Apparent metabolizable energy of crude glycerin originating from different sources in broiler chickens

An energy balance experiment was conducted to determine the AME(n) of various crude glycerin samples, and to generate an equation to predict AME(n) of crude glycerin based on its chemical composition. Dietary treatments consisted of a corn-soybean meal basal diet with no added glycerin and a basal diet supplemented with 6% glycerin. Crude glycerin samples were obtained from biodiesel production facilities throughout the United States, which use a variety of lipid products as their initial feedstock. Two identical energy balance trials were conducted. In each trial, 864 male broilers (Ross × Ross 708) were fed a common starter diet until 17 d of age when they were switched to 1 of 12 experimental diets (6 replicates per treatment) from 17 to 22 d of age, with a 48-h collection period on d 21 and 22. Nitrogen-corrected apparent metabolizable energy values of crude glycerin samples were estimated by difference, whereby AME(n) of the basal diet was subtracted from the complete diet containing the test ingredient. The AME(n) of the basal diet and US Pharmacopeia-grade glycerin were determined to be 3,085 and 3,662 kcal/kg, respectively, whereas the AME(n) of the 10 crude glycerin samples ranged from 3,254 to 4,134 kcal/kg. Two crude glycerin samples had high levels of fatty acids compared with the other samples (24 and 35% vs. <0.30%), and even though their AME(n) was higher than that of the other glycerin samples (3,806 vs. 3,611 kcal/kg, P < 0.01, respectively), their AME(n) as a percentage of gross energy (GE) was lower than that of the other samples (65.5% vs. 97.4%, respectively; P < 0.01). Including all of the glycerin samples, the stepwise regression equation to predict AME(n) was determined to be: [AME(n) (kcal/kg) = 1,605 - (19.13 × % methanol) + (39.06 × % fatty acid) + (23.47 × % glycerin)]; (R(2) = 0.25; SE = 379; P ≤ 0.01). These data indicate that glycerin is a good source of energy for broilers, and the AME(n) of glycerin is dependent on fatty acid, methanol, and water contents.