Correctie van de melkgift van koeien voor verschillen in leeftijd, seizoen en lactatiestadium

To correct milk records for differences due to age and season of calving, the milk records of 4000 cows, with lactation length of 290-320 days, were analysed. The magnitude of the influences of age, season and herd on test-day, cumulative and lactation yield were determined. There was no interaction between age and season. The age effect on test-day yield explained 47% of the total sum of squares at 30 days and decreased to 5% at 270 days. The effect of age on lactation yield was 39%. The influence of season was about 0.5% at 30 days and 8.5% at 270 days. Herd effect was 25% on lactation yield and increased from 15 to 31% on test-day yield.

To standardize lactation and test-day yield, correction factors for age and season were tested with an independent sample of 2000 records. There were no differences whether lactation yield was corrected for season by additive or multiplication factors. For test-day yield additive factors had some advantage. Age differences can be corrected with multiplication factors over herd levels (in spite of a interaction between age and herd level).

Test-day yields have to be standardized for stage of lactation by multiplication factors within herd level.

Factors to be used for extrapolation of current lactations were calculated within subclass of age and season; three methods of extrapolation were tested in sample of 2000 records. The method with regression of remainder of lactation on last test-day yield had the smallest absolute difference and the highest correlation coefficient between estimated and calculated lactation yield. In practice the extrapolation parameters have to be calculated within subclass of age, season and herd level.

Data of incomplete records were analysed. The length of the records was related to the season of calving. The peak yield was positively related to the length of the records and the test-day yield for records with less than about 200 days decreased faster than for complete records.