Can the relation between fat and protein in milk be changed by selective breeding
Authors: Lonka T. 
Published: July 2018

In Finland the protein content of the feeds is in general so low that it restricts the milk yield particularly in herds of high production capacity. This is especially significant for the reason that owing primarily to climatic factors the quality of the feeds cannot be notably improved in this respect. Therefore an effort should be made to change the composition of milk by means of breeding in such a way that it would correspond to the possibilities of feed cultivation in Finland better than at present. This would mean that it should be endeavoured to breed cows whose milk is very fat-rich but at the same time poor in protein, in which case more of the commercially valuable butterfat could be produced on the feed grown on the estate. Since a correspondingly greater amount of milk of poor protein percentage could be produced than that of protein rich milk, the breeding of such cows would not decrease the yield of protein, which is of such great food value, but as great an amount in kilograms of milk protein would be available as earlier for domestic purposes as well as dairies. We have aimed at this goal in our country in general by endeavouring to raise the fat percentage of the milk by means of breeding. It is known that protein percentage does not increase exactly as markedly as the fat percentage, wherefore the protein yield per kilogram of fat decreases as the percentage rises. The influence of breeding would however be incomparably greater if low protein percentage were selected simultaneously with high fat percentage. In the investigation we have dealt with the possibilities of such selection in Finnish cattle, in connection with which we have striven to make clear how much the protein percentage of the milk varies independently of the fat percentage. The material comprises 54 West-Finnish Native cows; the fat and protein percentages of their milk yield has been determined once a month during one lactation. The relation of fat and protein is not the same throughout the whole lactation, but it changes so that at the end of the lactation the protein percentage as compared with the fat percentage is relatively greater than at the beginning of the period (Table and Figure 1). The averages of the fat and protein percentages and likewise the correlation between them depend consequently upon the length of the calving interval. Therefore the said averages have been estimated only on the basis of 2—7 production months. Fy means of these averages r = +0.60 ± 0.09 was derived as the coefficient of correlation between the fat and protein percentages y = 1.53 + 0.39 x as the regression equation, x = fat percentage and y = protein percentage. The individual cows diverge very much from the general rule set by the regression equation, which can be concluded already from the comparatively low value of the coefficient of correlation. The protein percentage of the milk yield of the West- Finnish Native cows diverges at least about one per cent independently of the fat percentage when judging on the basis of the extreme divergencies in the material. Although a part of the divergency evidently is caused by external factors, the role of the genetic factors is nevertheless so great that there appear to be great possibilities for breeding cows producing fat-rich milk by means of breeding but having a protein which would be considerably lower than the present average.