Significance of Intestinal Digestion of Dietary Protein

In the last twenty years, elaborate models have been developed to formulate diets with greater accuracy in meeting the protein and amino acid requirements of lactating dairy cattle. Models for feeding protein to dairy cattle in the USA evolved from basic crude protein (NRC, 1978) to more complex systems based on rumen undegraded protein (RUP) and intestinal digestion of RUP (NRC, 2001). The NRC (1989) recognized that intestinal digestion of protein supplements differs; however, empirical data were lacking and as a result, a constant value of 80% was used for all feeds. The main problem was the lack of reliable techniques for estimating intestinal digestion of proteins. With improved techniques, the NRC (2001) assigned estimates of intestinal digestion to RUP fractions for each feedstuff. Data were obtained mainly from literature using the mobile bag technique and the three-step in situ/in vitro procedure of Calsamiglia and Stern (1995) using pepsin-pancreatin to predict digestibility of RUP. Values used in the French Protein System were adopted for feeds with limited or no data. Because a considerable amount of variation in intestinal protein digestion has been observed among and within feedstuffs using the three-step in situ/in vitro procedure, there is a need to improve the procedure so that a greater number of samples can be analyzed more rapidly.