A simulation model for mixed infections of gastrointestinal nematodes in lambs was used to compare with a series of extended drenching programmes. The model output concurs with the results of field trials showing that preventive drenching will control parasites adequately except when autumn conditions are particularly favourable for development and migration of larvae onto pasture. The model suggests that the level of pasture contamination in spring will have a significant influence on parasite control throughout much of the year. A reduction in pasture contamination was simulated by the addition of a post-lambing ewe drench prior to the preventive lamb drenching programme. This gave better parasite control and increased production compared with extended drenching of lambs in the autumn. This is likely to be the result of reduced larval challenge. These model predictions will require validation in the field.
Proceedings of the New Zealand Society of Animal Production, Volume 50, , 181-190, 1990
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