Relationship between length of productive life and the milk production in the Spanish Assaf sheep breed

The Spanish Assaf sheep breed has a National Breeding Program based on increasing milk production per lactation and its quality. This research attempts to determine the relationship between milk yield and length of productive life (number of lactations registered to culling) of the breed, using historic data from official dairy recording system until 2011. For each individual (27.170 dead and 36.268 alive animals) the average value of 150d standardized milk yield was calculated. Analysis of variance (GLM procedure of SAS) of milk production was performed using different models based on the state of sheep (alive or dead), flock, average type of birth, average number of dry days, and productive life. The average value of productive life was 3.76 lactactions/sheep (55.3% of the animals had between 2 and 3 lactations recorded to low). The results showed an increase in milk production over the years and a reduction of productive life.