The Effect of Machine Milking upon the Leucocyte Count and the Chloride Content of Milk

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Abstract
In an earlier paper (2) it was shown that the leucocyte count and the chloride content of milk are not reliable indices of udder infection. Indi- vidual differences between cows with respect to these two tests and differ- ences in results caused by factors other than infection frequently are greater than the changes that occur when the udder becomes infected. Any factor that causes udder irritation doubtless will cause a rise in the leucocyte and chloride values. The extent to which machine milking may influence leucocyte counts and chloride determinations, or other mastitis tests that reflect udder irritation, such as pit and catalase tests, is not clear. Munch-Petersen (4) reviewed the literature on mastitis and found opinion about equally divided with respect to milking machines as a factor contributing to mastitis. Dahlberg (1) found that leaving machines on the cows too long contributed to udder troubles and resulted in a drop in production early in the lactation period. He presented data indicating that reducing to 4 or 5 minutes the time inter- val that the machines were left on the cows was accompanied by a drop in the leucocyte count of the mixed herd milk. Hucker (3) reported that the leucocyte count of milk from machine-milked cows was higher than that of milk from hand-milked cows. In his machine-milked group there was a higher incidence of streptococcic infections than in his hand-milked group, a factor which probably exaggerated the difference he found between the two groups with respect to leucocyte counts. In studies on the effects of milking machines on tests for mastitis there has not to the author's knowledge been any report which deals separately with infected and uninfected cows. If milking machines cause irritation which predisposes the udder to infection, one should expect detectable in- creases in the leucocyte count and chloride content of the milk when a cow is on machine milking as compared with values found for the same cow when she is on hand milking. On the other hand, if the machine is only a fomite spreading the infection without itself predisposing the udder to the infection, high leucocyte counts and chloride values would be found in the milk of machine-milked cows because of the high incidence of infection and not directly as a result of injury caused by the machines. The purpose of this