COMPARISON OF 2 METHODS FOR PRE-ADJUSTING COMPLETED SHORT LACTATIONS IN GENETIC EVALUATION OF DAIRY-CATTLE

This study presents a comparison of the effect on dairy cattle genetic evaluation of 2 different methods, taking into account completed lactations of less than 305 d. The reference method, in use in the French evaluation system, consists of a multiplicative pre-adjustment of the total yield by a factor function of length of lactation (385/(length +80)), whereas in the second method, short completed lactations are systematically projected. in a first step, projection factors based on partial yield and duration of lactation were derived from individual test days recorded in 1989 in the Normande breed. Two different evaluations were then conducted according to the French procedure of genetic evaluation (BLUP applied to an animal model) with the two different pre-adjustments for the short lactations. Systematical projection caused a decrease in the proofs variability (-6.7% for bulls and -8.5% for cows) and in the genetic trend (-6.3% after standardization of proofs for within-year variance); however, limited changes in ranking of bulls were observed A negative correlation (-0.40) was observed between the bulls breeding values (even after projection of short lactations) and the percentage of short lactations of their daughters. Thus, early eliminations are not randomly distributed but concern mainly the progeny of the inferior bulls. It was concluded that the present method for adjusting short lactations should be kept to avoid an overestimation of bulls with low genetic breeding value.