Feeding yeast to dairy cattle has been a common practice for decades. The use of yeast, however, is most common in a variety of other industries such as baking, brewing or winemaking. It is likely if you walk into your kitchen and open the cabinet, there will be some of the small packets there; yeast use is just that common. Beside the industries listed, yeast is used in a wide variety of other applications where a fermentation process is required. Other microbia such as different bacteria or fungi are also used, but as noted, yeast is very common.
As a reminder, yeasts are very small, single-celled organisms which, when put into the presence of a food source (liquid and oxygen), begin to grow and reproduce. They then consume the food substrate and subsequently reproduce rapidly (make more yeast cells) as well as produce a variety of metabolic products. This may be a gas such as carbon dioxide, which causes bread to rise, or alcohol, as in brewing or wine making.
Yeast feeding applications in the dairy industry have grown and evolved over the years. Originally, feeding a basic yeast culture was the most common. A typical culture has only a very small amount of actual yeast, and this yeast is not “active.”
A variety of mechanisms have been suggested to explain changes in the activity of the rumen and improvements in performance when ruminants are fed different yeast products. For example, yeast may have a “buffering” effect in the rumen by mediating sharp drops in rumen pH, which follows feeding of higher-concentrate rations. As such, yeast may help to reduce excess lactic acid production when ruminants are fed higher-concentrate diets. While yeast products are not actual buffers per se, their function and activity appear to provide support to rumen pH.
At higher pH levels, however, when feeding yeast, research has found increased numbers of rumen cellulolytic (cellulose-digesting) bacteria and improvements in fiber digestion. One of the most common types of yeast fed, Saccharomyces cerevisiae, has been shown to stimulate the growth and activity of rumen bacteria. The response is different depending on the exact strain of the yeast.
There has been debate on the efficacy of specific forms of yeast products. The two broadest categories are seen as live yeasts and inactive or dead yeasts. Live yeasts are fairly well-defined. A culture of a specific yeast is grown, collected, freeze-dried and packaged. This yeast can then be rejuvenated as it is added to a ration or feed mix and exposed to moisture and air. At this point, it begins its activity. While different claims are made, the general understanding of the effect of a live yeast in the rumen is that it will scavenge (consume) oxygen from the system, thus making the rumen bacterial activity more efficient.
Secondly, a true live yeast has a limited shelf life after being taken out of its packaging. In most cases, a live yeast needs to be fed very soon after unpackaging to ensure its activity and to achieve the greatest result in the animal. In circumstances where a live yeast product is blended into a premix that is fed over a period of time (days or even weeks), it is highly likely the yeast will lose most or all of its activity. These types of products are also not tolerant of pelleting. More recently, companies have developed protected-live-yeast products that can tolerate longer periods in a premix environment and possibly even pelleting, but this should be evaluated closely.
Finally, live yeast products are normally fed at a much lower level (1 to 2.5 grams per head per day) and are much more expensive per unit of product.
Inactive yeasts come in a variety of forms – the most common being the yeast culture. A yeast culture is a blend of yeast cell materials, metabolites from yeast growth and respiration, and the media the yeast is grown upon. These yeast cells are not viable, meaning they do not become active/reproductive and have metabolic activity in the rumen. The yeast culture is essentially a nutrient source for the rumen microbial with the carrier, cellular material and metabolites believed to provide specific nutrients (prebiotic) to the rumen. This is the most common form of yeast product fed. It is commonly less expensive per unit when compared to a live yeast but is fed at a higher level, commonly from 28 to 56 grams per head per day.
Another form of yeast which is purer, although still inactive or nonviable, is often referred to as a functional dried yeast. This purer yeast product contains little or no media on which the yeast was grown. A functional dried yeast is commonly sourced from the fermentation of cane molasses or other substrates into ethanol. The yeast is recycled through the fermentation process, and the cells are collected periodically. The yeast cells are then processed (autolyzed, hydrolyzed, etc.) and finally dried and packaged. The functional dried yeast is similar to yeast culture in that it can act as a prebiotic, providing specific nutrients to the rumen bacteria, but the nutrient levels are typically higher. For example, protein levels in these products may range from 35% to 40% protein and are often in the form of amino acids or peptides. This greater nutrient density commonly leads to greater responses by the rumen microbia.
Because the product is largely made up of the complete cell, it may also include cell wall contents, such as mannanoligiosaccharides (MOS) and beta glucans (BG), which may be indigestible in the rumen. For this reason, it is important for these products to be autolyzed (mechanical) and/or hydrolyzed (enzymatic) to break down this cell wall and provide greater access to the cell contents. The MOS and BG component is valuable in the small intestine, as research has shown these functional fibers have multiple benefits including the binding and agglutination of pathogens and mycotoxins, and they can stimulate the epithelial lining of the gut, reducing inflammation, improving immune response at that location and improving absorption. Given the ruminal and intestinal effects, these products may provide broader effectiveness in the animal.
Feeding rates of this product are more moderate (10 to 20 grams per head per day for lactating cows) and are also moderate in cost.
Besides the debate on the type of yeast product to feed, there are questions on how to feed them. Some products on the market guarantee high numbers of live yeast cells (e.g., 20 X 1010 colony forming units per gram) with low recommended feeding rates (5-20 grams per day or less). Other products suggest that live organisms are not required for beneficial effects and that the end products (metabolites produced by the yeast cells) are the active ingredients. No doubt these debates will continue. It has also become increasingly common to feed a combination of live and inactive yeast products.
Feeding yeast can be a valuable part of a well-designed dairy feeding program. Primary end effects include the reduction of stress, improvement of performance, and improvement in gut and overall animal health. Since there are a fairly large number of different types of yeast products on the market, it is important to carefully research which products may be the most effective and economical for any given dairy.
