Introduction
Use of Saccharomyces cerevisiae in dairy nutrition is gaining popularity day by day. In the last few years, several dairy nutrition companies have commercially launched different strains of Saccharomyces cerevisiae to be used in ruminant feeding. Quantum effect of yeast supplementation depends on many factors such as diet composition, forage to concentrate ratio, type of forage fed, yeast dose, feeding strategy and stage of lactation. The ability of different strains of Saccharomyces cerevisiae to stimulate the viable count of bacteria in the rumen appears to be related to their ability to remove oxygen from the rumen fluid since anaerobic mutants of Saccharomyces cerevisiae were not able to stimulate bacterial numbers (Newbold et al., 1996). An increase in the bacterial count in the rumen is the most reproducible effect of dietary yeast supplementation, and it has been suggested that the increased bacterial count is the key reason behind improved ruminant productivity (Wallace & Newbold, 1992). In market two types of products are available, one as active live yeast and another as yeast culture. Composition and mode of action is the differentiating feature between these two types.
Differences in active live yeast (ALY) and yeast culture (YC)
Extensive research has been done on Saccharomyces cerevisiae both in vitro and in vivo (Lynch H and Martin S, 2002) to determine its effect in animal models. Commercially there are several ALY and YC products from Saccharomyces cerevisiae yeast available in the market. The products are classified based on the active ingredients and their mode of action (Poppy GD et al., 2012). Active live yeast products are dried fermentable living yeasts and contain at least 15 × 109 live yeast cells per gram while a YC is produced by fermenting cereal grains in a selected liquid with baker’s yeast then drying the whole medium culture (Poppy GD et al., 2012 & Lynch H and Martin S, 2002). The YC contents include yeast cell wall (β-glucans and mannan-oligosaccharides), cell solubles, , proteins, vitamins, peptides, amino acids, nucleotides, lipids, organic acids, esters and alcohols, B vitamins, polyphenols, organic acids and anti-oxidants (Callaway ES and Martin SA, 1997 & Jensen GS et al., 2008) all of which may have positive effects on performance and health when incorporated into the diet of animals. The composition of each of the above bioactive compounds in Saccharomyces cerevisiae yeast have not been characterized (Kim MH et al., 2011) and as a result, the effects of Saccharomyces cerevisiae yeast are mostly attributed to the yeast wall components. The active live yeast is believed to offer mainly probiotic effect while the yeast culture components are regarded as having both probiotic and prebiotic effect. Fuller R (1989) defined probiotics as live microbial supplements that beneficially affect the health and well-being of the host animal by improving its gastrointestinal balance. Recent research has shown that the two products might not have significant differences in their mode of action on rumen fermentation (Lynch H and Martin S, 2002). Saccharomyces cerevisiae supplementation in dairy ration has multiple effects which ultimately results in improved production.
Impact on fiber degradation
Most of the fiber degrading bacteria in the rumen are anaerobic i.e. oxygen is toxic to them and it is well proven in pure culture by different scientists (Loesche, 1969; Marounek and Wallace, 1984). Oxygen inhibits the adhesion of cellulolytic rumen bacteria to cellulose (Roger et al, 1990). Yeast cells in the rumen use available oxygen on the surfaces of freshly ingested feed to maintain metabolic activity (Newbold et al., 1996). Oxygen enters the rumen during feed intake, mastication and water intake causing an increase in redox potential (Mathieu et al., 1996). Yeasts are well known for their high respiratory rate. Published values for oxygen uptake by (200 to 300 mmol/min per g) (Bartford and Hall, 1979) suggest that they have respiratory rates several orders of magnitude greater than rumen fluid. Addition of yeast to ruminant diet improves fiber digestion (Elghandour et al., 2014a, b; Salem et al., 2015). Different strains of Saccharomyces cerevisiae differ in their potentiality to enhance bacterial numbers in the rumen (Jouany et al. 1991; Newbold et al. 1995). The ability of different strains of Saccharomyces cerevisiae to enhance the viable count of bacteria in the rumen seems to be related to their potentiality to remove oxygen from the rumen fluid since respiration-deficient mutants of Saccharomyces cerevisiae were found unable to stimulate bacterial numbers (Newbold et al., 1996). Oxygen scavenging effect of yeast in the rumen was explained by Rose even earlier, in 1987. Different Saccharomyces cerevisiae cultures have stimulated beneficial changes in activity and numbers of the rumen microbes with special interest with cellulolytic bacteria (Kumar et al., 2013; Pinloche et al., 2013). Increase in the bacterial count in the rumen is the most reproducible effect of dietary yeast supplementation, and it has been proposed that the increased bacterial count is the key factor behind the action of the yeast in improving ruminant productivity (Wallace & Newbold, 1992). Some studies have shown that yeast also provides vitamins (especially thiamin) to support the growth of rumen fungi (Chaucheyras et al., 1995). Oxygen scavenging effect of yeast is depicted in the below figure.
Impact on ruminal pH
Optimization of ruminal pH provides a great advantage for lactating dairy cows. Ruminal acidosis is most of the times a consequence of consumption of readily fermentable carbohydrates, which results in lactic acid accumulation and marked post-prandial fall in ruminal pH.Saccharomyces cerevisiae compete with other starch utilizing bacteria for fermentation of starch (Lynch and Martin, 2002) leading to the prevention of lactate accumulation in the rumen and has ability to provide growth factors, such as peptides, organic acids, vitamins or cofactors thereby stimulating ruminal populations of cellulolytic bacteria and lactate utilizing bacteria (Chaucheyras et al., 1995& Girard, 1996).As the pH goes down, lactate-producing bacterial species Streptococcus bovis overpowers the lactate utilizing species Megasphaeraelsdenii and Selenomonasruminantium. Protozoal population also disappears, and bacterial diversity is largely affected. If the pH decline continues, lactobacilli replace S. bovis which results in excessive lactate accumulation. Yeast stimulates lactate utilizing bacteria, increasing their population and serves as a competitor with lactate producing bacteria. Lactic acid being the primary cause of acidosis in dairy cattle, reducing its concentration can have a significant effect on the pH.
Impact on weight gain
Weight gain is an outcome of improved dry matter intake which in turn is an outcome of improved ruminal ecosystem. Fiems (1994)reported that on an average the addition of Saccharomyces cerevisiaeto the diet leads to a 9.5% increase in live weight in calves and a 7.8% increase in live weight gain in growing adult cattle. Similarly, Spedding (1991) reported that Saccharomyces cerevisiaestimulated weight gain inbulls fed a high cereal diet by almost 19%while the response in bulls fed a high foragediet was 6.7%.Wallace and Newbold (1993) notedthat responses in cattle fed corn silage tendedto be higher than responses recorded in trialsusing diets based on grass silage.
Impact on dry matter intake and digestion
Improved rumen ecosystem in yeast supplemented animals leads to increased dry matter intake. Different factors such asthe strain of yeast, the nature of the diet or the physiological status of the animal (Chaucheyras-Durand F, Walker ND, Bach A, 2008), dose and feeding strategy (Lesmeister KE, Heinrichs AJ, Gabler MT, 2004 &Magalhães VJA et al., 2008) influence DMI of the animal. Feed additives containing Saccharomyces cerevisiae enhance feed intake rather than alter feed conversion efficiency.Many factors are found responsible to influence appetite like palatability, the rate of fiber digestion, the rate of digesta flow, and protein status. The fungal products certainly have a pleasant smell and the ability of yeast to produce glutamic acid could benefit the taste of feedstuffs supplemented with yeast culture. Some studies have shown increased dry matter intake and milk production when yeast was fed during periods of heat stress, possibly reflecting the role in aiding appetite during time of stress. Fiems (1994) noted that the increases in productivity were often associated with an increase in feed intake while Williams et al (1991) observed larger responses in milk yield in response to Saccharomyces cerevisiaeaddition as the ratio of concentrate to forage in the ration increased.
Impact on milk yield responses
Yeast supplementation in lactating cows has always led to increased milk yield. Harris and Lobo (1988)and Gunther (1990) found that the response tothe inclusion of the yeast was greater in earlyas opposed to mid or late lactation.Fiems (1994)reported that on an average the addition of Saccharomyces cerevisiaeto the diet lead to a 3.9% increase in milk yield in lactating cattle. Results obtained from 22 trial studies with Yea-Sacc®1026 (a natural feed additive containing metabolically active Saccharomyces cerevisiaestrain 1026) involving more than 9039 lactating dairy animals were analyzed by Dawson andTricarico (2002).Theyconcluded an average increase in milk production of 7.3% in yeast-supplemented animals. Yeast supplementation responses were variable and ranged from 2 to 30% increase in milk production.
Impact on rumen maturity in calves
The stomach of a newborn calf is just like that of amonogastric animal due to its small and nonfunctionalrumen. Calves are germ-free at the time of birth but the contact with mother’s saliva, feces and that of other animals the neonate acquires a microflora rapidly. Early weaning in intensive dairy systems leads to an imbalanced microbial flora making the young ruminant more prone to suffer from different infections. Gastrointestinal diseases are one of the most important sources of economic loss in dairy calves. Cellulolytic rumen microorganisms’ establishment is faster in calves and lambs receiving Saccharomyces cerevisiaedaily. The cellulolytic bacteriaare more stable in the supplemented animals. Protozoa colonize the rumen once the bacterial population is present as they feed on rumen bacteria. There are studies that show that protozoa appeared earlier in calves supplemented with Saccharomyces cerevisiae. Also, the ciliate protozoa appeared more rapidly in the rumen in the presence of active dry yeast. The maturation of the microbial ecosystem gets accelerated in the presence of the yeast.
Impact on somatic cell count in the milk
The somatic cell count in the milk is directly related to the infection status of the udder. If an infection occurs in the udder, it recruits leucocytes (somatic cells) to the scene to get rid of the bacteria causing problems. The somatic cell count in milk reflects its quality. High somatic cell count in the milk is indicative of udder infection. Microorganisms fail to adapt well to fluctuations in the rumen pH. Endotoxins liberated from dying bacteria in the rumen cause inflammation of the hoof and udder. Since yeast stabilizes the ruminal pH, the somatic cell counts in milk decrease.
Impact on immune system
Digestive tract provides the site for nutrients absorption andis also the first line of defense against pathogens andother harmful substances for the animal (Zaworski EM et al., 2014). It remains as the main anatomical location that Saccharomyces cerevisiaeyeast might play asignificant role in immunomodulation. Several complexpolysaccharides found in the yeast cell wall such as β-glucans and mannan-oligosaccharides have been identifiedas the modulators of immunity (Jensen et al., 2008). The in vitroexperiment by Jensen et al., (2008) showed that a yeast culture productcould provide anti-oxidant, anti-inflammatory andimmuno-modulatory activities. It has been suggested that cells of Saccharomyces cerevisiaeyeast cannot penetrate the intestinal endothelium barrier while only pure β-glucan have immunomodulatory effects (Wójcik R, 2014). Therefore, it is possiblethat the whole yeast cell might not stimulate the immunesystem, but either β-glucan fragments or otheralternative mechanisms are involved.
Conclusions
Saccharomyces cerevisiaeyeast seems to offer benefits to dairy animals through improvement in DMI,stabilization of rumen pH, stimulation of certain cellulolytic bacteria, enhanced fiber degradation, improved immunity, and increased milk production. Field studies so far indicate that although positive effects on milk or meat production can be obtained, the animal response to such feed additives may be quite variable, depending ondifferent factors (nature of the diet, level of productivity, animal physiological and genetic factors, dose and strain of yeast used, etc.). Yeast supplementation may prove more beneficial in a highly stressed and malnourished animal as compared to the one which is in a well-managed production. It is going to be more important to better understand the nature of interactions between the yeast probiotic, the autochthonous flora and the dietary componentsso that the impact of such a probiotic in ruminant nutrition can be properly predicted. Anyway, yeast supplementation in dairy animals could serve as a solution to persisting acidosis problems and provide some extra profit in cattle production.
by Dr Vimlesh Chandra Sharma, Kemin Industries South Asia Pvt. Ltd.
