182. Variations in the protein content of milk during lactation

Abstract

1. It was planned to study the variations of the protein content of milk during the lactation period.2. The total protein nitrogen and the casein nitrogen were determined by approved methods in about 380 weekly samples of milk taken from twentyseven individual cows of different breeds and at different stages of lactation, from February to July 1937. The albumin+globulin nitrogen was calculated in each case by difference.3. The statistical analysis of the figures was made by the methods of “factorial arrangement” and by “pairing”, and also by the estimation of the 95% probability range of the mean of the figures available for each week of lactation.4. It was found that the percentage of total protein nitrogen decreases very significantly from the beginning until the 4th week of lactation, and then rises slowly until the end of lactation, the rise being more pronounced to wards the end. The same was true for casein nitrogen and for albumin+globulin nitrogen, but with the latter only the decrease at the beginning and the rise at the end were sharp. The rise for the rest of the lactation curve, although it did exist, was very slow.5. The lactation curves for the yield of total protein nitrogen, casein nitrogen and albumin + globulin nitrogen were practically parallel with the lactation curve for the yield of milk.6. The correlations between the yield of milk and the percentages of total protein nitrogen, casein nitrogen and albumin + globulin nitrogen are studied and the correlation tables with all details are given for each. It is noteworthy that the regression lines are not linear and that with a poor milker an increase of 1 lb. in yield of milk corresponds to a relatively high decrease in the percentage of protein nitrogen, whereas with a high milker this decrease is relatively much smaller.7. The correlations between the yield of milk and the yield of total protein nitrogen, casein nitrogen and albumin + globulin nitrogen are examined and the correlation tables are given. It is shown that there is a close correlation between the yield of milk and its protein content, from which the conclusion is drawn that a very low protein food does not decrease the percentage of protein in milk as one might expect, but decreases the yield of milk and increases the percentage of protein in milk. On the other hand, an increase of adequate protein in the ration may raise the yield of milk if it is not at its maximum level.8. It is emphasized that the above conclusions are drawn from a study carried out on a group of cows, and may not be strictly applicable to individual cows.