Genetic parameters for nematode resistance in periparturient ewes and post-weaning lambs in Uruguayan Merino sheep

Abstract

Gastrointestinal (GI) parasites are one of the main sanitary and economic constraints for sheep production worldwide. In Uruguay, resistance to GI parasites has been included in genetic evaluations of Merino and Corriedale breeds since 1994, using Faecal worm Egg Count measured in post-weaning lambs (lambFEC) as a selection criterion. Although adult categories are more resistant to GI parasites, a temporary loss of acquired immunity is present in the periparturient period (=periparturient rise). The purpose of the present study is to estimate the genetic parameters of resistance to GI parasites in periparturient ewes and post-weaning Merino lambs. A total 2110 faecal samples of 748 periparturient ewes (eweFEC), the progeny of 107 sires, were collected in 2009 and 2010. 9458 lambFEC records from 7506 lambs born between 2001 and 2009 were analysed as well. (Co)variance components and systematic effects were estimated using a multi-trait animal model, with a Bayesian analysis using the Gibbs sampler algorithm. Direct and correlated responses (DR and CR) of eweFEC to selection using eweFEC and lambFEC respectively, were also estimated. Posterior medians (posterior standard deviation) for heritability (h2) and repeatability were 0.25 (0.03) and 0.34 (0.02) for lambFEC, and 0.08 (0.03) and 0.18 (0.03) for eweFEC, respectively. Posterior median for genetic correlation between both traits was 0.81 (0.11). CR obtained by selecting for lambFEC was two times more efficient than DR from selection by eweFEC. In conclusion, eweFEC has a lower h2 than lambFEC while their genetic correlation is moderate to high. Therefore, indirect selection by using lambFEC will be more effective than direct selection on eweFEC, and will produce ewes that eliminate less worm eggs, resulting in less pasture contamination.