Rumen degradability

The extent to which dietary protein is degraded to ammonia in the rumen, and conversely the extent to which it escapes rumen degradation and is subsequently digested in the small intestine, is discussed in Chapters 10 and 13. At this point it is sufficient to emphasise that with most diets, the greater part (and sometimes all) of the protein reaching the ruminant s small intestine will be microbial protein of reasonably constant composition. The lesser part will be undegraded food protein, which will vary in amino acid composition according to the nature of the diet. 

Animals fitted with a rumen cannula can be used to estimate the proportion of a food that is digested in the rumen, or rumen degradability. Typically, food samples (3 5 g of dry matter) are placed in small bags made of permeable synthetic material with a standard pore size (400 1600 pm ), which are then inserted into the rumen... 

The digestibility of foods may be reduced by nutrient deficiencies or excesses, particularly in ruminants. For example, a deficiency of rumen-degradable nitrogen or... 

Table 13.8 Composition, rumen degradation and intestinal digestion of protein fractions...

Fraction Composition Rumen degradation (%/hour) Intestinal digestibility (%)... 

The microbial demand for protein is stated in terms of effective rumen-degradable protein (ERDP) and foods have to be evaluated in the same terms. The ERDP of a food is calculated as follows ... 

Protein requirements for ruminant animals are stated in terms of metabolisable protein, and protein supply to rumen microbes are expressed in terms of effective rumen-degrad-able (ERDP) protein or effective rumen-degradable nitrogen (EDN). 

The amino acid composition of forages is clearly important when the feeds are used as protein sources for non-ruminants. For ruminants, however, the most important characteristics of forage proteins are their rumen degradability and their overall digestibility. In immature forages, both measures are commonly very high (0.7-0.8),... 

Table 19.10 Variation in rumen degradability of the protein of silages...

The digestibility and intake of straws may be improved by treatment with sodium hydroxide or ammonia (as a gas or derived from urea). Such treatment is expensive, and the more appropriate way of making the best use of straws is to supplement them, especially with a source of rumen-degradable protein. 

Blood meal is a chocolate-coloured powder with a characteristic smell. It contains about 800 g/kg of protein, small amounts of ash and oil, and about 100 g/kg of water. It is important nutritionally only as a source of protein. Blood meal is one of the richest sources of lysine and a rich source of arginine, methionine, cystine and leucine, but it is deficient in isoleucine and contains less glycine than fish, meat, or meat and bone meals. Owing to the poor balance of amino acids, its biological value is low in addition, it has a low digestibility. It has the advantage, in certain situations, that its protein has a very low rumen degradability (about 0.20). 

AA = amino acids, ADIN = acid detergent insoluble nitrogen, DUP = digestible undegradable protein, ERDN = effective rumen degradable nitrogen, LCFA = long-chain fatty acids,... 

Gressley, T.F. and L.E. Armentano, 2007. Effects of low rumen-degradable protein or abomasal Ihictan infusion on diet digestibility and urinary nitrogen excretion in lactating dairy cows. J. Dairy Sci. 90, 1340-1353. 

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