The currently recommended allowances of total digestible nutrients (TDN) and digestible protein (DP) for milk production and their bases were examined. It was concluded that the uncertainties of estimating the body gain or loss of energy and protein from body weight changes preclude the use of the conventional feeding trial to estimate requirements. As a consequenee, in the present studies the milk production requirements for TDN were estimated by the digestion-trial method and those for DP from published nitrogen-balance data. The TDN values of seven diets ingested at levels of intake ranging from one to five times the maintenance level were determined in 265 individual-cow trials. Per increment of intake equivalent to that of maintenance, the TDN value of the diets declined at rates ranging from 2.1 to 6.2% (average rate, 4.0%). These data indicated that the requirements for TDN (determined at maintenance) increase with increasing feed intakes (or milk outputs). Accordingly, the TDN requirements were computed for the average rate (4%) of depression in the TDN value observed. An examination of nitrogen-balance data indicated that the average rate of utilization of DP for the combined functions of milk production and maintenance of cows in body nitrogen equilibrium is of the order of 65%. This observation suggests that milking cows require an average minimum amount of DP in the diet equivalent to 154% of the quantity of protein in the milk. For usual 4% FCM, the requirement would be 24.4 g of DP per pound of milk. Since the DP value declines at about the same rate as the TDN value of the diet with increasing daily inputs of feed, the effect of feeding level was superimposed on the apparent net requirement for DP. This indicated that for cows in nitrogen equilibrium, the requirement would range from 25.3 to 30.0 g per pound of 4% FCM for daily yields of 10 and 100 lb, respectively. The data also suggest that milk production requires a DP:TDN ratio of 1:5.7.

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