Protein metabolism in the gastro-intestinal tract of lactating dairy cows
Chapter 1 – Introduction
One of the main purposes of improving our knowledge of nutrient requirements and supplementation to ruminants is to optimize resource (recourse) utilization and minimize production costs. Therefore, to predict production responses in dairy cows, effective and reliable feed evaluation systems are required.
The knowledge of protein feeding and protein evaluation of diets for lactating dairy cows have increased considerably during the last 40 years, from use of total or digestible crude protein (DCP) to the new protein evaluation systems (Madsen, 1985; NRC, 1985; Vérité et al., 1987; AFRC, 1992; Tamminga et al., 1994). The major disadvantage of the DCP system is that it does not take into account protein degradation and microbial protein synthesis in the rumen, and thus makes it difficult to optimize the protein supply to the rumen microbes and to predict the net supply and of absorbable amino acids (AA) to the host animal. Consequently, in the new protein evaluation systems prediction of amounts of AA absorbed in the small intestine orginates from dietary protein escaped rumen degradation and from microbial protein synthesized in the rumen. Although these systems differ in terminology and detail, they are conceptually similar in their aim to describe the N metabolism in the gastro-intestinal tract of ruminants. In Norway, the AAT/PBV system (Madsen et al., 1995) was officially introduced for use in practice from January 1993. Although the system has weaknesses, it has improved the efficiency of protein feeding of dairy cows, and it has become an important tool to reduce the N-excretion at the animal level, and thus reduced the potential risk of N-pollution. However, still 60 to 80% of ingested N in dairy cows is excreted in feces and urine (Volden, 1999, VII). Therefore, to further improve our protein evaluation system more information about factors influencing the protein metabolism in the gastro-intestinal tract is needed. Firstly, one of the main challenges is to more accurately predict the amount of energy available for microbial growth, e.g. information is needed on the relationship between ruminal retention time and ruminal digestion of organic matter. Secondly, better prediction is required on the microbial AA composition and the variation in the yield of microbial protein synthesis. Thirdly, more information is needed on the proportion, the AA profile and the intestinal digestibility of the ruminally undegraded protein (RUP). Fourthly, the efficiency of AA conversion into milk protein needs to be known. On this background the main aims of the experiments in this thesis has been to study:
1. The chemical composition and AA profile of rumen microorganisms. 2. Rumen microbial protein synthesis. 3. Ruminal degradation and escape of dietary protein. 4. Ruminal degradation and escape of intraruminally supplemented free AA. 5. Intestinal digestibility of RUP.
When studying protein metabolism in the gastro-intestinal tract the results obtained may to some extent depend on the experimental methods applied. Sometimes the choice of methods may not seriously affect the relative differences between dietary treatments but rather the absolute values, which, however, is of rather great importance when comparing results from different studies. To enable improvements of our protein evaluation systems, we need to develop new and better methods as well as improve existing methods for studying protein digestion and metabolism in ruminants. Therefore, a further objective of this study has been to evaluate methods for measuring microbial chemical composition, microbial protein synthesis, ruminal degradation of dietary protein and intestinal digestibility of RUP.
Pr. desember 2004: Harald Volden, Postboks 5003, 1432 Ås
