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Understanding feeding fermented versus live yeast

Yeast is a common inclusion in dairy rations, says Dr Alison Bond, Rumenco’s commercial nutritionist; however, it comes in many guises, so it’s important to work with your nutritionist and ask the right questions. 

In recent years, yeast has become a staple part of the majority of dairy diets; with a reputation for complementing modern TMR rations and improving rumen efficiency.

The common yeast organism, Saccharomyces cerevisiae, is extraordinarily versatile; however, there are critical differences between products that incorporate live, active yeast versus manufactured products derived from yeast fermentation. They can offer very different benefits and work in contrasting ways.

Farmers need to be aware of the key differences so that they are equipped with the knowledge when working with their nutritionist to choose the most appropriate and cost effective yeast, explains Dr Bond. “Although yeast feeding has been around a long time, there is still some confusion concerning the difference between various yeast products on the market,” she notes.

Rumen activity

Unlike products consisting of live, active yeast, fermented yeast products have already undergone fermentation in a controlled manufacturing environment. Depending on the manufacturing parameters this process can yield a wide variety of beneficial fermentation metabolites by-products.

“These metabolites make an important difference,” notes Dr Bond. Extracellular metabolites from the yeast fermentation process contain proteins, antioxidants and organic acids, amongst other potentially beneficial compounds.

“A complex yeast product is produced by pairing the metabolites with the intracellular yeast nutrients and the natural media that is used during the fermentation process.

“And it’s these beneficial nutritional metabolites that can support the growth of rumen bacteria, and as a result, rumen efficiency is improved because it is working to capacity.”

In contrast, live active yeast consists of just that, live, viable, active yeast cells mixed with a carrier. There can be variation in the yeast viability counts, and this can add to the confusion surrounding yeast products, cautions Dr Bond.

“For example, a live yeast product will often only contain 20% to 25% ‘active’ dry yeast colony forming units (cfu). The remainder will be diluents and carrier; it’s not a simple case of just considering ‘a yeast is a yeast’,” she adds.

“Live active yeasts need to undergo the fermentation process in the cows’ rumen, so performance can be affected by a wide range of factors, including the cow's level of stress, and the pH and condition of the rumen. There are many elements that need to be right for the yeast to fully ferment in the rumen, including temperature and the composition of the rumen fluid.”


As live active yeast undergoes fermentation within the rumen, oxygen is absorbed in the process, and this is what can affect the rumen efficiency long term, and is sometimes considered one of the main benefits of live yeast. However, Dr Bond points out that for significant oxygen absorption to occur, the live yeast needs to reach the rumen in a viable condition.

By contrast, Dr Bond notes, the metabolites in fermented yeast products directly support the rumen bacteria, which increases rumen efficiency. “When feeding live yeast, this is not the case. The viable live yeast cells have to undergo fermentation within the rumen, and this can delay the yeast in providing a benefit.” 

Fermented yeast products has been proven to beneficially impact the dairy cow at all stages of production, from heifer calves right through to all stages of lactation, explains Dr Bond. She notes, however, that there also are crucial differences among yeast fermentation products. “Differences mainly arise from the quality of raw materials and the technology of the manufacturing process.”

New peer reviewed research*1 revealed that by feeding Rumenco’s fermented yeast product, XP, during the transition period, positively affected the metabolic and immune status of dairy cows in early lactation, as indicated by lower serum cortisol concentrations.

Dr Bond adds that the research demonstrated that during the first four weeks postpartum, feeding XP versus a control, decreased milk somatic cell counts (SCC) and increased milk production and serum phosphorus concentrations. “The research identified that cows fed XP had SCC halved compared to the control group.”

The research also highlighted improvements in milk yield with the cows fed XP having, on average, 4.6kg per day more milk produced which was of a comparable composition.


Handling and storage varies between live and fermented yeast products, explains Dr Bond. “Fermented yeast products such as XP can be included in steam conditioned and pelleted feed, as it is not degraded by processing heat or humidity.” The nutritional metabolites in fermented yeast products are very stable in all phases of feed manufacturing.

However, on the other hand, Dr Bond points out that, live yeast products can be damaged or destroyed by rough handling, prolonged storage and the high temperatures of some manufacturing processes. “The live element of the yeast means that they respond to environmental changes in different ways.”

There are a plethora of ‘yeast’ products on the market, notes Dr Bond, so it is vital farmers are aware of what they are feeding to their cows. “Quality fermented yeast products deliver reliability and act to deliver more than just a yeast to support the cow so they can meet the challenges of modern milk production,” concludes Dr Bond.

Recently published research shows that feeding XP increases milk production while lowering the milk’s somatic cell count*1

Effect of feeding XP on milk yield and composition in Holstein cows during the first 4 weeks postpartum


0g XP/day/cow

56g XP/day/cow

Amount kg/day)






3.5% FCM















Composition (%)















SCC (cells/µL)





*1 Zaworski et al (2014) J Dairy Sci 3081–3098