Validity of NRC method for estimating metabolizable energy value of laboratory dry canine diets

An equation to estimate the metabolizable energy (ME) content of practical dry canine diets, [metabolizable energy (MENRC, kcal/g) = 3.50 x crude protein + 8.46 x acid ether extract + 3.50 x nitrogen-free extract] has been recommended by the National Research Council (NRC), which assumes fixed digestibility for each nutrient. This estimation method is much more convenient than that of nitrogen-corrected metabolizable energy (MEn) following the equation MEn = digestible energy-1.25 x digestible crude protein. This study aimed to assess the validity of MENRC through a comparison with MEn determined by using 11 diets with 4 mature male Beagle dogs. The relation between MENRC and MEn was expressed as a quadratic equation (MENRC = 0.83MEn2 - 5.43MEn + 12.36, r2 = 0.956, P < 0.01), suggesting that MEn is overestimated when the NRC method was applied to lower energy diets. It was also suggested that the strict estimation of MEn by means of fixed digestibility coefficients was impossible, because of the relatively wide variation in digestibility among dry canine diets.