RESISTANCE TO NEMATODE PARASITES IN MERINO SHEEP - CORRELATION WITH PRODUCTION TRAITS

Merino sheep representing a range of bloodlines in resource flocks loc ated across Australia were tested for resistance to gastro-intestinal nematodes. These flocks included the JB Pye Flock (Camden, NSW), Katan ning Base Flock (Katanning, WA), Turretfield Merino Resource Flock (Ro sedale, SA), and the CSIRO Finewool Flock (Armidale, NSW) and included a total of 328 sire groups. Resistance to nematodes was measured by f aecal egg count (FEC). Data were also available for greasy and clean f leece weight (GFW and CFW, respectively), fibre diameter (FD), and bod y weight (BW) at a range of ages from weaning to 21 months. Variance c omponents were estimated by restricted maximum likelihood, fitting an animal model and estimating covariances in a series of bivariate analy ses. Phenotypic correlations between FEC0.33 and production traits wer e 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 fo r 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, an d 0.04 for 10-month FD, 16-month FD, and 21-month FD. When estimates w ere pooled for all fleece traits and all BW traits, the genetic correl ations 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 w ould lead to an unfavourable correlated response in FEC0.33 Of approxi mately 1% per year.