Correlated responses following genetic selection to change faecal egg count in Romney

Genetic studies of nematode parasite-related traits were carried out in New Zealand Romney sheep between 1979 and 2006. Breeding lines were selected for high or low faecal worm egg count (FEC), and managed alongside a control unselected line. FEC data were collected from lambs post-weaning, whilst exposed to continuous, natural, mixed-species, nematode parasite challenge under grazing conditions. Overall there were 33,314 records of lamb FEC, taken at ‘FEC1’ and ‘FEC2’ (first and second post-weaning sampling times, separated by a drench treatment). Genetic parameter estimates are reviewed, including correlations of FEC with lamb, anti-parasite antibody and ewe performance traits. The heritability of FEC in lambs was 0.26 ± 0.01 overall, with a repeatability of 0.40 ± 0.01. The genetic correlation between FEC1 and FEC2 was 0.85 ± 0.02. Trends were for low FEC to be associated genetically with slight reductions in post-weaning growth, and in ewe and lamb fleece weights, minimal change in autumn and yearling live weights, and increased weaning weight, dag score and anti-parasite antibody levels. In practice, ram breeders would normally use index selection procedures, to achieve higher production at the same time as a lower FEC, although a slower rate of change in FEC would be expected.