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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.

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.

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.

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.

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.

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.

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), CSIRO Finewool Flock (Armidale, NSW), and the Trangie D Flock (Trangie, NSW). Faecal egg count (FEC) was used to measure relative resistance of sheep to nematode parasites after either natural or artificial infection with Haemonchus contortus and Trichostrongylus colubriformis. Differences in FEC 0' 33 between strains and between and within bloodlines were examined and the heritability of this trait was estimated. A low proportion of the total variation in parasite resistance could be attributed to strain and bloodline effects (1 and 3.5%, respectively) after either natural or artificial infection. The major source of genetic variation was found within bloodlines (22.2% of total variation), with individual sires showing a wide range in parasite resistance. Paternal half-sib heritability estimates for FEC 0' 33 were significant (P < 0.05) in 9 of the 11 analyses and ranged from 0.07 to 0.42, with a weighted average of 0.22. The influence of the environmental effects of sex, age of dam, birth-rearing rank, and day of birth were also investigated, and were found to be only occasionally significant, accounting for a small proportion (0.3-2.2%) of variation. Management group effects both prior to and at the time of measurement were often significant, and accounted for 2.2-19.4% of variation in FEC. Correction of FEC for effects other than management group would seem to add little to precision of selection. These results have demonstrated that significant genetic variation for nematode parasite resistance exists within a wide range of Merino bloodlines, and within-flock selection of resistant sires appears to be an effective method of improving this trait in Merino sheep.

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), CSIRO Finewool Flock (Armidale, NSW), and the Trangie D Flock (Trangie, NSW). Faecal egg count (FEC) was used to measure relative resistance of sheep to nematode parasites after either natural or artificial infection with Haemonchus contortus and Trichostrongylus colubriformis. Differences in FEC 0' 33 between strains and between and within bloodlines were examined and the heritability of this trait was estimated. A low proportion of the total variation in parasite resistance could be attributed to strain and bloodline effects (1 and 3.5%, respectively) after either natural or artificial infection. The major source of genetic variation was found within bloodlines (22.2% of total variation), with individual sires showing a wide range in parasite resistance. Paternal half-sib heritability estimates for FEC 0' 33 were significant (P < 0.05) in 9 of the 11 analyses and ranged from 0.07 to 0.42, with a weighted average of 0.22. The influence of the environmental effects of sex, age of dam, birth-rearing rank, and day of birth were also investigated, and were found to be only occasionally significant, accounting for a small proportion (0.3-2.2%) of variation. Management group effects both prior to and at the time of measurement were often significant, and accounted for 2.2-19.4% of variation in FEC. Correction of FEC for effects other than management group would seem to add little to precision of selection. These results have demonstrated that significant genetic variation for nematode parasite resistance exists within a wide range of Merino bloodlines, and within-flock selection of resistant sires appears to be an effective method of improving this trait in Merino sheep.

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), CSIRO Finewool Flock (Armidale, NSW), and the Trangie D Flock (Trangie, NSW). Faecal egg count (FEC) was used to measure relative resistance of sheep to nematode parasites after either natural or artificial infection with Haemonchus contortus and Trichostrongylus colubriformis. Differences in FEC 0' 33 between strains and between and within bloodlines were examined and the heritability of this trait was estimated. A low proportion of the total variation in parasite resistance could be attributed to strain and bloodline effects (1 and 3.5%, respectively) after either natural or artificial infection. The major source of genetic variation was found within bloodlines (22.2% of total variation), with individual sires showing a wide range in parasite resistance. Paternal half-sib heritability estimates for FEC 0' 33 were significant (P < 0.05) in 9 of the 11 analyses and ranged from 0.07 to 0.42, with a weighted average of 0.22. The influence of the environmental effects of sex, age of dam, birth-rearing rank, and day of birth were also investigated, and were found to be only occasionally significant, accounting for a small proportion (0.3-2.2%) of variation. Management group effects both prior to and at the time of measurement were often significant, and accounted for 2.2-19.4% of variation in FEC. Correction of FEC for effects other than management group would seem to add little to precision of selection. These results have demonstrated that significant genetic variation for nematode parasite resistance exists within a wide range of Merino bloodlines, and within-flock selection of resistant sires appears to be an effective method of improving this trait in Merino sheep.

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), CSIRO Finewool Flock (Armidale, NSW), and the Trangie D Flock (Trangie, NSW). Faecal egg count (FEC) was used to measure relative resistance of sheep to nematode parasites after either natural or artificial infection with Haemonchus contortus and Trichostrongylus colubriformis. Differences in FEC 0' 33 between strains and between and within bloodlines were examined and the heritability of this trait was estimated. A low proportion of the total variation in parasite resistance could be attributed to strain and bloodline effects (1 and 3.5%, respectively) after either natural or artificial infection. The major source of genetic variation was found within bloodlines (22.2% of total variation), with individual sires showing a wide range in parasite resistance. Paternal half-sib heritability estimates for FEC 0' 33 were significant (P < 0.05) in 9 of the 11 analyses and ranged from 0.07 to 0.42, with a weighted average of 0.22. The influence of the environmental effects of sex, age of dam, birth-rearing rank, and day of birth were also investigated, and were found to be only occasionally significant, accounting for a small proportion (0.3-2.2%) of variation. Management group effects both prior to and at the time of measurement were often significant, and accounted for 2.2-19.4% of variation in FEC. Correction of FEC for effects other than management group would seem to add little to precision of selection. These results have demonstrated that significant genetic variation for nematode parasite resistance exists within a wide range of Merino bloodlines, and within-flock selection of resistant sires appears to be an effective method of improving this trait in Merino sheep.

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), CSIRO Finewool Flock (Armidale, NSW), and the Trangie D Flock (Trangie, NSW). Faecal egg count (FEC) was used to measure relative resistance of sheep to nematode parasites after either natural or artificial infection with Haemonchus contortus and Trichostrongylus colubriformis. Differences in FEC 0' 33 between strains and between and within bloodlines were examined and the heritability of this trait was estimated. A low proportion of the total variation in parasite resistance could be attributed to strain and bloodline effects (1 and 3.5%, respectively) after either natural or artificial infection. The major source of genetic variation was found within bloodlines (22.2% of total variation), with individual sires showing a wide range in parasite resistance. Paternal half-sib heritability estimates for FEC 0' 33 were significant (P < 0.05) in 9 of the 11 analyses and ranged from 0.07 to 0.42, with a weighted average of 0.22. The influence of the environmental effects of sex, age of dam, birth-rearing rank, and day of birth were also investigated, and were found to be only occasionally significant, accounting for a small proportion (0.3-2.2%) of variation. Management group effects both prior to and at the time of measurement were often significant, and accounted for 2.2-19.4% of variation in FEC. Correction of FEC for effects other than management group would seem to add little to precision of selection. These results have demonstrated that significant genetic variation for nematode parasite resistance exists within a wide range of Merino bloodlines, and within-flock selection of resistant sires appears to be an effective method of improving this trait in Merino sheep.