Utilization of year-round data in the estimation of genetic parameters for internal parasite resistance traits in Dorper sheep

The objective of this study was to evaluate the effect on the estimates of heritability and permanent environmental effects as a proportion of phenotypic variance for FAMACHA score, fecal worm egg count and hematocrit value when year-round records are used. Records from 1008 Dorper sheep in a private South African flock comprised 17,711 FAMACHA scores, 6837 fecal worm egg counts (FEC; practically only Haemonchus contortus), and 4209 hematocrit (packed cell volume-PCV) values that were collected from 1997 to 2000. Animal models were used to conduct single-trait analyses. Data were analyzed in two sets: 1) warm season records only and 2) year-round records. Treatment (with anthelmintic) status was investigated as a 2-level fixed effect in both sets; additionally records of treated sheep were removed for another analysis of both data sets. In analyses of warm season records, estimates of heritability and permanent environmental variance as a proportion of phenotypic variance for FAMACHA score were 0.33 +/- 0.03 and 0.04 +/- 0.02, respectively, when treatment status was modeled, and 0.41 +/- 0.02 and 0, respectively, when treated records were excluded from the analysis. Heritability estimates for PCV were 0.22 +/- 0.06 (treatment status modeled) and 0.28 +/- 0.07 (treated records excluded), while permanent environmental variance as a proportion of phenotypic variance was, respectively, 0.13 +/- 0.05 and 0.09 +/- 0.06. Fecal worm egg count heritability estimates were 0.10 +/- 0.03 (treatment status modeled) and 0.13 +/- 0.04 (untreated records only). Permanent environmental variance for FEC was 0.04 +/- 0.03 when treated records were excluded and 0.05 +/- 0.03 when treatment status was included in the analyses. In analyses of year-round records, estimates of heritability and permanent environmental variance as a proportion of phenotypic variance for FAMACHA score were 0.32 +/- 0.03 and 0.03 +/- 0.02, respectively (treatment status modeled) and 0.36 +/- 0.03 and 0.02 +/- 0.02 (treated records excluded). Packed cell volume heritability and permanent environmental variance were, respectively, 0.19 +/- 0.05 and 0.18 +/- 0.05 in the analyses of untreated records, and 0.18 +/- 0.04 and 0.15 +/- 0.04, when treatment status was modeled. Heritability estimates for FEC were the same (0.11) for untreated records only, and when treated records were included. Permanent environmental variance was 0.04 +/- 0.02 (treated records included) and 0.03 +/- 0.02 (treated records excluded) for FEC. Collection and inclusion of cool season (that is, outside of the regular worm proliferation season) records in analyses may not substantially change estimates of genetic parameters for these traits.