True metabolizable energy of fats at low level dietary inclusion.

When assayed at a 2.5% level of inclusion in a corn-soybean meal basal diet with 40 replications per treatment, no significant differences could be demonstrated between the true metabolizable energies (TME) of corn oil and two samples of tallow varying in stearic acid (18:0) content. All values were in excess of the gross energy of fat, suggesting an improvement in the absorption of other dietary constituents. When assayed at a 15% level in a purified basal diet, the TME of corn oil was significantly higher than that of the tallows; all values were below the gross energy of fat. Use of the practical corn-soybean meal basal improved the TME of the fats by 22.4% (corn oil), 34.9% (low 18:0 tallow), and 43.0% (high 18:0 tallow). A portion of the improvement in the TME of the tallows is presumed to be accounted for by an interaction with fatty acids in the practical basal ingredients that may be obscured at higher levels of inclusion. It is concluded that with adequate replication it is feasible to assay the TME of fats at low levels of dietary inclusion. A comparison of fatty acid absorption in Single Comb White Leghorn roosters during a TME study and in 8 to 9 week broilers on a full-feed regimen suggests that the conditions imposed during a TME assay provide a satisfactory model for evaluating the fat absorption of chickens reared under normal conditions.