GENETIC-PARAMETERS FOR FECAL EGG COUNT FOLLOWING MIXED, NATURAL, PREDOMINANTLY OSTERTAGIA-CIRCUMCINCTA INFECTION AND RELATIONSHIPS WITH LIVE WEIGHT IN YOUNG LAMBS

Faecal egg counts and live weights were measured on approximately 200 predominantly twin-born Scottish Blackface lambs each year for 3 years , from 2 to 6 months of age. Measurements were made at 4-week interval s following anthelmintic treatment. Heritability estimates (with s.e.s ) of log transformed faecal egg count at each age were 0.01, 0.00, 0.1 2 (0.10), 0.14 (0.12), 0.15 (0.07) and 0.22 (0.13), for ages 1 to 6 mo nths respectively. Therefore, genetic variation exists for acquired bu t apparently not for innate resistance to infection. Maternal common e nvironmental effects (with s.e.s) were 0.36 (0.11), 0.20 (0.05), 0.27 (0.09), 0.06 (0.08), 0.15 (0.09) and 0.16 (0.08), for ages 2 to 6 mont hs respectively. Genetic correlations between faecal egg counts in lam bs older than 3 months were not significantly less than 1.0, indicatin g that faecal egg counts at different ages are expressions of the same trait. Phenotypic correlations between faecal egg counts were general ly positive but small. Measurement error contributed one-third of the observed variation for individual egg counts. The heritability of mean faecal egg count from 3 to 6 months was 0.33 (s.e. 0.15), indicating that selection decisions can be made more accurately using multiple eg g counts per animal. Phenotypic correlations between faecal egg counts and live weight were generally negative but close to zero. However, g enetic correlations between faecal egg counts and live weight in lambs older than 3 months were close to -1.0, indicating that resistance to gastrointestinal parasites may be an important genetic determinant of growth rate in this environment.