Identification of dairy and beef lines of Guzerat bulls using cluster analysis

Progeny testing of the Guzerat breed started in 1994 with dairy bulls, in addition to genetic evaluations for growth traits performed by the Brazilian Association of Zebu Breeders (ABCZ). Considering that the bulls employed in beef cattle herds are also used in dairy and dual-purpose herds, the objectives were to identify differences in the growth and milk production patterns of Guzerat bulls by cluster analysis and to group the animals according to the desired selection objective. Two-trait analyses of cumulative milk yield at 305 days with weights at 120 days, weaning, yearling, postweaning and 24 months of age were performed. The databases analyzed were provided by ABCZ and contained 97,394; 65,181; 50,443; 40,425 and 31,279 records, respectively. The animal model included the fixed effects of contemporary groups (herd, sex, year and month of birth, year and month of weighing, feeding regimen, and rearing condition for weights and herd, year and month of calving season for milk), age of dam at calving (linear and quadratic covariate), and random direct additive genetic, permanent environmental (maternal for weights; direct for milk) and residual effects. The variance components were estimated by the restricted maximum likelihood method using the WOMBAT software. Estimated breeding values were used to perform hierarchical cluster analysis by the Ward method, which resulted in the division of bulls into two groups according to similarity in the genetic evaluations of the animals. Non-hierarchical analysis was then performed by the K-means method in which the bulls belonging to the groups formed by hierarchical analysis were identified, resulting in a genetically superior and a genetically inferior group. New hierarchical and non-hierarchical analysis was performed with each of the two groups, resulting in four groups of bulls. A major difference was observed in the profiles of the breeding values studied, with the identification of bull lines with a high potential of exclusive selection for milk production, bulls with a high potential of beef production, and bulls that can be used when the objective is dual-purpose selection. The use of bulls evaluated for the objective of specific selection will provide better results in the genetic improvement of the breed.