Heritability and genetic correlations for scrotal circumference at different ages in Brahman bulls raised under tropical conditions

Abstract

Beef herds in the tropics are based on the utilization of Bos indicus and its crosses bred naturally. Genetic progress has been traditionally restrained by many factors including social, climatic, nutritional and management, among others. Besides, there is little selection of sires through variables correlated to their profitability, such as scrotal circumference (SC). This fact justifies attention to and research on those valuable characteristics as selection criterion to benefit genetic improvement in those systems. This paper aimed to determine the heritability index and genetic correlations for SC at different ages in Brahman bulls, to establish its use as a robust selection criterion at early ages in sires raised under tropical conditions. Heritability index (h2

) and genetic correlations (rg

) for SC at different ages were determined in 485 full blood Brahman bulls distributed in 8 herds from the North dry Pacific region of Costa Rica. Monthly SC measurements were performed by the same operator from 7 to 24 months of age. The genetic estimates were determined with a bivariate random regression animal model (SAS, ver 9.3, 2010). The model considered the fixed effects of herd, year and season of birth, nutritional plane, body weight, age at weaning and calving number of the mother. In addition, the random permanent effect of environment and animal additive genetic effect were considered. The genealogy data base included 3000 animals distributed in 7 ancestors ́ generations. The average h2

index for SC was 0.58 (range 0.48-0.72), being

highest at 20 months of age. In addition, SC h2

was higher in the period comprising 14 to 20 months of age compared to younger ages. The h2 index found in this study in Brahman bulls is higher than previous reports in crossbred zebu sires in Australia (Brahman and Sahiwal x Shorthorn; h2

=0.40 and 0.45 at 18 and 24 months respectively; Fordyce et al., 1996. Aust J Exp Agric, 36:9-17). In contrast, they are lower than those published for most Bos taurus breeds raised under sub-tropical climate. Coulter for instance (Coulter et al., 1976. J Anim Sci, 43:9-12), reported average h2

=0.68 in Holstein bulls under conditions of

an experimental station. The SC rg

coefficients obtained in this study in Brahmans aged 7 to 24 months

ranged from 0.43 to 1. Furthermore, rg

among yearling and bulls 18, 20, 22 and 24 months of age were higher (0.95, 0.92, 0.89 and 0.84 respectively) than those obtained among 7 months-old bulls and the same age range (0.43, 0.43, 0.44 and 0.45). These findings indicate that selection of Brahman sires by their SC can be performed as early as 12 months of age with high confidence (>80%) of their SC when adults (24 months). In addition, the lower h2 and rg for SC obtained among bulls <11 months and older ages (20-24 months), suggest that the initial post weaning is a stressful and adaptation period in Brahman steers. This fact should be considered by breeders and practitioners when choosing the right age for selection of prospective Brahman sires. This study was funded by the Andrology Section-UNA.Los rebaños de vacuno de los trópicos se basan en la utilización de Bos indicus y sus cruces criados de forma natural. Tradicionalmente, el progreso genético se ha visto limitado por muchos factores, como los sociales, climáticos, nutricionales y de gestión, entre otros. Además, hay poca selección de sementales a través de variables correlacionadas con su rentabilidad, como la circunferencia escrotal (CE). Este hecho justifica la atención e investigación de esas valiosas características como criterio de selección en beneficio de la mejora genética en esos sistemas. El presente trabajo tuvo como objetivo determinar el índice de heredabilidad y las correlaciones genéticas para SC a diferentes edades en toros Brahman, para establecer su uso como criterio de selección robusto a edades tempranas en toros criados bajo condiciones tropicales. Se determinó el índice de heredabilidad (h2 ) y las correlaciones genéticas (rg ) para SC a diferentes edades en 485 toros Brahman de sangre completa distribuidos en 8 hatos de la región del Pacífico Norte seco de Costa Rica. Las mediciones mensuales de SC fueron realizadas por el mismo operador desde los 7 hasta los 24 meses de edad. Las estimaciones genéticas se determinaron con un modelo animal de regresión aleatoria bivariada (SAS, ver 9.3, 2010). El modelo consideró los efectos fijos de rebaño, año y estación de nacimiento, plano nutricional, peso corporal, edad al destete y número de parto de la madre....