb liveweight gain compared with an all-lamb treatment. For low and moderate initial parasitic burdens in the 80%:20% cattle:sheep ratio, 25% of lambs reached 37 kg and 100% gained weight until week 18. This simulation indicated that in organic lamb finishing systems, alternate grazing with cattle provides substantial benefit but only when the initial parasite burden was low or moderate.