Yeast products are widely utilized as feed additives for ruminant animals in many parts of
the world. There are a number of corporate groups that produce these products, which
are marketed under a wide variety of trade names. While the number of yeast products
that have undergone substantive evaluation in controlled research studies is somewhat
limited, there is a widespread belief among dairy and beef producers, and ruminant
nutritionists, that yeast products are beneficial by enhancing dry matter (DM) intake and
overall animal performance. Since yeast products are generally modestly priced,
economic barriers to their use are low.
Mechanisms have been proposed to explain why yeast products could stimulate DM
intake and productivity in growing and lactating cattle. Perhaps the oldest hypothesis is
that the yeasts are able to grow, at least for a short period of time, in the rumen thereby
directly enhancing fiber digestion and/or producing nutrients that stimulate growth of
rumen bacteria, which do the bulk of the fiber digestion. It has also been suggested that
the yeasts utilize nutrients, such as lactic acid that, if allowed to accumulate in the rumen,
could suppress bacterial growth and/or suppress DM intake by driving rumen pH down.
A more recently suggested possibility is that growth of yeast in the rumen utilizes the
trace amounts of dissolved oxygen, particularly at the interface of the cellulolytic bacteria
and fiber, thereby stimulating growth of rumen bacteria, to which oxygen is toxic. It
seems clear that for these mechanisms to be operative, yeasts in the product have to be
viable, in the sense of being able to grow for at least a short period of time in the rumen.
Hence the origin of the debate between ‘live’ and ‘dead’ yeast products.
The alternate mechanism is that it is the yeast culture itself, which is created in the yeast
fermentation process, provides a mixture of micro-nutrients which stimulates bacterial
growth in the rumen thereby facilitating increased fermentation of fiber and/or utilization
of the end-products of fiber fermentation to prevent their accumulation in the rumen.
Supporters of this theory point to a limited research base showing that when cultures of
live brewers or fermentation yeasts are fed to ruminants, there are few, if any, changes to
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rumen fermentation and/or animal performance. Indeed the view that live fermentation
yeasts are ineffective is so widely held that little research has been completed in the area.
The objective of this article is to summarize studies that have been published in the
scientific literature that have examined the impact of specific commercial yeast products
on rumen fermentation, fiber digestion and/or animal performance. The mechanisms by
which yeast cultures could be efficacious will only be addressed indirectly.