A digestion trial was conducted to study the effect of age and level of feeding on the digestibility of feed. The 36 Holstein heifers used were part of a growth experiment in which an attempt was made to calculate the total digestible nutrients (TON) required to obtain 0.8, 1.2, and 1.6 pounds average daily gain (low, medium, high). The digestibility values were determined by either the total collection or chromic oxide indicator method, using a diet of two parts hay to one part grain mixture. Feed allowances were based on average TON values of corn (75%) and hay (50%). Three age levels, 7-, 14-, and 19-months, were investigated to compare age effects on digestibility of feed at each of the three feeding levels. Twelve heifers at each age level were divided into low, medium, or high level of feeding. There were no significant differences (P\u3e.05) between the digestion coefficients of energy, crude protein, organic matter, or in TDN at any one of the three levels of feeding, but crude fiber digestibility was significantly lower at the low level of feeding. Significant differences (P\u3c.05) were noted in the digestibility of crude protein, crude fiber, organic matter, energy, and in TDN between the 7 month-old heifers and both of the older groups
