Genetic parameters for production traits, resistance and resilience to Nematode parasites under different worm burden...

Characterized as the most important gastrointestinal parasite (GIP) in the world, Haemonchus contortus is also the most predominant and pathogenic genus in Uruguay, causing high morbidity and mortality rates in sheep flocks. For approximately 26 years resistance to GIP has been included in the National Genetic Evaluation (NGE) of Corriedale and Merino breeds, using Faecal Egg Count (FEC) as the selection criterion. The aim of the present study was to estimate genetic parameters for FEC, post-weaning Body Weight (BW), Packed Cell Volume (PCV), FAMACHA© and Body Condition Score (BCS), to determine the possibility to include these traits in the NGE of GIP resistance and resilience, as complementary selection criteria and to investigate the sensitivity of breeding values to different environmental worm burden challenges. Data were collected on 19,510 lambs from 2000 to 2009 in 24 Corriedale studs that participated in the NGE of sheep in Uruguay. (Co)variances components were estimated using a multi-trait animal model performing a Bayesian analysis. Posterior means estimated for heritability (h²) were of low to moderate magnitude (between 0.10 and 0.33) for the traits analysed. Genetic correlations (r_g) were not significantly different from zero for BW- Log_e (FEC + 100), BW-FAMACHA©, BW-PCV, Log_e (FEC + 100)-PCV, BCS-FAMACHA©, moderate favourable r_g for Log_e (FEC + 100)-FAMACHA and PCV-BCS (0.55, 0.41) and high favourable r_g for BW-BCS (0.66). These results indicate the possibility to complement selection programs incorporating BW, Log_e (FEC + 100), FAMACHA©, PCV and BCS in order to select resistant and resilient animal to GIP. Furthermore, genetic parameters for BW and Log_e (FEC + 100) to different environmental worm burden challenge was investigated. Two contemporary groups of low worm environment (Log_e (FEC + 100) below 6.40) and a high worm environment (Log_e (FEC + 100) above 6.75) were considered. In this model, h² estimated for BW and Log_e (FEC + 100) were found to be not statistically different between low and high environmental worm burden, furthermore, high correlations between traits at both environments were also presented.