Direct and correlated responses to selection for high or low faecal nematode egg count in Perendale sheep

Abstract

A selection experiment with Perendale sheep was established in 1986, with lines selected solely on the basis of high or low faecal nematode egg count (FEC) in lambs after weaning. Ranking for FEC involved a natural mixed‐species challenge in all years, although in early years this was augmented by an artificial challenge with Haemonchus contortus larvae. Faecal samples were taken from each animal for FEC on two occasions, separated by an anthelmintic drench. A total of 1840 lambs were recorded for FEC in the high and low selection lines from 1986 up to the 2002‐born lamb crop. Direct responses to divergent selection for or against FEC were estimated, along with indirect responses in live weights (lambs, yearlings, and adult mixed‐age ewes), in fleece weights (yearlings and ewes) and in breech soiling ("dag") scores. Analyses of both lines across all years were carried out using animal‐model restricted maximum likelihood techniques and also fixed‐effects models. The realised heritabilities of loge(FEC + 100) at the two sampling times were 0.22 ± 0.03 and 0.16 ± 0.03; the genetic correlation estimates between loge(FEC1 + 100) and yearling live weight and fleece weight were 0.36 ± 0.17 and 0.54 ± 0.16, respectively. Averaging the breeding value results for the 1998–2002 crops, the back‐transformed FEC means averaged 556 and 114 eggs/g for the high and low lines, respectively, representing a 4.9‐fold line difference. When exposed to equal parasite challenge, lambs from the high FEC line were heavier than low‐line lambs by 6–12% (breeding ewes 8%), with all differences being significant (P < 0.001); corresponding figures for fleece weight were 24–26% (breeding ewes 15%), again with all differences significant (P< 0.001), and dag scores averaged 0.55 units higher in low‐line animals (P < 0.001). It is concluded that, under natural mixed‐species parasite challenge on pasture, small rates of genetic change for FEC were achieved in small, closed populations of Perendales. There were unfavourable correlated responses to selection to reduce FEC, comprising lower weights, reduced fleece weights, and more dags. The prospect of index selection to break the unfavourable genetic correlations with FEC is discussed.