The Effect of an Incomplete Removal of Milk from the Udder on the Quantity and Composition of the Milk Produced during the Immediate Subsequent Milkings

Summary The difference between the averages of the respective percentage fat lactose and protein + ash contents and the yields of fat and milk, as determined from the preliminary and subsequent milkings reported for each cow in tables 1 to 15, are in most cases not very great. There is, however, a general trend in these differences (table 17) which indicates that they are due to the presence of some common disturbing factor, that factor being the leaving of milk within the udder. The differences in the percentage lactose and protein + ash contents are for the most part not as great as those found in the percentage fat content and yields of fat and milk. On the other hand the extent of the variation within the percentage fat content and yields of fat and milk, as measured from one milking to another, figures 1 to 15, is on the average more than twice as great as the equivalent variation within the percentage lactose and protein + ash contents. Hence taking into consideration the degree of variation as measured from one milking to another, it is obvious that comparatively smaller differences in the percentage lactose and protein + ash contents are just as significant as larger differences in the percentage fat content and yields of fat and milk. Considering the averages of all the increases and decreases in the respective percentage fat, lactose, and protein + ash contents and the yields of fat and milk, as determined for each trial (table 17), it will be found that the average of all the increases and decreases in trial II is greater than the average of all the increases and decreases in trials I or trials III to X. This difference in the averages of all the increases and decreases in the percentage contents and yields as found in the various trials can be attributed to the influence of one of two factors; namely, (1) the amount of milk left in the udder; and, (2) the length of time the milk is left in the udder. In trials I and II the cows were milked twice a day, but in trial 1 one-fourth of the average milk yield was left in the udder, whereas in trial II one-half of the average milk yield was left in the udder. Hence by increasing the amount of milk left in the udder, the effect upon the percentage fat, lactose and protein + ash contents and the yields of fat and milk was increased. In trials III to X the cows were milked 4 times a day and one-half of the average milk yield was left in the udder. In trials III to X the same amount of milk was left in the udder as in trial II, but the milk remained in the udder only 6 hours in trials III to X as compared to twelve hours in trial II. We find that the average of all the increases and decreases in the various percentage contents and yields in trials III to X is less than the equivelant average in trial II, the decrease being due to the shortening of the length of tine the milk was left in the udder. The number of cows in each trial showing a significant average increase or decrease in the percentage fat, lactose, and protein + ash contents and the yields of fat and milk (table 16) also manifest the influence of the above mentioned factors in each trial. Trial I included 4 cows and each cow had the opportunity of showing a significant average increase or decrease in the percentage fat, lactose, and protein + ash contents and the yields of fat and milk. If each cow in every trial showed a significant average increase or decrease in every percentage content and yield, there would be 20 significant averages increase or decreases in trial I, 15 significant average increases or decreases in trial II and 40 significant average increases or decreases in trials III to X. Hence the ratio of the actual number of significant average increases or decreases to the total possible number of significant average increases or decreases in each trial will give the amount of disturbance in the various percentage contents and yields due to the leaving of milk within the udder. In trial I we find 10 actual significant average increases or decreases out of a possible 20 significant average increases or decreases, a ratio of 1:1. In trial II we find 10 significant average differences out of a possible 15 significant average differences, a ratio of 2:1. In trails III to X we find 19 actual significant average differences out of a possible 40 significant average differences, a ratio of less than 1:1. Trial II again stands out as showing the most marked disturbance in the various percentage contents and yields due to the amount of milk left in the udder and the length of time it was left in the udder.