EFFECT OF THE HERD LEVEL OF PRODUCTION ON THE MONTHLY TEST DAY AND CUMULATIVE PRODUCTION IN DIFFERENT LACTATIONS

The first 8 monthly test days and cumulative I production at 244 days of 10 623 lactations of Holsteins with one, two, three or more calving s were used to study the effect of the herd level of production on mon thly test days and cumulative production. Individual linear models for calvings including year (7 in the first lactation and 6 in the 2nd, 3 rd or more lactations), season (bimester), herd level of production (l ow, medium and high) year x season interaction, linear regression, age of calving and quadratic regression on the calving first monthly test day interval. Dairy units with > 2500 kg from 2501 to 3100 kg and < 3 100 kg were considered as low, medium and high level production units , respectively. Lactation was analysed through the persistency express ed as the ratio of production in the second 100 days and the productio n in the first 100 days with linear models per calving described previ ously. The production level of the herd explains 44 and 86% of the dif ferences between dairy units in the monthly test days while cumulative production were 53 and 86%. The first lactation was the most persiste nt with values between 83.7 and 84.3% compared to values between 73.0 and 77.7% for the second, third or more lactations. The production lev el of the herd did not affect the persistency in the first lactation b ut it affected the second third, or more lactations, although with ver y low relative weights (1.09 and 0.89, respectively). It is concluded that although the production level of the herd considerably affects mo nthly test day and cumulative production, its influence on the lactati on curve is poor and should not be taken into consideration on extendi ng part lactations.