Protein Digestion

Some rumen microbes will effect considerable degradation of dietary protein, depending upon the susceptibility of that protein to digestion in the rumen. Proteins are initially reduced to constituent free amino acids and limited amounts of peptides of varying chain length, by the action of microbial proteases, followed by the further reduction of a significant proportion of the degraded amino acids to ammonia by microbial deaminases. Both ammonia and amino acids, and in some instances short chain peptides, can be used by the rumen microflora for the synthesis of microbial protein. The most commonly occurring plant protein is ribulose 1.5 bicarboxylase, often referred to as Rubisco or fraction 1 protein. It is the principal photosynthetic enzyme of green plants and is found extensively in grasses and legumes. It has high inherent solubility and as such is readily degraded by rumen microbes, which accounts for the increased levels of rumen ammonia that often accompany large intakes of fresh forage. Other proteins are also found in forages, including membrane proteins but these tend to be less degradable although they can make some contribution to the overall supply of nitrogen to the rumen microbes.

Some forages, especially certain legumes, contain significant amounts of tannins and after ingestion these can react with some of the proteins in those forages to form insoluble complexes which are not capable of being degraded by the rumen microbes. This is a naturally occurring rumen protection and in specific forages such as sain-foin and lotus, research has shown that this may lead to marked increases in the amount of protein which ultimately reaches the small intestine. Sain-foin and lotus are however not widely grown and most forms of ruminal-protected protein that are fed have been produced by pre-treatment of the feed. One obvious example of this is formaldehyde-treated soya bean meal, sold in some markets whilst an alternative approach has relied on the treatment of protein feeds with wood sugars. These contain significant amounts of xylose and xylitol and promote the formation of some products which are equally resistant to rumen degradation. Ruminal-protected protein feeds can be used to enhance total supply of protein to the intestines and to the animal when microbial derived protein is considered to be limiting.

As indicated earlier, ammonia is a key substrate for microbial protein but certain bacteria show considerable propensity to produce ammonia in excess of their own specific requirements. These are known as the high ammonia producing (HAP) bacteria and together with rations which contain high levels of soluble protein, can contribute to high rumen ammonia levels which are in considerable excess of the ability of the microbes to assimilate ammonia into microbial protein. The concept of rumen synchrony was proposed several years ago in which rations would be developed to provide balanced amounts of ruminal degradable carbohydrate and protein. Whilst this approach had sound scientific grounds, subsequent research was unable to establish sizeable benefits and has now generally been abandoned. However, it is pertinent to note that total mixed ration feeding will provide some opportunities in this respect although direct research evidence does not exist.

Author: Denis Dreux