Estudi de la longevitat i la supervivència fins al deslletament en bovins de muntanya de la raça Bruna dels Pirineus.

Abstract

The most economically important traits in the production system of the Bruna dels Pirineus beef breed are: calving interval, prolificity, cow longevity, calf survival until sale, weaning weight, growth rate during fattening and carcass conformation. Within these, cow longevity is a desirable quality, because its improvement would allow to reduce the replacement costs and increase the percentage of cows in more productive ages. Calf survival up to weaning increases the incomes from calf sales and reduce significantly the costs. The aim of this work is to study the factors that afect longevity of Bruna dels Pirineus beef breed cows and survival up to weaning of its calves using survival analysis techniques. The first chapter is a bibliographic revision that gives a general view of the situation and evolution of survival analysis techniques in its application to animal production. The second chapter is the analysis of factors that influence cow´s length of productive life and replacement rates. The average length of productive life is approximatelly of 9 years, which implies an annual replacement rate of 11%. Using a proportional hazard model, we have observed that the daily culling rate increases with late ages at first calving, increasing calving difficulties, very large calf birth weights, very small weight gains until weaning and very long calving intervals. Furthermore, this increase also depends on the sequence of values of the variable in previous calvings. A higher daily culling rate increases the annual replacement rates, in spite of the loss in productivity by reducing the percentage of cows in more productive ages is not very important. The third chapter analyses the genetic and environmental factors that afect calf survival up to weaning, which is 96.9%. Using a proportional hazard model, we have observed that the daily mortality rate is not constant during all the lactation but within intervals: is higher during the first to weeks, diminishes the next two weeks and after the first month of life is nearly depreciable. This rate increases for calf born in the second part of the calving period (from the beginning of march to august), born from young cows, with high calving difficulty and small birth weight. From a genetic point of view, despite the additive sire variance is relativelly high (0.3), the heritability of the trait at weaning is only 0.037. Finally, the fourth chapter is a validation via simulation of an approximate two-step approach to estimate genetic correlations between survival and linear traits for its aplication to genetic evaluation. In the first step, additive variances are estimated and a genetic evaluation is done separatelly for each trait with its ideal univariate model to obtain the pseudo-record for each animal and its weight. In the second step, these pseudo-records are used in the joint genetic evaluation of all traits using linear models and BLUP and REML methods in the genetic evaluation and the variance components estimation. The mean of the genetic correlations and accuracies obtained in simulated data confirmed the suitability of the approach to estimate genetic correlations in a wide range of situations (different genetic correlations, heritabilities and censoring levels).

Date of Award	31 Jan 2006
Original language	Undefined/Unknown
Supervisor	Vincent Ducrocq (Director) & Jesús Piedrafita Arilla (Director)