Estimation of Genetic Trend of Resilience Indicator over 37 years in Caracu Heifers

Abstract

Caracu heifers (Bos taurus taurus) raised in extensive pasture-based systems under tropical climates, face environmental challenges that may affect their growth and development. Therefore, the objective of this study was to evaluate the genetic trend of the resilience indicator defined as the logarithm of the residual variance (LnVar) in Caracu heifers. Data from 999 Caracu heifers born between 1982 and 2019, from the herd selected for post-weaning weight at the Instituto de Zootecnia (São Paulo, Brazil), were used. The resilience indicator, LnVar, was obtained from the residuals of observed and predicted weights recorded from birth to 18 months of age, with predicted values estimated using a repeated measures linear model. Estimated breeding values (EBVs) of LnVar were obtained via Bayesian inference using a single-trait animal model, with contemporary group (year and month of birth) as a fixed effect and additive genetic, as random effect. The pedigree matrix included 1,368 animals. The genetic trend of LnVar was assessed by regressing EBVs on year of birth using SAS 9.4. Additionally, phenotypic and genetic means from two periods (1982-1985 and 2016-2019) were compared using the t-test. The regression coefficient ± standard error was 0.00007610 ± 0.00008130 (p = 0.3495), indicating no significant genetic trend over time. The phenotypic and genetic differences between periods were not significant (t = 0.29; p = 0.7744; Satterthwaite method), differences ± standard deviations were 0.0589 ± 0.9064 and 0.00117 ± 0.0242, respectively. These results suggest that selection for post-weaning weight did not compromise the resilience of Caracu heifers. The stability of the resilience indicator over time may reflect the inherent adaptability of the Caracu breed, composed of taurine animals subjected to intense natural selection in tropical environments. However, further studies are recommended to better understand the interaction between resilience and productive and reproductive traits within genetic improvement programs.