Genetic parameters for traits associated with resistance to Haemonchus contortus in a South African Dohne Merino sheep flock

Abstract

Abstract Data collected from 2011 until 2017 on a South African Dohne Merino flock were used for the estimation of genetic parameters for parasite resistance indicator traits. Nine to twelve recordings of faecal egg count (FEC), Famacha© score (FAM) and body condition score (BCS) were done annually from January (after weaning) until the end of June on all the lambs in the flock. Monthly body weight, as well as fleece traits at selection age (14 months of age), were also recorded. Various animal models were applied to estimate genetic parameters for and correlations among the resistance traits (FAM, BCS, FEC), as well as correlations between the resistance and production traits. Moderate univariate model heritabilities of 0.29\u2009±\u20090.05, 0.29\u2009±\u20090.05 and 0.26\u2009±\u20090.05 were estimated for FAM, BCS and log transformed faecal egg count (LFEC) respectively, when all data recordings were included. Heritabilities obtained for FAM, BCS and LFEC averaged for the 1\u2009st, 6th and 9th recordings were much lower than the heritabilities obtained under univariate analyses where all available data were included. Considering the variance components and genetic parameters for the resistance traits over 1 to 12 recordings, it is evident that direct heritability increased with 10% to 15% for all the traits when data from all 12 available recordings were included. Direct heritabilities estimated with repeatability animal models for all the traits were lower than those obtained with the univariate models. FAM had a high genetic correlation and moderate phenotypic correlation with LFEC, while body weight, fleece weight and the coefficient of variation of fibre diameter had favourable genetic correlations with FAM. BCS had a moderate genetic correlation with LFEC and significant favourable genetic correlations with body weight, fleece weight and coefficient of variation of fibre diameter. Body weight, fleece weight and coefficient of variation of fibre diameter had favourable genetic correlations with LFEC, while fibre diameter and staple length was unfavourably correlated with LFEC. Standard errors of these correlations were, however, high and more data need to be collected to confirm these correlations. The genetic parameters estimated for this flock will be used to design the most suitable protocol incorporating FAM, BCS and FEC for the selection of sires and dams resistant to H. contortus.