Effects of Vacuum Level and Milking Duration on Guernseys and Holsteins Differing with Respect to Lactation Number and Status of Udder Health. II. Milk Yield, Milking Time, and Rate of Milk Flow

Summary Forty-eight animals from a Streptococcus agalactiae -free herd, after a 4-week standardizing period immediately after calving, were assigned to be milked from the fifth to the 44th week of lactation at either 10, 13.5, or 17 in. of vacuum and for a “normal” or “twice normal” duration. Of these animals, 24 were Guernseys and 24 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, six were selected for being “clean” of mastitis and six for being “infected.” The following results were obtained: 1. No appreciable differences in milk yield and FCM were observed between vacuum levels. Milking time and time to reach peak rate of flow decreased in a linear manner and rate of milk flow increased. The least amount of time spent in stripping was at the 13.5 in. vacuum level, and the least amount of strippings, expressed as a percentage of the total yield, was at the 10-in. level. 2. Leaving the milking machine on the udder for an extended duration (twice normal) resulted in slightly greater yields of FCM (1.01b. per cow per day) and decreased stripping time. 3. In this experiment, mastitis infected animals gave greater milk yields and had slower rates of milk flow than did the mastitis clean animals. 4. The differences between first-calf heifers and animals with one or more lactations and between Guernseys and Holsteins agreed in general with previously published studies.