Plasma nitrogen changes with protein supplementation in sheep

Abstract

Ruminant metabolism is essentially dependent upon the presence of rumen microorganisms. These can digest plant fibre, otherwise indigestible, ferment carbohydrate and hydrolyse protein and non-protein compounds, both dietary and endogenously secreted, to degradation products (Hungate 1966).

Ingested protein is initially converted in the rumen to peptides and amino acids and these are degraded to ammonia and fatty acids (Hungate 1966). Ammonia is also produced from the non-protein nitrogen fraction of the diet and from endogenous nitrogen compounds.

Some of these degradation products may be absorbed through the wall of the gastrointestinal tract, chiefly in the rumen, but most are used for the synthesis of microbial protein, both enzymic and structural (Weller et at. 1958, McDonald and Hall 1957).

The synthesis of microbial protein is known to be closely connected to carbohydrate fermentation (Hungate 1966).

The protein presented for gastric digestion in the abomasums is mainly bacterial and protozoal and is of a constant biological value (Purser & Puechler 1966). In the acid conditions of the abomasums, which is analogous to the conventional mammalian stomach, the bacterial and protozoal cells are killed proteolysis occurs; amino acids are released for subsequent absorption in the small intestine (Harrison & Hill 1961).

In particular bacteria synthesize the essential amino acids of the ruminant, a function inherently lacking in the tissues of mammals (Downes 1961). The essential amino acids are absorbed in the small intestine and have a special significance for wool growth (Little & Mitchell 1967).

Extensive ruminal degradation of dietary proteins is inefficient for ruminant nutrition, particularly proteins with a suitable balance of amino acids for significant nitrogen retention and wool growth, as wastage occurs through absorption of ammonia from the rumen and subsequent excretion in the urine as urea (Little & Mitchell 1967, Mcdonald 1948). Protection of dietary protein with formaldehyde prevents ruminal microbial attack and increases the essential amino acids reaching the small intestine for absorption (Ferguson, Hemsley & Reis 1967).

The experimental work described here was designed to study aspects of nitrogen metabolism in sheep fed casein and formalin-treated casein supplementary to a basal maintenance diet. To attain an overall picture of nitrogen balance in the sheep, excretion of nitrogen in the faeces and urine must be determined in addition to nitrogen intake and utilisation. However the work described here is limited to the study of plasma urea nitrogen levels and amino acid levels; in effect the sequestering of dietary nitrogen into specific chemical components of the blood plasma.