Selecting for resilience in Romney sheep under nematode parasite challenge, 1994–2007

Selective breeding for increased resilience to nematode parasite challenge in New Zealand Romney sheep is reported for the period 1994-2007. Resilience was defined as the age at which a first post-weaning anthelmintic treatment was required to maintain acceptable growth in lambs grazing nematode-contaminated pasture. An experimental line selected for resilience (Rsl) was established in 1994. This was expanded in 1997 for a study comparing animal performance under conventional and low-chemical management systems. An elite resilient line (ER) was re-established in 1999. Selection increased the average 'age-at-first-drench' in ER lambs by 23.6 days relative to their control line counterparts. This was accompanied by a 4.5 kg increase in 6-month live weight, and a significant reduction in breech-soiling at 5 months of age. No significant change in faecal worm egg count occurred. Although the Rsl lambs still required some anthelmintic intervention under low-chemical management, the study nevertheless provided useful insights into the potential value of resilience to the sheep industry. Some practical difficulties currently associated with on-farm selection of lambs for resilience may be overcome if appropriate selectable genetic markers or biomarkers can be identified.