Rate of Maturity of Dairy Females

Mature equivalent lactation yields of 18,869 Pennsylvania Holstein cows were analyzed for changes with age. Each record was a deviation from herdmates, and the difference between a later deviation and the first lactation deviation of the same cow was used to measure change to maturity. Five definitions of maturity, approximating changes to 3, 4, 5, 6, and 7 yr-old cows, were examined. Sire effect was highly significar~t in 9 of the 10 analyses with fat yield at the oldest age being the exception. An analysis of first lactation deviations of milk and fat yields of cows in each maturity study was parallel. All ten analyses indicated a highly significant sire effect. Heritabilities were generally one-third to one-half as large for may maturity analysis as for the corresponding first lactation data.