Genetic parameters for FAMACHA score and related traits for host resistance/resilience and production at differing severities of worm challenge in a Merino flock in South Africa.

The objectives of this study were to estimate genetic parameters for FAMACHA score and related traits at different levels of worm challenge in sheep and to assess the effect of different methods for modelling records from treated lambs on estimates of genetic parameters. Data were collected over five consecutive Haemonchus seasons from a total of l671 Merino lambs using the FAMACHA clinical evaluation system, and anaemic individuals were treated as needed, until flock health necessitated mass treatment at the peak of the worm season. Records of each sampling occasion were classified into low, moderate, or peak levels of worm challenge. Animal models were run separately for traits within each data set. Alternative analyses were conducted in which records of treated lambs were (1) included without adjustment, (2) included along with a fixed effect representing treatment status of the lamb for each record, and (3) included after application of a penalty to offset any phenotypic improvement or advantage due to that treatment. Estimates of heritability for individual FAMACHA data sets ranged from 0.06+/-0.04 to 0.24+/-0.05, the highest estimates being obtained for peak worm challenge data. Estimates of genetic correlation for FAMACHA with other traits varied, but were always near negative unity for FAMACHA score with haematocrit value. When data of treated lambs were penalised, higher estimates of heritability were obtained than when not penalised, hence this may be an effective method for allowing for early treatment of overly susceptible animals before the level of worm challenge is at an optimum level for BLUP (Best Linear Unbiased Prediction) evaluation. The estimate of genetic correlation for FAMACHA score in moderate worm challenge with that in peak worm challenge was almost unity. This suggests that estimation of breeding values for this trait using data from moderate worm challenge may be as effective as that from peak challenge.