There is considerable interest in the use of microbial additives such as yeasts in the nutrition of ruminants. The prohibition of the antibiotics as growth promoters in animal feeds increased the interest to investigate the effects of yeasts as natural additives on the gastrointestinal ecosystem and animal productive behavior. The effect of yeast-based preparations on the rumen environment and on the growth performance of ruminants has been well documented and has generated considerable scientific attention in the last two decades. However, the precise action modes by which the yeast cultures improve nutrient utilization and livestock production are still under study. Therefore, the objective of this chapter is to deepen into the action mechanisms of the yeasts at the ruminal level and at the productive level for their use as additives in animal feeding.
- natural additives
The yeasts are not part of the ruminal ecosystem, and their presence is mainly due to the ingestion in feeds and water. Marrero  and Castillo-Castillo et al.  showed that supplementing ruminant feeds rich in yeasts, they survived for longer time in the rumen and improved the conditions that favored the dry matter (DM) use by microorganisms that inhabit in it. These results corroborate the approach made by Galves  that when certain allochthonous microorganisms are deposited in a new habitat with nutrients, they survive and are able to use part of the resources of the environment in which they were deposited; in this case, the yeasts use the little oxygen existing in rumen and favor the conditions of anaerobiosis  that facilitate or potentiate the growth of other anaerobic microorganisms as the cellulolytic bacteria.
Research reports where yeasts are a natural alternative to manipulate ruminal microbial fermentation and animal productivity are shown below. Yeasts have shown to improve the digestibility of DM and neutral detergent fiber (NDF) , feed consumption , milk production , and live weight gain . However, not all yeast cultures have been shown to modify ruminal metabolism or improve animal productivity [9, 10]. These inconsistencies could be related to the type of yeast strain used , the specificity of the different commercial additives , or to the diet composition . The objective of this review was to deepen into the mechanisms of action of the yeasts as well as their effects at the ruminal level and at a productive level for their use as animal feed additives.
Yeasts are unicellular microorganisms that ferment carbohydrates, and they are reproduced by budding. Most commercial yeast-based products contain a mixture of varying proportions of living cells of
3. Mechanisms of action of yeasts in the rumen
One of the proposed action modes is that living yeasts through their aerobic respiration allow the elimination of the small percentage of oxygen (1%) that enters to the rumen when the animal ingests their feeds, thus facilitating the growth of the most anaerobic microorganisms as cellulolytic bacteria and fungi [4, 15].
Another proposed mechanism of action is that yeast cultures provide vitamins (specifically thiamin), glucans, mannano-proteins, and organic acids, which stimulate the growth of microorganisms that digest fiber and use lactic acid [16, 17, 18].
An additional effect is that yeast cultures are rich in organic acids (mainly malic acid) that stimulate the growth of
Yeasts also produce changes in the bacterial flora by competition and growth stimulation, increasing the growth, and activity of the acetogenic populations that compete with the methanogenic ones by the use of the metabolic hydrogen . This decrease the energy losses in the animal caused the methane formed in rumen, which decreases the negative effect of this greenhouse gas on the environment .
4. Effect of the addition of yeasts in the ruminal fermentation
Yeast cultures based on
The yeast products available in the market vary widely depending on the strain of
Interestingly, it has been observed that not all strains of
In vitro and in vivo studies found no effect of yeasts on ruminal pH [4, 20, 21, 22, 23, 24]. However, results where pH decreased were reported by Williams  when they supplemented
On the other hand, regarding to the concentration of ammonia-nitrogen (N-NH3), Lila et al.  and Erasmus et al.  showed that the inclusion of yeast does not affect the levels of this compound. Contrary to the above, Lattimer et al.  reported a decrease in the N-NH3 concentration when they evaluated the effect of a yeast culture on the in vitro fermentation of a high fiber diet. Also, Moallem et al.  reported a decrease in ruminal N-NH3 from dairy cows supplemented with yeast.
Other effects of yeasts include increase in metabolites as VFA [12, 18], decrease in lactic acid concentration , reduction of methane production [22, 25], increase in growth of cellulolytic bacteria and fungi [4, 16, 22] as well as acetogenic bacteria , and increase in fiber degradability [12, 22].
5. Use of yeasts in animal feeding
The use of feed additives is important in the feeding of ruminants. Yeast proteins have a high nutritional value, characterized by a balanced amino acid profile with a high content of lysine and threonine, which gives it an extraordinary potential for use as a supplement to animal diets, since these could be deficient in these amino acids .
The yeast of
Baiomy , in a study carried out with lactating sheep, mentioned that supplementation with live yeasts of
6. Other non-
Saccharomycesstrains of yeasts with potential commercial use as probiotics
A joint project carried out by Cuban and Mexican researchers has developed additives based on native yeasts adapted to their local conditions, showing a good potential to be utilized as growth enhancers in ruminants and could be economically competitive in the international market. Several studies have been conducted to examine the potential use of these non-saccharomyces yeasts on animal nutrition [35, 36]. Additionally, Marrero et al.  studied the effects of the addition of two strains, Levazoot 15 (
The results in the use of yeast as feed additives are not conclusive; however, in rumen, they stimulate the growth and activity of total and cellulolytic bacteria, improving fiber digestion, synthesis of microbial protein, enhances feed intake and growth performance of ruminants. Yeasts also use ruminal oxygen, facilitating the growth of obligate anaerobes. In addition, yeasts may stimulate the bacteria that consume the lactic acid produced in rumen and may contribute to modulate the pH in rumen, reducing the risks of acidosis, and it can contribute significantly to the reduction of production of methane in the rumen.