A study of variation in milk yield and butterfat has been undertaken, using the test station records of almost 5 000 first-calf cows of three dairy breeds.
The general object of the investigation was to provide a background for a more comprehensive examination of the important productive characteristics of dairy cattle, and of the use of different sire-testing procedures for their improvement; the specific object was to consider the possible causes of large sire components and heritability estimates that have been observed in all analyses of test station production data, and the reasons why the station and field tests of a bull do not always correspond.
Large heritability estimates for milk yield, fat yield and fat percentage were obtained in the Danish Red and Black Pied breeds, but the corresponding estimates in the Jerseys (0·31, 0·48 and 0·58 respectively) were similar in size to the values normally obtained for these traits in field data.
The pre-selection of heifer recruits to the stations, and breed differences in the intensity of selection applied to the productive characteristics, were considered and dismissed as causes of abnormal sire components, in the light of results obtained from field data which indicated that the heritability of milk yield in the Jerseys was not lower than in the other two breeds (0·26, 0·37 and 0·36 in Danish Reds, Black Pied and Jersey).
It was suggested that differences between records made in test stations and records made in farmers' herds may be caused by differences in milking efficiency, by differences in culling practices in the two environments, and by the effects of age, season at calving and contemporaneity. Other investigations have established that these factors may have a large influence on the variance components of the production traits, and all of them would repay further investigation.