PSII-5 Genetic parameters for new approaches to stayability in early-challenged females of the Nellore breed and their correlations with economically important traits.

Abstract Over the past 12 years, there has been a significant increase in the number of Nellore females in Brazil exposed to the reproductive season during the 11-to-14-month period. This is particularly important, given that zebu breeds were considered late maturing. The cow’s longevity in the herd is closely related to economic and reproductive sustainability, directly impacting the profitability of the entire production system. Traditionally, the stayability (STAY) trait is defined as a female giving birth to three calves by 76 months of age. However, this criterion may include females that experience reproductive failures throughout their lives, potentially reducing reproductive efficiency, slowing genetic progress, and impacting producers economically. Collecting phenotypic information from younger females could improve genetic progress for traits related to reproductive efficiency and herd longevity. This study aimed to estimate the variance components, heritabilities, and genetic correlations between different categories of STAY with two or three calves (STAY48-2, STAY48-3, STAY54-2, STAY54-3) in relation to weight at 240 days of age (W240), weight at 450 days of age (W450), scrotal circumference at 365 days of age (SC365), age at puberty in males (APM), stayability (STAY76), probability of precocious calving at 30 months of age (PPC30), rib eye area (REA), rump fat thickness (RFT), residual feed intake (RFI), dry-matter intake (DMI), residual live weight gain (RG), and frame score (FRAME). Records from 80.958 females born between 2000 and 2020, exposed to mating starting at 10 months of age, raised on pasture from 187 farms in the central-west and southeastern regions of Brazil that participate in the National Association of Breeders and Researchers (ANCP), were analyzed. The (co)variance components were estimated by Bayesian inference in a bi-trait animal model employing Gibbs sampling as implemented in the gibbsf90+ software. The posteriori means of heritability estimates for STAY48-2, STAY48-3, STAY54-2, and STAY54-3 were moderate to low, 0.20, 0.20, 0.22, and 0.22, respectively. The traits related to different categories of stayability showed low genetic correlations with male reproductive traits (-0.22 to 0.19), feed efficiency (-0.03 to 0.13), carcass (0.11 to 0.18), and body composition (-0.09 to -0.6), low to moderate with growth (0.04 to 0.29), and high with sexual precocity (0.88 to 0.93) and fertility (0.65 to 0.73). The heritability estimates of the different categories of stayability indicate genetic improvement for longevity in early challenged females. The genetic correlations with sexual precocity and fertility showed that its use as selection criteria is recommended for production systems of younger females challenged in the reproductive season rather than traditional stayability to increase the probability of stayability on the herd.