Study of stayability in nellore cows using a threshold model.

• Grant number: 07/04403-1
• Support Opportunities: Regular Research Grants - Publications - Scientific article
• Start date: September 01, 2007
• End date: October 31, 2007
• Field of knowledge: Agronomical Sciences - Animal Husbandry - Genetics and Improvement of Domestic Animals
• Principal Investigator: Joanir Pereira Eler
• Grantee: Joanir Pereira Eler
• Host Institution: Faculdade de Zootecnia e Engenharia de Alimentos (FZEA). Universidade de São Paulo (USP). Pirassununga , SP, Brazil

Abstract

Objectives of the present study were to assess the feasibility of using stayability traits to improve fertility of Nellore cows, and to examine the genetic relationship among the stayability at different ages. Stayability was defined as whether or not a cow calved every year up to the age of 5 (Stay5), 6 (Stay6) or 7 (Stay7) yr of age or more given that she was provided the opportunity to breed. Data were analyzed based on a maximum a posteriori probit threshold model to predict breeding values on the liability scale, while the Gibbs sampler was used to estimate variance components. The EBV were obtained using either all animals included in the pedigree or bulls with at least 10 daughters with stayability observations, as well as average genetic trends were obtained in the liability and transformed to the probability scale. Additional analyses were performed to study the genetic relationship among stayability traits, which were compared by contrasting results in terms of EBV and the average genetic superiority as a function of the selected proportion of sires. Heritability estimates and SD were 0.25 (0.02), 0.22 (0.03), and 0.28 (0.03), for Stay5, Stay6 and Stay7, respectively. Average genetic trends, by year, were 0.51, 0.34 and 0.38 % for Stay5, Stay6, and Stay7, respectively. Estimates of EBV SD, in the probability scale, for all animals included in pedigree and for bulls with at least 10 daughters with stayability observations were 7.98 and 12.95, 6.93 and 11.38, and 8.24 and 14.30 % for Stay5, Stay6, and Stay7, respectively. A reduction in the average genetic superiorities in Stay7 would be expected if the selection were based on Stay5 or Stay6. Nonetheless, the reduction in EPD, depending on selection intensity, is on average 0.74 and 1.55 %, respectively. Regressions of sires’ EBV for Stay5 and Stay6 on sires’ EBV for Stay7 confirmed these results. The heritability and genetic trend estimates for all stayability traits indicate that it is possible to improve fertility with selection based on a threshold analysis of stayability. The SD of EBV for stayability traits show that there is adequate genetic variability among animals to justify inclusion of stayability as a selection criterion. The potential linear relationship among stayability traits indicates that selection for improved female traits would be more effective by having predictions on Stay5 trait. (AU)