Abstract Current equations for calculating the net energy (NE) of feed ingredients fed to pigs were developed 30 to 40 years ago based on animal work conducted before that time. The equation most often used in the United States is an equation that simply adjusts the digestible energy (DE) of the feed ingredient for concentrations of energy containing nutrients in the ingredient with a positive contribution from lipids and starch and negative contributions for fiber and crude protein. Specifically, in this equation it is remarkable that the largest negative contribution is for the protein fraction, which results in feed ingredients with high concentrations of protein having low calculated values for NE. Because the equation is based on work conducted around 40 years ago, the equation reflects the utilization of the energy containing nutrients by the genotypes of pigs used at that time. Specifically, the older genotypes of pigs had a retention of nitrogen of about 40 to 45% of ingested nitrogen, whereas currently used genotypes of pigs have retention of nitrogen that is between 65 and 70% of ingested nitrogen indicating that modern pigs are more efficient in converting dietary protein into body protein. As a consequence, the energy contribution from the protein fraction in diets is greater if the diet is fed to modern genotypes of pigs compared with older genotypes. The calculated increase in NE in soybean meal due to the increased nitrogen retention is around 170 kcal per kg. In addition, based on a large number of experiments, it is also clear that current book values underestimate the DE in soybean meal by more than 200 kcal per kg, which also translates to an underestimation of around 170 kcal per kg NE. Therefore, the improved nitrogen retention by modern genotypes of pigs, and the corrected values for DE combined indicate that soybean meal contains at least 340 kcal per kg more NE than previously believed. The NE in corn is believed to be close to 2,700 kcal per kg whereas the NE of soybean meal is believed to be only around 2,100 kcal per kg or 78% of that in corn. However, with the updated values for DE in soybean meal and taking the increased efficiency of nitrogen utilization into account, the updated calculated NE in soybean meal is around 2,430 kcal per kg, which is close to 91% of the NE in corn. Indeed, results of a large number of recent experiments in the United States, Canada, Brazil, Mexico, and China, indicate that NE in soybean meal is within 10% of the NE in corn, and there is, therefore, strong evidence that the NE of soybean meal is greater than currently believed.
Hans H Stein (Wed,) studied this question.