Continued selection of Romney sheep for resistance or susceptibility to nematode infection: estimates of direct and correlated responses

Divergent breeding lines of Romney sheep, selected as lambs for consistently high or low faecal worm egg count (FEC) following natural multi-species challenge by nematode parasites, were established in New Zealand at Wallaceville Animal Research Centre in 1979 and at Rotomahana Station in 1985. In 1988 the Rotomahana lines, including an unselected control line maintained tinder the same management conditions, were transferred to Tokanui Station where they remained for 4 years. In 1993 elite high and low FEC animals from Tokanui, along with the controls, were transferred to Wallaceville, where merged lines have since been managed together. Selection responses from the lines at Rotomahana and Tokanui, and from a further 5 years of divergent selection in the merged lines, are reported here. For the two most recent lamb crops (1996 and 1997 birth years), log-transformed FECs of the high and low lines were 1.27 and -1.46 phenotypic standard deviation units from the control. After backtransformation to the original scale, where the FEC for control line lambs averaged 1255 eggs per g, the means for the high and low lines were 3.05 and 0.27 times the control mean. Animal-model restricted maximum likelihood estimates of heritability and repeatability for single-record FEC (following separate infections) were 0.28 (s.e. 0.02) and 0.42 (s.e. 0.01), respectively. Correlated responses in production traits include significantly decreased post weaning weight gain and increased dags (breech soiling) in lambs, and decreased fleece weight in yearlings and ewes in the low FEC line, compared with those in the high line. However the low FEC line had proportionally 0.11 more lambs weaned per ewe mated than the high FEC line (P < 0.01). It is concluded firstly that selection for high or low FEC in Romneys has achieved an 11-fold difference between the divergent lines. Secondly, it will generally be necessary in a commercial environment to apply index selection for a combination of increased productivity, decreased FEC and possibly decreased dags, when potential candidates are recorded under conditions of nematode challenge.