Rumen degradable nitrogen

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

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). 

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

Non-protein materials such as urea are used to provide a supply of degradable nitrogen for the rumen bacteria and thus protein for the host ruminant. Care is needed to ensure an adequate supply of energy and sulphur for the bacteria. 

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. 

The chemical nature of crude protein in feedstuffs is the primary factor determining how rapidly it is degraded to ammonia or escapes microbial degradation. To compare feedstuffs, feed nitrogen can be divided into NPN, true protein, and unavailable fractions, which Pichard and van Soest (1977) labeled as the A, B, and C fractions, respectively (Fig. 18.3). The A fraction is rapidly attacked by rumen bacteria and converted to ammonia. Approximately 20% of the crude protein in SBM is in the A fraction and is degraded in the rumen at a rate of 300%/h (NRC, 1996). In contrast, a more undegradable protein source like distillers grains has 6% of the crude protein in the A fraction. 

The unavailable or C fraction nitrogen is estimated by measuring the amount of acid detergent insoluble protein (van Soest, 1991). This fraction is assumed to have zero availability in the rumen and small intestine, and thus has no nutritional value. SBM is a highly digestible protein source with only 2% of the protein in the C fraction (NRC, 1996). In contrast, many slowly degraded protein sources have 10—20% of the protein in the C fraction. 

In the case of ruminants, the tables now include the digestible amino acids in the intestine, calculated according to the method published by Rulquin et a/. (1993 and 2001). In addition to updating the in sacco degradability values for nitrogen in the rumen and the intestine, we have indicated the kinetic parameters of in sacco degradation of dry matter and starch in the rumen. We have also included, when the data were available, the levels of absorbed phosphorus for ruminants. 

The degradative and synthetic processes taking place in the rumen are of major importance in the nitrogen economy of the host animal since they determine the nature of the amino acid mix made available for protein synthesis at tissue level. The series of changes undergone by dietary protein between mouth and body tissue in the ruminant animal is illustrated schematically in Fig. 13.2. 

Satisfying the demands of the rumen microorganisms for readily available nitrogen is a major fimction of the diet, and to this end a certain proportion of the nitrogen fraction must be degradable by the rumen microorganisms. 

Nitrogen fractions within the diet will vary in their susceptibility to breakdown, from immediately degraded to undegradable, and from 0 to 1 in the extent to which they are degraded in the rumen and digested when they reach the small intestine (see Table 13.8). 

The extent to which a nitrogen fraction is degraded in the rumen will depend upon its innate degradability and the time it spends in the rumen, i.e. rate of passage. As the rate of passage increases, so the extent of ruminal breakdown is reduced. 

This involves incubation of the food in synthetic fibre bags suspended in the rumen, as described in Chapter 10. The degradability is calculated as the difference between the nitrogen initially present in the bag and that present after incubation, stated as a proportion of the initial nitrogen ... 

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