Parâmetros e tendências genéticas para características de desempenho e de escores visuais das raças Hereford e Braford

Abstract

This study aimed to estimate (co)variances and genetic parameters of Hereford and Braford herds participating in the PampaPlus genetic evaluation program, using the Bayesian inference (BI) methodology; to analyze trait genetic progress through genetic trends and predict expected genetic gain per generation (ΔG). Records of 87,933 animals were used to evaluate objective traits and visual scores: birth weight (BW), weaning weight adjusted to 205 days (WW205), yearling weight adjusted to 550 days (YW550), post weaning gain adjusted to 345 days (PWG345), cow weight at weaning (CWW), scrotal circumference at yearling (SCY), muscling at weaning (MUSW) and yearling (MUSY), frame at weaning (FRW) and yearling (FRY), body structure at weaning (BSW) and yearling (BSY), navel size at weaning (NSW) and yearling (NSY), and cow body condition score at weaning (BCCW). Bayesian inference was based on Markov Chain Monte Carlo methods (MCMC) with 440,000 cycles, after 40,000 burn-in cycles, and the samples were stored every 40 cycles, resulting 10,000 samples. Animal model was used, including contemporary group, classes for age of cow and linear and quadratic coefficients of animal age at evaluation time as fixed effects, and random direct additive genetic effects, maternal and maternal permanent environment. The posteriori mean heritability (h²) estimates and their respective standard deviations of the yearling objective traits (BW, WW205, YW550 and PWG345) resulted in low magnitude, (0.14 ± 0.02; 0.13 ± 0.02; 0.13 ± 0.02; 0.12 ± 0.03, respectively), for CWW and SCY traits h² were (0.26 ± 0.08 and 0.18 ± 0.02, respectively). For visual scores (MUSW, MUSY, FRW, FRY, BSW, BSY, NSW, NSY and BCCW) h² estimates were 0.07 ± 0.01; 0.18 ± 0.04; 0.10 ± 0.02; 0.11 ± 0.02; 0.04 ± 0.01; 0.14 ± 0.03; 0.08 ± 0.01; 0.11 ± 0.02; 0.09 ± 0.02, respectively. Overall, heritability values were of moderate to low magnitude and below compared to other studies. Nevertheless, additive genetic variances were in accordance with most part of the literature reviewed, while environmental variances were higher than those found in the literature demonstrated that greater control over the environmental factors could increase values of h². Positive genetic correlations (rG) were estimated between weight traits, and rG between visual scores at weaning and yearling were positive and of high magnitude. Genetic trends calculated by linear regression using the average breeding values (BV) for each trait according to the year of birth, showed positive values indicating reasonable gains for traits of interest. The genetic qualifying index (GQI) presented a 10% of annual progress, and WW205 YW550 traits presented an increase of 243 and 398 grams/year, respectively. Considering a herd of 504 dams, with a yearly replacement rate of 20%, with 1 bull for each 28 cows and weaning rate of 78.1% (annually weaning 197 males and 197 females), expected ΔGs were 5.81, 7.72 and 5.97 kg for WW205, YW550, PWG345, respectively, per generation. However, for SCY trait, the ΔG indicates the possibility of increasing 1.01 cm per generation in direct selection for only this trait. The results demonstrated the existence of genetic variability for the studied traits, allow their successful use in animal breeding programs. Satisfactory results of genetic progress should be achieved by using the breeding values generated by PampaPlus as selection criteria in Hereford and Braford.