Resistance to nematode parasites in Merino sheep: correlation with production traits

Merino sheep representing a range of bloodlines in resource flocks located across Australia were tested for resistance to gastro-intestinal nematodes. These flocks included the JB Pye Flock (Camden, NSW), Katanning Base Flock (Katanning, WA), Turretfield Merino Resource Flock (Rosedale, SA), and the CSIRO Finewool Flock (Armidale, NSW) and included a total of 328 sire groups. Resistance to nematodes was measured by faecal egg count (FEC). Data were also available for greasy and clean fleece weight (GFW and CFW, respectively), fibre diameter (FD), and body weight (BW) at a range of ages from weaning to 21 months. Variance components were estimated by restricted maximum likelihood, fitting an animal model and estimating covariances in a series of bivariate analyses. Phenotypic correlations between FEC0.33 and production traits were all close to zero (-0.09-0.02). Genetic correlations between FEC0.33 and production traits were -0.20, -0.18, and -0.26 for weaning weight, 10-month BW, and 16-month BW, respectively; 0.21, -0.06, and 0.21 for 10-month GFW, 16-month GFW, and 21-month GFW; 0.21, -0.05, and 0.07 for 10-month CFW, 16-month CFW, and 21-month CFW; and -0.09, -0.12, and 0.04 for 10-month FD, 16-month FD, and 21-month FD. When estimates were pooled for all fleece traits and all BW traits, the genetic correlations between FEC0.33 and GFW, CFW, FD; and BW were 0.15, 0.10, -0.06, and -0.21, respectively. Using pooled estimates for CFW, FD, and BW, selection for a breeding objective based on production traits alone would lead to an unfavourable correlated response in FEC0.33 Of approximately 1% per year.