Feed grade fat was included at levels of 2 to 6% in corn-based laying hen rations and at levels of 3 to 6% in laying hen rations containing 8% wheat middlings. Added fat did not affect rate of egg production or average egg weight during the 196-day experiments. Feed consumption and efficiency of feed utilization were improved by added fat. Ration metabolizable energy (ME), as measured experimentally, was increased more than expected on the basis of commonly accepted ME values of feed grade fat when fat levels of 4 and 6% were used in cornbased rations. This effect of fat was most pronounced with the rations containing 8% wheat middlings. Assuming that the entire increase in ration ME's associated with added fat was due to fat per se, estimates of the ME content of fat were 8675 and 9015 mcal/kg when used in cornbased rations. When used in the ration containing 8% wheat middlings, the apparent ME of fat exceeded its gross energy content considerably, indicating an interaction effect between added fat and ingredient composition of the ration. Dietary fat level did not change the proportion of ME consumed that was converted to egg energy. Dietary ME to egg energy conversions ranged from 25.7 to 27.9%. The proportion of ME used for weight gain was increased slightly by added fat while no consistent changes in energy estimated as heat increment were observed.1979 Poultry Science 58:900-905

The influence of an indigestible and noncombustible material, silica gel (SG) on the excretion of energy by roosters and on the true metabolizable energy (TME) of corn was determined. Energy excretion by fasted roosters increased linearly with each increment of SG force-fed. The equation, energy excreted (kcal/24 hr) = 9.87 + (.47 x g SG) described this effect of SG. The data also showed that 24 hr was insufficient time for SG to completely clear the digestive tract, irrespective of amount force-fed. The TME of corn was 4.05 kcal/g dry matter when the grain was force-fed alone to roosters, when corn was force-fed in mixtures containing increasing proportions of SG, the TME of corn decreased with each increment of SG in the mixture. It was concluded that the extra energy excreted as a consequence of the presence of SG in mixtures with corn caused an underestimation of corn's TME. The regression coefficient relating energy excretion to amount of SG forced-fed in the previous trial was used to correct energy excretion by roosters fed corn-SG mixtures, and corrected TME's for corn were calculated. The average corrected TME of corn was 3.97 kcal/g dry matter. These data illustrated that an indigestible material passing through the gastrointestinal tract of the chicken changed the amount of fecal metabolic energy excreted and, consequently, influenced the TME value of companion feedstuffs when the conventional TME assay procedure was used.