This paper reports on backfat depth and muscling comparisons of sheep selected for nematode parasiterelated traits. Results are from two AgResearch projects, the first located at Kaitoke Farm (Upper Hutt), in which experimental Romney lines wereselectively bred for susceptibility (S) or resistance (R) to nematode infection (high or low faecal egg count), or for resilience to nematode challenge whilst maintaining acceptable growth with minimal anthelmintic treatment (RL line); these lines grazed alongside an unselected control line (C). The second project was located at Ballantrae (Northern Wairarapa), in which Romney breeding lines were selected for resilience, with two replicate farmlets under 'Conventional' parasite management (with anthelmintic treatment), and with two further replicate farmlets under 'Low-Chemical' management. The four replicate lines at Ballantrae were serviced by rams of equivalent genetics, from elite RL-line sires bred at Kaitoke or Ballantrae. Ultrasonic backfat depth and eye muscle dimensions (length, depth and area) of ewe lambs from each project were assessed at 8-9 months of age (234 C-, S-, R- and RLline lambs born in 2003 at Kaitoke, and 154 resilient lambs born in 2004 at Ballantrae). Adjustments were made for significant fixed effects including covariate adjustment for live weight. No significant differences were found among the R, C and RL lines for either trait, but the S line had 5-10% smaller eye-muscle dimensions (P < 0.05) than the other lines following an extended period of natural nematode challenge with minimal drench treatment. The two replicate lines under Low-Chemical management were 3.8 ± 0.6 kg (11.8%) lighter in May than the replicates under Conventional management (P < 0.001). After adjustment for live weight, lambs in the Conventional treatments had 15% larger eye-muscle areas (P < 0.001) but not backfat depths than those in the Low-Chemical treatments. Results suggest that muscle dimensions of the Sline lambs were more affected than those of their R-line counterparts by parasite infection, whilst the C-, R- and RL-line lambs did not differ significantly. At Ballantrae, even at equivalent live weights, the two Conventional replicates had more muscling but not fat depth than the two Low-Chemical replicates.
Proceedings of the New Zealand Society of Animal Production, Volume 67, Wanaka, 198-203, 2007
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