Faecal nematode egg counts in lactating ewes from Romney flocks selectively bred for divergence in lamb faecal egg count

Faecal nematode egg counts (FECs) were examined in lactating ewes from divergent flocks of Romney sheep which had been selectively bred from 1979 to 1996 for or against FEC in 4 to 7 month old lambs. Faecal samples were obtained from the ewes while under normal grazing management, 1 to 2 months after lambing in spring for each of 6 years between 1987 and 1996 (no. = 785 records; 298 animals). Analyses were carried out on log(e) (FEC+ 100)-transformed data using animal-)model maximum likelihood procedures, accounting for repeated records on ewes, within and between lactations. An examination of non-genetic effects indicated that there was no significant effect of age class of ewe on FEC but ewes which gave birth to single lambs had significantly lower post-parturient FECs than those bearing twins (back-transformed means of 184 v. 276 egg per g, respectively; P < 0.001). In relation to genetic effects, post-parturient FECs were significantly lower in ewes from the flock bred for low lamb-FEC than in their counterparts from the flock bred for high lamb-FEC, with ewes from the most recent birth years (1991 to 1994) showing a nine-fold difference (back-transformed means of 33 and 305 eggs per g faeces respectively,. P < 0.001). This was equivalent to 69% of the divergence observed between log(e) (FEC + 100) in their lambs in the same years. Heritability and repeatability estimates for log(e) (FEC + 100) in ewes were 0.37 (s.e. 0.06) and 0.46 (s.e. 0.03) respectively. Genetic correlation estimates between a ewe's post-parturient log(e) (FEC + 100) and her log(e) (FEC + 100) as a lamb, based on analysis of (co)variance or realized responses, were 0.70 or 0.58 respectively. The phenotypic correlation between a ewe's log(e) (FEC + 100) and that of her Iamb(s) in the same lactation was 0.29 (s.e. 0.06) (P < 0.001). From the results it is clear that substantial genetic changes in post-parturient FECs of breeding ewes can be induced through a correlated response to selective breeding for or against reduced FEC in lambs. This may have important implications for the epidemiology of nematode parasite infections in spring-born lambs, a possibility which is currently being investigated.