I generally recommend feeding live yeast or yeast culture (Saccharomyces cerevisiae) to transition and early lactation cows. It's difficult to measure responses to yeast supplements on the farm with the many variables that can confound results.
Research on yeast supplementation has had variable results, but meta- analyses that combine the results of many experiments have been positive for production.
There are several theories on the mode of action, such as stimulating cellulolytic microbes and lactate utilizing microbes or providing certain nutrients or "growth factors” in the rumen that have not been identified.
It seems to me that if yeast supplements are going to be economically beneficial, feeding them in the transition period and early lactation, when cows are going through greater physiological and nutritional challenges is the best bet.
Yeast culture helped prevent milk fat depression in research published in the January 2009 Journal of Dairy Science. The researchers challenged cows by an abrupt increase in fermentable carbohydrate. The ruminally cannulated cows were fed either zero or .12 lbs. of Diamond V XP yeast culture. Dry ground corn fed at 36% of the dry matter was replaced by the same amount of finely ground high moisture corn to suddenly increase the amount of fermentable starch in the diet.
Milk fat decreased from 3.34% to 3.03% on the control diet, but was not affected by the dietary challenge when yeast culture was fed. Interestingly, the effect of yeast culture on milk fat was not related to its effect on ruminal pH. None of the other factors measured in the trial helped explain the mechanism of action of the yeast culture.
Other research with live yeast supplements has shown that rumen pH is maintained during nutritional challenges. So, although the exact mechanisms still seem to elude researchers, it appears that yeast supplements can help prevent milk fat depression.
Reference: Journal of Dairy Science Vol. 92 No. 1, 2009
--Rick Lundquist is an independent nutrition and management consultant based in Duluth, Minn. You can contact him at firstname.lastname@example.org.
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