RELATIONSHIP BETWEEN NUMBER OF DAYS IN MILK IN FIRST LACTION COWS AND THEIR LIFETIME MILK YIELD.

Some farmers with herds producing high milk yields extend the lactation cycle to more than 305 days. This is related to reproductive management, which then obviously results in long open days. One argument is that high producing first lactation cows will have higher lifetime milk yields with increasing milk yield per day of lifetime. The question is if this assumption is correct and therefore the authors cited aimed in their study to determine if the effect of duration of the first lactation period has a significant effect on lifetime performance.

Some 155 000 lactation records of Holstein cows from the Milk Recording Scheme with first lactation calving dates between 2001 and 2008 were used, which ensured that these cows completed their lifetime lactations. The duration (number of days in milk) for cows in first lactation was used to determine the effect on the lifetime milk production of the cows. Lactation records were divided into categories comprising 10 day intervals starting from 215 to 425 days in milk. Within each lactation period the mean lifetime (in days) milk yield, lifetime efficiency (LTE = lifetime milk yield divided by lifetime) and productive lifetime efficiency (PLE = Lifetime milk yield divided by productive life) of all cows was estimated. The lifetime (LT) of cows was derived from birth date to the end of the last lactation period which was regarded as each cow’s cull date, as actual cull date was not available. Productive lifetime was the sum of days in milk for all lactations.

The results showed that the productive lifetime, lifetime of the cows, LTE an PLE increased significantly as first lactation length increased from 215 to 305 days. For first lactation cows with extended lactation periods (longer than 305 days) lifetime milk yield increased. However, for first lactation periods more than 305 days in milk there was no change in productive lifetime, lifetime of cows, LTE and PLE, and the biological differences were minimal, i.e. ranging from 28.1 kg to 28.3 kg milk per day and TLE increased from 12. 2 kg milk to 13.0 kg milk per day.