Sources of Variation in Milk Flow Characteristics

Kymograph milking rates on 87 cows at morning and afternoon milkings at periodic stages of lactation were studied for effects of stage of lactation and milk yield. Two machine types were compared, one with 30 to 33cm mercury vacuum (low), 48 pulsations per min, and a 1:1 milking to rest pulsation ratio; and a second with 38 to 41cm mercury vacuum (high), 60 pulsations per min, and a 2.5:1 milking to rest pulsation ratio. Traits highly related to milk yield (yield to .45 kg/min, yield during maximum rate, and total yield) had the highest variation among stages of lactation. Milking rate traits (maximum rate, duration of maximum rate, strip time, time to reach maximum rate, total time, average rate, and time to reach 45 kg/min) had least variation from stage-to-stage for the same cow. There was a significant interaction between machine type and stage of lactation for all traits except yield to .45 kg/min and total yield. For maximum rate, the high-vacuum machine had a much flatter lactation trend than the low-vacuum machine. As lactation progressed, the high-vacuum machine had progressively less milking time than the low-vacuum machine. However, the high-vacuum machine left more shippings than the low-vacuum machine. Regressions on milk yield were significant for all traits except time to maximum rate.