metabolic activity, and it usually contains approximately 15 billion colony forming units per gram (CFU/g) of viable fer- mentable cells. Alternatively, a YC contains only a small num- ber of viable cells and it is a fermented product containing a dried culture medium and yeast cell wall components (such as β-glucans and mannanoligosaccharides). An ADY is consid- ered to be a probiotic (or a DFM), whereas a YC is considered to be a prebiotic. These two terms are often used inter- changeably; however, it is important to distinguish between the two because they have different modes of action in vivo. Various strains of Saccharomyces cerevisiae (SC) are well ac- cepted as DFMs in the livestock industry. Many have been registered as gut microflora stabilisers in the European Union and accredited in North America by the Generally Recognised As Safe (GRAS) list. To date, they have been predominantly advertised as suitable feed additives for mature ruminants and monogastric animals to improve health and production by preventing subacute ruminal acidosis (SARA) and enteric infections, respectively. Emerging studies may provide evidence that ADYs are also beneficial for young ruminants.
Rumen microbial balance Feed additives containing SC have been predominantly used in ruminant nutrition to promote a symbiosis of the rumen microflora, which harbours communities of bacteria, fungi and protozoa. However, ruminants are not born with such a complex rumen microflora; instead, the resident microbial community has to be established through a series of
profound changes that occur during the pre-weaning phase. This succession of changes includes the sequential establish- ment firstly of aerobic bacteria, then anaerobic bacteria and eventually ciliate protozoa. Attaining a stable and diverse ru- men microbial community is essential for initiating efficient digestion of dry feed and the absorption of fermentation products. In spite of this, in some production systems young ruminants may be completely weaned from milk before a sta- ble rumen microflora is established. To overcome this, it is proposed that ADYs containing SC may promote earlier matu- ration of the rumen microflora. This theory is investigated by using germ-free lambs that were reared in a sterile environ- ment (gnotobiotic). All lambs were inoculated with cellulolyt- ic bacteria at specific time points. They found that the group receiving a SC supplement had a more rapidly established and stable community of cellulolytic bacteria present in the rumen within the first 42 days after birth. This was verified by another study in which the effect of SC on microbial colonisa- tion in the rumen of conventional lambs was investigated. It was found that the number of lambs with cellulolytic bacteria present in the rumen at two and four days after birth was far greater in the SC group than in the control group. Additional- ly, by ten days after birth, 60% of the lambs receiving SC had protozoa present in the rumen, whereas protozoa were only first detected two days later in the control group. Ciliate pro- tozoa do not become established in the rumen unless a bac- terial population is already present, so the presence of protozoa may be indicative of the stability and maturity of
▶ ALL ABOUT FEED | Volume 28, No. 3, 2020
ADY products containing ei- ther SC or SCB have the poten- tial to become recognised as a suitable feed ad- ditive for calves.
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