Sources of variation in residual milk and fat in dairy cows : their relation to secretion rates and persistency of lactation.

Abstract

Duplicate determinations of residual milk and fat were made at milkings following intervals of 10, 14, and 24 hr at two stages of lactation in 12 cows. The data obtained thus represented 144 milkings. The amount of residual milk was correlated with total yield whether differences in total yield arose from differences between cows, stages of lactation, milking intervals, or error variation. The effect of changes in total yield was greater in early lactation than later, but within either stage of lactation the same relationship persisted approximately over most of the variables examined. However, between different milking intervals, change in total yield produced somewhat less effect on residual milk than when total yield varied from other causes. There were also differences between cows in the degree to which residual milk was affected by variation in total yield. For early and late lactation, and for milkings at 10 and 14-hr intervals, the mean relative amount of residual milk was constant at 17.8 per cent. of total yield. For 24-hr milkings it was 13.9 per cent. Independently of differences in yield, cows differed significantly in amount of residual milk. These differences were highly correlated with differences in persistency of lactation, low persistency being associated with high proportion of residual milk. Seventy-five per cent. of variation in persistency was accounted for by differences in residual milk. Fat percentages of residual milk differed between cows, between stages of lactation, and between milking intervals. The data were used to calculate rates of secretion of milk and fat, and fat percentage of milk secreted, during intervals of 10, 14, and 24 hr. With cows normally milked at intervals of 10 and 14 hr, secretion rate was not significantly less during the longer interval, and any difference which may exist must be extremely small. Fat percentage of milk secreted increased when the milking interval was lengthened. Imposition of a single 24-hr interval on cows normally milked twice daily reduced milk secretion rate by only 15 per cent., but there was also an aftereffect of the long interval upon immediately subsequent secretion rate.