Yeast and yeast cultures have been fed to ruminants for more than 60 years. The first studies on the use of Saccharomyces cerevisiae in dairy cattle and beef diets began in the decade between years 1940 and 1950. Since then, a huge amount of research on live and dead yeast has been done, but mechanisms to explain their role in ruminant nutrition is still poorly known.
To be effective, yeast cultures fed to ruminants should have some characteristics, such as a high concentration per head per day (more than 1 billion UFC) and to be stable in rumen.
It has been suggested that the yeast cultures are able to grow in the rumen, at least for a short period of time; to do that, yeasts should use the trace amounts of dissolved oxygen thereby stimulating growth of rumen bacteria, to which oxygen is toxic.
However, for some, yeasts are not able to live in the rumen due to the lack of oxygen; the ideal range for the development of Saccharomyces cerevisiae is between pH 4.3 and 4.6, while at pH 6.5 (average pH of rumen environment), their reproduction is markedly reduced. Under these pH conditions, the Saccharomyces cerevisiae cell slows down its reproductive activity, but reacts to the adverse conditions by producing a greater quantity of nucleotides, amino acids and vitamins, together with a very large quantity of enzymes that contribute to autolytic effect on its cell wall.
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