LAUSR

The hypothesis tested in this experiment was that the effects of gastrointestinal nematodes (GIN) on growth of meat-breed lambs would be absent during the suckling period and greatest after weaning. The experiment was a single factor design conducted on five farms over 2012–2014, using meat-breed lambs (Poll Dorset × Border Leicester/Merino), which were either GIN-suppressed (SUP) or not (NSUP). The experiment commenced at lamb marking and continued for 5 months. SUP lambs were serially treated with short- and long-acting anthelmintics. NSUP lambs were not treated before weaning and at weaning received a short-acting multi-active drench with subsequent treatments varying between farms. Worm egg count (WEC) of NSUP lambs were significantly higher (P < 0.0001) than SUP lambs at weaning (1170 and 10 epg) and thereafter (P < 0.0001) reaching 920 epg 2 months after weaning. Haemonchus contortus was the predominant species present in NSUP lambs with smaller contributions from Trichostrongylus spp. There was no effect of GIN control on lamb growth during the pre-weaning period (SUP 17.2 kg vs NSUP 17.0 kg, P = 0.093), whereas GIN reduced growth of lambs by 0.5 kg (SUP 7.1 kg vs NSUP 6.6 kg, P = 0.002) in the last month of observation. There was no effect of GIN control on lamb mortality. GIN did not affect growth of suckling meat-breed lambs despite high WEC at weaning indicating resilience to GIN infection in lambs growing at 200 g/day. In contrast, post-weaning growth was reduced by GIN infection, despite effective treatment at weaning and movement to GIN ‘safe’ pastures. The susceptibility of weaned lambs to the effects of GIN infection highlights the importance of effective control measures at this time.