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An animal model was used to estimate heritabilities of faecal egg count (FEC) and packed cell volume decline (PCVD) in lines of Merinos selected for divergent levels of resistance to Haemonchus contortus. The estimated heritability of FEC was 0-23 (s.e. 0-03) and of PCVD was 0-21 (s.e. 0-03). A cube root transformation was found to be effective in normalizing FEC data and reducing the range of within selection line-birth year variances from 118fold to 10-fold. Transforming FEC data increased the heritability estimate to 0-29 (s.e. 0-03) and reduced selection bias due to heterogeneity of variance, but the further step of pre-adjusting the transformed data to a common variance had only a trivial effect. Heritability estimates for FEC in years with high means and variances were similar, as were estimates for transformed FEC. The estimated genetic correlations between measurements in years with high and low means and variabilities were 0-77 to 0-91.PCVD and pre-adjusted transformed FEC had a phenotypic correlation of 0-48. Estimates for the genetic correlation ranged from 0-76 in the decreased resistance line to 1-00 in the increased resistance line and 0-87 with pooled data. Other effects on resistance included birth type and dam age, with twins and progeny of maiden dams being more resistant than their cohorts. Sex effects were unimportant for FEC but males had higher PCVD than females in most, but not all, years. Younger animals had higher FEC and PCVD than older animals. Maternal genetic effects were found to be unimportant, as were the effects of low levels of inbreeding.

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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.