RESISTANCE TO NEMATODE PARASITES IN MERINO SHEEP - SOURCES OF GENETIC-VARIATION

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), CSIRO Finewool Flock (Armidale, NSW), and the Trangie D F lock (Trangie, NSW). Faecal egg count (FEC) was used to measure relati ve resistance of sheep to nematode parasites after either natural or a rtificial infection with Haemonchus contortus and Trichostrongylus col ubriformis. Differences in FEC(0.33) between strains and between and w ithin bloodlines were examined and the heritability of this trait was estimated. A low proportion of the total variation in parasite resista nce could be attributed to strain and bloodline effects (1 and 3.5%, r espectively) after either natural or artificial infection. The major s ource of genetic variation was found within bloodlines (22.2% of total variation), with individual sires showing a wide range in parasite re sistance. 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 environme ntal effects of sex, age of dam, birth-rearing rank, and day of birth were also investigated, and were found to be only occasionally signifi cant, accounting for a small proportion (0.3-2.2%) of variation. Manag ement 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. Co rrection 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 ex ists within a wide range of Merino bloodlines, and within-flock select ion of resistant sires appears to be an effective method of improving this trait in Merino sheep.