Effects of Vacuum Level and Milking Duration on Guernseys and Holsteins Differing with Respect to Lactation Number and Status of Udder Health. I. Udder Health

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Abstract
Forty-eight animals, after a 4-week standardizing period immediately after calving, were assigned to be milked from the 5th to the 44th week of lactation at either 10, 13.5, or 17 inches of vacuum and for a "normal" or "twice normal" duration. Of these animals, 24 were Guernseys and 24 were Holsteins. Within each breed group, 12 were first-calf heifers and 12 had one or more previous lactations. Within each breed-lactation group of 12, 6 were selected for being "clean" of mastitis and 6 for being "infected." The latter was based on the occurrence of a clinical case of mastitis and/or the occurrence of more than 500,000 leucocytes /ml of milk and/or the presence of mastitis organisms in the milk during the standardizing period. The following results were observed: Animals milked at 13.5 inches of vacuum had greater weekly increases in chloride content of the milk than did those milked at 10 and 17 inches. No effect of vacuum level on any of the other criteria studied could be ascertained. Twice normal milking duration resulted in greater positive weekly changes in leucocyte number and chloride content of milk tha''n did the normal milking duration. Animals classified clean with respect to mastitis and first-calf heifers had fewer leucocytes, less pronounced weekly rates of increase in leucocytes, greater percentages of milk samples free of mastitis organisms and lower udder fibrosis scores than did animals classified infected and those with one or more previous lactations, respectively. Holsteins in this experiment had on the average a higher chloride content in their milk samples than did Guernseys. Teat-end morphology as indicated by scores had a tendency to change with an increase in vacuum level and milking duration; however, the differences in the scores were not statistically significant.