the rumen microflora. SC may promote the growth of highly anaerobic cellulolytic bacteria in the rumen by scavenging oxygen and providing vitamins, amino acids and peptides that stimulate microbial growth. It appears that no research has been conducted to look at the effects of SC on the matu- ration of the rumen microflora of calves. However, two stud- ies have looked at the effect of SC on calf performance. In one study it was observed that calves receiving SC had improved dry matter intake (DMI) and growth. In the other study there were no observed differences in the growth rate of calves. However, there was a significantly higher DMI in the calves receiving SC. Furthermore, it was found that calves in the group fed SC had higher concentrations of ammonia, propi- onate and butyrate in the rumen. The study suggests that these results may be influenced by an earlier maturation of rumen microflora, which would subsequently enhance rumen fermentation, improve digestion and promote growth.

Enteric microbial balance The subspecies S. cerevisiae boulardii (SCB) is more common- ly used as an ADY in monogastric nutrition for the purpose of preventing and treating enteric infections. Young ruminants are often regarded as having a digestive tract that is physical- ly and functionally similar to monogastric animals and it is therefore possible that SCB may confer the same health ben- efits on calves. As a result, most of the emerging studies have focused on the use of this species in calf nutrition. To begin with, in a study it was observed that calves receiving milk re- placer supplemented with SCB experienced fewer days with diarrhoea. In addition, it was reported that calves receiving SCB were less likely to experience severe diarrhoea and there- fore received fewer antibiotic treatments. It was also found that diarrhoeic calves receiving the SCB supplement man- aged to maintain the same DMI and growth rate as the non-diarrhoeic calves. Supplementing SCB to the milk replac- er may promote symbiosis in the enteric microbial communi- ty, which may prevent diarrhoea by inhibiting the growth of pathogenic bacteria and by promoting the development of the immune system. This is of great importance to neonatal calves since they rely on non-specific innate immune re- sponses and attenuating maternal antibodies as a defence mechanism against invading pathogens. In a study it was found that calves receiving SCB had a greater proportion of Lactobacilli (bacteria that promote enteric health) in their faeces. This validates the theory that SCB is able to promote the growth of beneficial micro-organisms in the GI tract. In addition, it was found that the supplementation of SCB influ- enced colon histomorphology by significantly reducing crypt depth and width. This may have reduced the surface area available for attachment by micro-organisms, thus reducing the risk of pathogen translocation. It has also been found that SCB has an effect on the modulation of the immune system in calves. It was discovered that the defence mechanisms of the

20 ▶ ALL ABOUT FEED | Volume 28, No. 3, 2020

In a study it was observed that calves receiving milk re- placer supplemented with SCB experienced fewer days with diarrhoea.

innate immune system such as phagocytosis and oxidative burst capacity, which are involved in engulfing and killing pathogens, were enhanced in calves receiving SCB.

Validity of yeast products Despite the positive results that have been observed in these studies, the benefits of using ADY products are not yet guar- anteed in calves. This is due to the fact that some studies have reported no improvements in calf performance when an ADY was administered. Therefore, further research needs to be conducted to understand why some studies can obtain positive results and others not. Some researchers speculate that this may be due to variation in the strain of yeast used, the dosage, the type of yeast product, the diet supplied, the physiological status of the animal and the animal husbandry practices employed. In one study, where Holstein calves re- ceived milk replacer supplemented with SCB, the researchers did not observe any improvement in calf growth and health. They reported that all the calves were in good health for the duration of the trial and suggested that an ADY supplement may be more beneficial to a calf when it is exposed to a disease challenge or stress.


Based on the studies mentioned in this review, ADY products containing either SC or SCB have the potential to become rec- ognised as a suitable feed additive for calves. They may im- prove calf performance by promoting symbiosis of either the rumen or enteric microbial communities. The most recent studies have focused mainly on ADYs containing SCB, show- ing that it may reduce the incidence and severity of diarrhoea by promoting the growth of beneficial GI microflora and im- proving the responses of the innate immune system. Despite this, the results obtained are variable between studies and further research is required to understand why there is a dis- parity. Additional studies should attempt to further investi- gate the modes of action of ADYs, particularly in calves that are exposed to stress. This may assist nutritionists in estab- lishing clear guidelines for the use of ADYs in calf nutrition.


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