Fungal Additives Yeast culture

Yeast cultures based on Saccharomyces cerevisiae are widely used in ruminant diets. Typically added to the diet of cattle at between 4 and 100 g/d, available products vary widely in both the strain of S. cerevisiae used and the number and viability of yeast cells present. Not all strains of the yeast are capable of stimulating digestion in the rumen. These differences are not related to the number of viable yeast cells in the preparations, although their ability to stimulate rumen fermentation may be related to dif ferences in metabolic activity.[8] Milk yield increased by an average of 4.5% and liveweight gain in growing adult cattle by 7.5% in response to yeast addition. However, responses were diet- and animal-dependent, with greater response reported in early lactation and in animals fed high-concentrate diets.[8] There is general agreement that production responses are the result of the action of the yeast within the rumen. An increase in the number of total culturable bacteria that can be recovered from the rumen would appear to be one of the most consistently reported responses to yeast addition. The increased bacterial count seems to be central to the action of the yeast (Fig. 1), driving both an increased rate of fiber degradation in the rumen and an increased flow of microbial protein from the rumen. What remains contentious is how small amounts of yeast in the diet can stimulate microbial numbers in the rumen. A number of possible modes of action have been proposed, including provision of vitamins or other stimulatory nutrients to the rumen microbial population, but to date, the only proposed mode of action that has been investigated in depth is the suggestion that yeast might scavenge oxygen from the rumen, promoting the growth of anaerobic bacteria therein.[8]

Other fungi

In addition to S. cerevisiae, products based on other fungi have also been described. Although preparations based on Aspergillus niger, Penicillium sp., and Trichoderma harianum and even the ruminal fungus Neocallimastix frontalis have been used experimentally, the only commercial products known to us are based on Aspergillus oryzae (AO). Production responses to AO are generally similar to those seen with S. cerevisiae and are certainly as variable.[8] Like S. cerevisiae, AO stimulates microbial numbers in the rumen. It has been suggested that vitamins and other nutrients in AO stimulate bacterial activity in the rumen in a manner similar to that postulated for S. cerevisiae. However, unlike S. cerevisiae, AO did not stimulate oxygen uptake by rumen fluid.[8] The wide range of polysaccharidase enzymes produced by Aspergillus spp. has led to the suggestion that enzymatic attack of plant fibers by Aspergillus may be an important factor in the stimulation of forage degradation in the rumen when AO was fed.[8]

Increased bacterial viability

Increased rate of cellulolysis

Increased flow of microbial protein

Improved productivity

Fig. 1 The central role of an increase in bacterial numbers in the rumen in driving production responses to fungal addition.

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