FECAL NEMATODE EGG COUNTS IN LACTATING EWES FROM ROMNEY FLOCKS SELECTIVELY BRED FOR DIVERGENCE IN LAMB FECAL 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 sam ples were obtained from the ewes while under normal grazing management , 1 to 2 months after lambing in spring for each of 6 years between 19 87 and 1996 (no. = 785 records; 298 animals). Analyses were carried ou t on log(e) (FEC+ 100)-transformed data using animal-)model maximum li kelihood procedures, accounting for repeated records on ewes, within a nd between lactations. An examination of non-genetic effects indicated that there was no significant effect of age class of ewe on FEC but e wes which gave birth to single lambs had significantly lower post-part urient FECs than those bearing twins (back-transformed means of 184 v. 276 egg per g, respectively; P < 0.001). In relation to genetic effec ts, post-parturient FECs were significantly lower in ewes from the flo ck bred for low lamb-FEC than in their counterparts from the flock bre d 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 equivale nt to 69% of the divergence observed between log(e) (FEC + 100) in the ir lambs in the same years. Heritability and repeatability estimates f or log(e) (FEC + 100) in ewes were 0.37 (s.e. 0.06) and 0.46 (s.e. 0.0 3) respectively. Genetic correlation estimates between a ewe's post-pa rturient log(e) (FEC + 100) and her log(e) (FEC + 100) as a lamb, base d 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 throu gh a correlated response to selective breeding for or against reduced FEC in lambs. This may have important implications for the epidemiolog y of nematode parasite infections in spring-born lambs, a possibility which is currently being investigated.