Effect of Blending and Level of Inclusion on the Metabolizable Energy of Tallow and Tower Rapeseed Soapstocks

Abstract

Experiments were conducted with prime tallow (PT) and Tower rapeseed soapstocks (TSS) to determine the possible synergism between these fats by the total collection and chromic oxide (Cr2O3) apparent metabolizable energy (AME) methods as well as the true metabolizable energy (TME) assay and to examine the effect of inclusion level on the metabolizable energy (ME) values of fats. Graded levels of 0, 3, 6, and 9% of each of PT, TSS, and a 1:1 blend of the two were fed to groups of adult roosters, and the ME values of the test fats and the mixture were evaluated from a linear regression of the ME of the diets and the levels of fat inclusion. The total collection and Cr2O3 methods gave essentially the same AME values for PT as well as for TSS when these fats were fed separately. The 1:1 blend of these fats gave a somewhat higher value with the index method than with the total collection procedure. A positive synergistic effect of blending PT with TSS was observed in all three methods. Increases in the observed ME of the mixture over the calculated ME were 3.94, 5.26, and 5.72%, respectively, for the total collection, Cr2O3, and TME assays. When calculated by difference, the ME values of the two fats and the blend varied widely with inclusion level in all three assay procedures. The implications of use of a single level assay to determine the ME content of fats are discussed.