Lactation Studies. II. Comparison of Techniques to Measure True Milk and Fat Secretion Rates in Dairy Cattle as Applied to Between-Milking Intervals of Various Lengths of Time ,

Abstract

Summary Twenty cows were used over a 2-yr. period to compare three techniques of measuring rates of milk and fat secretion. Apparent secretion rates (pounds of milk obtained at normal milking times divided by hours in the between-milking intervals to get pounds per hour) were compared to true secretion rates which where determined by two methods, using oxytocin to obtain complementary milk. Oxytocin was used at four consecutive milkings and at four milkings spaced so as to effect recovery between injections. Three milkings schedules were employed, i.e., (10–14), (11–13), and (12-12). Actual secretion rates were computed and these also were converted to relative rates which expressed the relation between day-time and night-time secretion rates. The true secretion rate measurement which was most consistent was the one in which oxytocin was injected at consecutive milkings. It was necessary to consider differences in complementary milk left in the udder after milkings, even when a 12-12 hr. milking schedule was used, in order to measure true secretion rates. True secretion rates of milk and fat on a within-interval basis were increased significantly by decreasing the between-milking interval by 4 hr. relative to the other interval in the same day (14 and 30% higher for milk and fat, respectively). The milk and fat secretion rates were higher by 5 and 10%, respectively, during the 11-hr. intervals, as compared to the 13-hr. intervals, but were not statistically different. It was not possible to ascertain whether the 24-hr. rate was affected by changes in milking schedule, because schedule and lactation trend effects were confounded.