Estimates of genetic parameters for indicator traits of resistance to gastrointestinal nematodes and growth traits in Santa Inês sheep

Abstract

Infections by gastrointestinal nematodes is a major problem in the sheep industry worldwide. Therefore, this study aimed: (1) to estimate genetic parameters for indicator traits of resistance to gastrointestinal nematodes such as packed cell volume (PCV), total plasma protein (TPP), color of the ocular conjunctiva by Famacha© method (FAM), and fecal egg count (FEC); (2) to estimate genetic parameters for growth traits such as body weight (BW), body condition score (BCS) and chest girth (CG); and (3) evaluate the FAM as an indicator trait of resistance to gastrointestinal nematodes in Santa Inês sheep. The dataset contained 3524 observations from 1176 animals and the pedigree presented 2011 animals. The genetic parameters were estimated by the animal model, in single- and multi-trait Bayesian analyses, using the THRGIBBS1F90 program. Stochastic Markov Chain Monte Carlo Simulation (MCMC) was used to obtain estimates of the parameters of interest, using Gibbs sampling. The a posteriori estimates were obtained with the POSTGIBBSF90 program. The sensitivity and specificity of the FAM were 66.7 % and 75.8 %, respectively. In general, the sensitivity and specificity values found herein do not invalidate the use of FAM. In the single-trait model, the heritability estimates for BW, CG, BCS, PCV, TPP, FAM, and FEC were 0.51 ± 0.02, 0.61 ± 0.03, 0.40 ± 0.04, 0.38 ± 0.03, 0.29 ± 0.03, 0.19 ± 0.05 and 0.15 ± 0.03, respectively. In the multi-trait model, the heritability observed for these traits were 0.73 ± 0.02, 0.64 ± 0.02, 0.35 ± 0.05, 0.38 ± 0.03, 0.29 ± 0.03, 0.20 ± 0.05 and 0.13 ± 0.04, respectively. Genetic correlations between growth traits (BW, BCS, and CG) were positive with high magnitude. In addition, growth traits exhibited positive genetic correlations with PCV and TPP, and negative correlations with FAM. The genetic correlation between PCV and FAM, and between FEC and BW were negative and favorable. The genetic parameters suggest that it is possible to obtain genetic gains for the evaluated traits by the response to direct selection. Unfavorable genetic correlations were not found between growth traits and indicator traits of resistance to gastrointestinal nematodes. The heritability of FAM was moderate, and genetic correlations were considered favorable between FAM and PCV. In addition, FAM is easy-to-measure and has a low cost. Therefore, FAM can be an alternative secondary selection criterion associated with body weight (main selection criterion), to increase resistance to gastrointestinal nematodes.