Effects of feeding high carbohydrate or fat diets. 2. Apparent digestibility and apparent metabolizable energy of the posthatch poult

An experiment was conducted with turkey poults to determine the apparent digestibility and derivation of ME from diets containing a high proportion of carbohydrate from corn (CHO; 60% of diet) or 10% supplemental fat from an animal-vegetable blended fat (FAT) or a synthetic medium-chain triglyceride (MCT). Poults fed the FAT diet consumed more feed from 6 to 8 and 9 to 11 d of age than poults fed the CHO diet, intake of the MCT diet was intermediate. From 3 to 11 d of age, the percentage apparent digestibility of nonlipid DM by poults fed the CHO diet was greater than that observed for either the FAT or MCT diets (P < or = 0.05). The percentage apparent digestibility of lipid was consistently greater for poults fed the MCT diet (> or = 90%) and could be attributed to the high apparent digestibility of C(8:0) (>95%), which accounted for 76% of total MCT dietary fatty acids. Over the course of the experiment, the mean percentage apparent digestibility of lipid in the CHO and FAT diets was 76.4 and 70.8%, respectively, and did not change with increasing age. The lower percentage apparent digestibility of lipid in the CHO and FAT diets was attributed to the low apparent digestibility of C(16:0) (70.4, 52.7%) and C(18:0) (58.4, 26.8%), respectively. The apparent digestibility of the polyunsaturated fatty acids, C(18:2) (n-6) and C(18:3) (n-3), was consistently high and ranged from 72 to 85% and 81 to 88%, respectively. The CHO diet resulted in an approximate 6% increase in determined AME(n) compared with either the FAT or MCT diets. The results of this study showed that commercial feed-grade fats are poorly digested by very young poults. The digestibility of polyunsaturated fatty acids, however, was shown to be quite high, and the data suggest that vegetable oils containing high proportions of polyunsaturated fatty acids or alternative fat sources containing high proportions of medium-chain fatty acids could be well utilized by the very young poult.