"Resilience" in sheep can be defined as the ability to withstand nematode challenge, and to maintain acceptable health and productivity, with minimal reliance on anthelmintic treatment. One heritable component trait is Total Drench Requirement (TDR), which is measured in lambs managed on a regime of "drench-on-demand" (where Trichostrongylus and Ostertagia are the predominant nematode genera in New Zealand sheep). A breeding experiment was established in 1994, selecting for increased resilience in Romney lambs. Currently, breeding ewes in the experiment are subdivided between Ballantrae, where four genetically equivalent sub-groups graze separate farmlets year-round, under Non-Chemical versus Conventional management (AgResearch’s Low-Chemical Farming Systems Programme), and Wallaceville, where an Elite Resilient line was re-established in 1999 alongside the Control Faecal Egg Count (FEC) line. Selection responses up to 1999/00 in the Elite Resilient-line lambs (relative to the Control line) included a 27% greater post-weaning weight gain (GAIN: Dec. to Apr.), a 0.48 unit reduction in dags, and a 45% reduction in TDR (Dec. to Apr.). A favourable genetic correlation (-0.54) was recorded between TDR and GAIN in lambs, whilst that between TDR and log FEC was -0.17 (not significant). Mean autumn weights in lambs at Ballantrae were 5.8 kg (18%) lower in the Non-Chemical than Conventional treatment groups (P<0.01). Through intense genetic selection, 51% of Elite-line ram lambs are now resilient enough for low-chemical or organic production systems.

CA, Morris, SA Bisset, A Vlassoff, AD Mackay, K Betteridge, MJ Alderton, CJ West, and BP Devantier

Proceedings of the New Zealand Society of Animal Production, Volume 61, Christchurch, 92-95, 2001
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