Implications of Increased Nitrogen Retention in Modern Pigs

Abstract Modern genotypes of pigs have greater daily protein synthesis and, therefore, greater requirements for digestible amino acids than older genotypes. However, a less obvious characteristic of modern pigs is that they utilize dietary protein with greater efficiency than older genotypes and nitrogen retention measured as a percentage of nitrogen intake is much greater in modern genotypes of pigs than in older genotypes. In fact, a linear increase in nitrogen retention has taken place over the last 60 years and nitrogen retention during that time has increased from less than 35% to more than 65%. To some degree, this increase may be a consequence of greater accuracy in diet formulation, but the largest increase is caused by genetic changes of the pigs because the continued selection for leaner genotypes has resulted in selection of pigs that are more efficient in converting dietary protein into body protein. The consequence of the increased nitrogen retention is that the energy needed to deaminate amino acids and excrete nitrogen via the urea cycle is reduced and more amino acids are used for protein synthesis, which has greater energy efficiency than deaminated amino acids. Therefore, the energy value of the protein fraction in diets is greater if diets are fed to modern genotypes than to older genotypes and equations to predict net energy of diets and feed ingredients need to be updated to reflect this change. Another consequence is that the net energy of high protein ingredients fed to modern pigs is greater than when the same ingredient was fed to older genotypes. Results from recent research also indicate that nitrogen retention is influenced by the type of diet used and pigs fed diets containing only corn and soybean meal or corn, soybean meal and 2 or 3 synthetic amino acids have greater daily nitrogen retention than pigs fed diets containing 4 or 5 synthetic amino acids even when all diets are formulated to contain the same quantities of digestible indispensable amino acids. It therefore appears that modern pigs are better at utilizing amino acids from intact protein than from synthetic amino acids. It is possible that this is a consequence of synthetic amino acids being absorbed faster than amino acids from intact protein and when these synthetic amino acids arrive at the cells in the body, there is a lack of other amino acids that are being supplied by the intact proteins, which prevents protein synthesis and nitrogen retention, but this hypothesis has not been experimentally verified. Nevertheless, it is clear that modern genotypes of pigs have greater nitrogen retention than older genotypes and because of that, net energy of diets is changed, and amino acids appears to be utilized differently.