Effect of Level of Intake on Digestibility of Dietary Energy by High-Producing Cows

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Abstract
Total digestible nutrients (TDN) values of 3 diets each containing 48 to 75% of concentrates were determined in 107 trials with cows ingesting 1 to 5 times the amount of feed required for maintenance. Baseline TDN values were determined with the same diets fed to mature steers at the 1 times maintenacne level of intake. The average rate of depression in the TDN value was 4.0% (range, 3.41 to 6.22%) per increment of intake equivalent to the maintenance requirement for TDN. If it is assumed that the metabolizable energy value is proportional to the TDN value of the diet, these observations suggest that the dietary energy requirements expressed as TDN measured at the maintenance level of intake) increase per unit of milk produced as the milk output or feed input per day increases. Based on the average data obtained, a 1400 lb cow in body energy equilibrium would require in addition to her maintenance requirement the following amounts (lb) of TDN (measured at the maintenance intake) /lb of 4% fat corrected milk (FCM) produced at the following levels (Ib/day), respectively: 0.317, 10; 0.322, 20; 0.327, 30; 0.333, 40; 0.339, 50; 0.346, 60; 0.353, 70; 0.361, 80; 0.370, 90; and 0.380, 100.