Amino Acid Composition of Microbial Protein

Formulation of non-ruminant diets for amino acid requirements is easier than that of ruminant diets since amino acid requirements of non-ruminant animals are provided only by dietary protein. In contrast, amino acid requirements of ruminants are met by a mixture of dietary protein and microbial protein. Briefly, the challenge is to predict: (i) what the requirements of the animal are, and (ii) the amino acids available for absorption in the small intestine. Therefore, the amino acid composition of rumen microbes is a crucial element of ruminant protein nutrition, as is the digestibility of microbial protein. A recent review (Hvelplund eta I., 2001) describes these issues in detail.

The calculations of the nutritive value of the amino acids in point (ii) focus mainly on the essential amino acids, principally lysine, methionine and histidine, which are first-limiting (Lobley, 1994). Microbial protein has a higher content of lysine and methionine than plant protein sources (Wallace, 1994), hence knowing its composition is vital. The general view for some time has been that rumen microbial protein has a fairly constant amino acid composition (Purser and Buechler, 1966; Bergen et a I., 1967). However, Hvelplund et al. (2001) provided new data based on 70 estimations (Table 15.3) suggesting that there is a genuine variation in amino acid composition of microbial protein. Unfortunately, the variation was particularly acute for lysine, for

Table 15.3. Dependence on energy substrate of the response in microbial protein yield (g) to amino acids in sheep. (From Chikunya eta!., 1996.)

Nitrogen source

Energy substrate



Grass hay



Sugarbeet pulp



which the highest measured concentration was almost twice the lowest concentration. Significant variation occurred also in methionine composition, exacerbating problems in diet formulation. Clearly an important research priority must be to explain, and then to predict, the reasons for this variation.

Ciliate protozoa have a greater content of lysine and methionine than bacteria (Czerkawski, 1976). However, protozoa are selectively retained in the rumen (Veira, 1986), which may explain why defaunation has been reported to have a minor effect on the composition of amino acids flowing to the small intestine (Merchen and Titgemeyer, 1992).

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