The influence of free fatty acid content and degree of saturation on the apparent metabolizable energy value of fats fed to broilers

To determine the influence of free fatty acid (FFA) content on the subsequent nutritive value of fats, the following were studied: tallow (FFA content of 138.4 g/kg) and tallow acid oil (FFA = 952.1 g/kg) in Experiment 1; palm oil (FFA = 57.5 g/kg) and palm acid oil (FFA = 917.5 g/kg) in Experiment 2; and soybean oil (FFA = 14.4 g/kg) and soybean acid oil (FFA = 683.4 g/kg) in Experiments 3. The two fats within each experiment were blended in proportions to give three mixtures of intermediary FFA content Thus, each experiment investigated five fats by substituting them into a basal diet at the expense of the energy-yielding ingredients at rates of 40, 80, and 120 g/kg. Experimental diets were evaluated for apparent fat availability (AFA) of broiler chicks 1.5 and 7.5 wk of age. Corresponding values for fats were obtained through extrapolation of the linear response, or interpolation of the quadratic response, of AFA of diets. The AME was calculated as the product of fat AFA and gross energy. The degree of saturation had a pronounced effect upon the AME of fats; values decreased in the following order: soybean oil, tallow, and palm oil. Overall AME of fats linearly decreased with increasing FFA content, and the decrease was more pronounced with the younger birds. This rate of reduction appeared to be influenced by degree of saturation of the fat, but only with younger birds. Thus, AME of fats declined from 30.9 to 20.4, 27.7 to 14.8, and 38.5 to 33.1 MJ/kg (1 MJ = .239 Mcal) for young birds in Experiments 1, 2, and 3, respectively. Corresponding data for older birds were from 32.9 to 28.3, 32.3 to 27.5, and 38.5 to 35.1 MJ/kg. Effects of rate of inclusion of fats were minor and, generally, were confined to fats of a higher degree of saturation with greater contents of FFA evaluation with younger birds.