96. Genetic Analysis of Longevity in Brazilian Santa Inês Sheep.

Abstract Santa Inês is the most common hair type sheep breed in Brazil and is highly relevant to commercial production systems, especially in the Northeast region due to its adaptability to tropical conditions and maternal ability. In these conditions, ewe longevity is a key functional trait because longer productive life can improve flock efficiency, lower replacement costs, and increase the number of lamb crops produced per ewe. Thus, this study aimed to estimate genetic parameters for longevity across age-at-lambing classes in Brazilian Santa Inês sheep. Longevity was assessed longitudinally as a binary trait, indicating whether each ewe lambed within 10 age-at-lambing classes, ranging from 9 to 128 months and grouped in 8-month intervals. After data quality control, 2,886 ewes born between 1998 and 2014 in 21 flocks and complete lifetime records (i.e., non-censored data) remained for analyses. These ewes were daughters of 421 sires and 2,037 dams and produced 8,135 lambs in 6,445 litters between 2000 and 2015. Genetic parameters were estimated using a single-trait random regression model with a third-order Legendre orthogonal polynomial under Bayesian inference with GIBBSF90+. Gibbs sampler comprised a chain of 1,000,000 iterations, with a burn-in of the first 500,000 iterations and samples stored every 20 cycles. Convergence was assessed using the Heidelberger-Welch and Geweke diagnostics criteria, as well as visual inspection of the chains using the boa R package. The model included age at first lambing class as a fixed effect and year-season as a fixed regression curve, whereas flock-year-season, direct additive genetic, and animal permanent environmental effects were fitted as random regressions, assuming homogeneous residual variance (0.995 ± 0.012). Heritability estimates for different ages ranged from low (h2 = < 0.001 ± < 0.001) to moderate (h2 = 0.202 ± 0.044) across the evaluated age. For instance, a value close to zero was observed at the earliest age-at-lambing groups (from 9 to 40 month classes), and values above 0.14 were obtained at ages higher than 65 months, indicating a gradual increase in heritability estimates over time. Lower heritability estimates in the first age-at-lambing groups reflect lower direct additive genetic variance. From 2.75 years of age onward, genetic correlations ranged from 0.71 to 1.00, indicating that longevity expressed at older ages is influenced by similar genetic background. Repeatabilities were also assessed and found to be low, reflecting a strong temporary environmental influence on reproductive performance. These results indicate that longevity in Santa Inês sheep is heritable and can respond to selection, particularly at older ages. Further studies are needed to determine the most appropriate stage of life for selection decisions to maximize genetic gain for longevity. For image description, please refer to the figure legend and surrounding text.