RELATIONSHIP BETWEEN SURVIVAL OF COWS AND MILK-PRODUCTION DURING FIRST LACTATION .2. INFLUENCE OF THE PRODUCTION LEVEL OF THE HEIFER

The influence of first lactation milk production at 244 days on the su rvival of Holstein heifers of three enterprises was studied in two sta ges: a) for 2 years of productive life where the survival was measured from first calving until 3.5 and 4 years of age (S 3.5 A and S 4 A) a nd until 1.5 and 2 years of productive life (S 1.5 PL and S 2 PL) with 17835 data and in b) for 3 years of productive life with 14180 observ ations where the survival was measured until 5 years of age and for 3 years of productive lire (S 5A and S 3 PL). Data per enterprise and to tal in each stage were analysed according to a model of fixed effects (least squares) with the effects of milk production classes (14), year (12), calving trimester, enterprise and double interactions. There wa s a significant influence of the production classes (P < 0.001) on the survival measurements. From the study of the linear, quadratic and cu bic regressions of milk production on survival for all enterprises, si gnificant quadratic curves were found which define the relationship be tween the milk production classes and the 5 3.5 A, S 4 A and S 1.5 PL, and with the rest of the survival measurements were cubic. With the i ncrease of the period of time considered from 1.5 years to 3 years of productive life the maximum classes for survival (3200-3800 kg of milk ) are better defined and the disadvantage of the classes of lower prod uction are more evidenced (< 2300 kg) with a range of 17 and 22% of su rvival between the best and worst classes at 5 year of age and 3 years of productive life, respectively, In the enterprise of the lowest pro duction level, the cows of the highest production level (> 4700 kg) ha d an extremely unfavoured survival rate with respect to the intermedia te (2400-4400 kg) level. The curvilinear relationships between milk pr oduction and survival accounts for the low phenotypic correlations fou nd in previous studies.