Inheritance of faecal egg counts during early lactation in Scottish Blackface ewes facing mixed, natural nematode infections

This paper presents an analysis of nematode faecal egg counts from Scottish Blackface ewes facing mixed, natural nematode infections (predominantly Te ladorsagia circumcincta). The data set comprised 1445 measurements on 421 e wes taken at 4 and 6 weeks post-lambing, over a 4-year period. The ewes, th emselves, were the progeny of 73 sires and 285 dams. Only Strongyle eggs we re consistently present, and faecal egg counts from this genera ranged from zero (0(.)4 of all measurements) to 3388 eggs per g. Faecal egg counts wer e significantly affected by the number of lambs reared and suckled (increas ing reproductive burden led to higher counts) by ewe age (older ewes had lo wer counts) and by previous selection history (ewes from a line previously selected for increased carcass fatness had lower faecal egg counts than ewe s from a conversely selected lean line). The heritability of log-transforme d faecal egg counts was 0(.)23 (s.e. 0(.)05) and the repeatability, includi ng both within- and between-year permanent environmental effects, was 0(.)2 5 (s.e. 0(.)04). The apparent presence or absence of infection had a herita bility of 0(.)15 (s.e. 0(.)07) on the observed scale and 0(.)39 (s.e. 0(.)1 6) when analysed as a binomial threshold trait. The genetic correlation bet ween average 4-week weight of lamb suckled by the ewe and faecal egg counts was 0(.)24 (s.e. 0(.)10). The results suggest that faecal egg counts durin g early lactation are heritable and influenced by the reproductive performa nce of the ewe. Selection to reduce nematode faecal egg output from ewes du ring this period should be successful in reducing pasture larval contaminat ion.