Variability of milk production within the contemporary groups in prediction of breeding value

Calving frequency within the year and variability of milk production within the herd-year-season (HYS) for cows in the 1st to 3rd lactation of the Czech Pied (C) and Holstein (H) cattle calved in 1996 to 1997 was determined. The herds were divided into seasons on the basis of the number of observations from 2 to 15. Systematic effects of the environment (HYS) and third degree polynomial regression for the age at 1st calving, for the service period, and previous calving interval were taken into account. With the increasing number of individuals within HYS, the season was prolonged and resulted in the increase of residual variability. In dependence on the number of individuals within HYS and lactation number, in C cattle the systematic effects explained 47 to 52% variability, the random environment accounted for 38 to 43% and genetic disposition for 10% of total variability of measured data. In H cattle, these data were 54 to 60%, 28 to 33% and 13%, respectively. In the 1st lactation, systematic effects explained a lower portion of variability than in the following lactations. Based on the number of observations in herds, it was possible to determine the lengths of seasons that guarantee the given number of individuals within HYS. The mean error of production deviation from the contemporaries decreased dependent on their number. Starting from the number of contemporaries equal to 7, further increase in their number has no effect on the mean error that reached approximately 3/4 of maximum value at one contemporary. Taking into account the parents reduced the mean error by 3 to 5%.