reduced reproductive performance (Tyrrell et al., 1970; But- ler, 1998). By adequately balancing energy and nitrogen sup- ply to the microbes and, thus, avoiding excessive dietary crude protein levels, this detrimental impact can be mitigat- ed. Use of feed ingredients designed to satisfy the nitrogen requirements of around 95% of rumen microbes can aid in providing a balanced ration.


Live yeast for anaerobic environment The majority of beneficial bacteria in the rumen rely on a sta- ble, anaerobic environment in order to function; as such, oxy- gen entering the rumen associated with feed particles poses a constant threat. Dietary inclusion of live yeast can rapidly promote an anaerobic environment, helping desirable, fibre- digesting microbes to proliferate and efficiently colonise feed particles and resulting in lower rumen lactate concentrations, higher overall pH and a reduced risk of acidosis. Live yeast works by metabolising excess oxygen entering the rumen via feed particles, thus helping to maintain an anaerobic rumen environment. There is also stimulation of lactate-utilising bacteria, which helps to reduce the acid load in the rumen and avoid significant drops in rumen pH that are detrimental to the sensitive cellulolytic bacteria. These cellulolytic bacte- ria then thrive and generate VFA, which are subsequently ab- sorbed through the wall. This creates a more efficient and complete digestion of the ration. particularly the fibre por- tion, leading to improved feed efficiency (Figure 1). Promo- tion of cellulolytic bacteria results in a shift in VFA production toward acetate, which drives milk fat production. As well as scavenging oxygen, live yeasts produce small peptides and co-factors that stimulate bacterial growth. There is much vari- ation in the range of stimulatory compounds or metabolites generated by different yeast strains (Kondo et al., 2014), and this contributes to some of the varied responses across indi- vidual strains. Fatty acids, sugars and amino acids make up a large proportion of these metabolites but are produced at


very different concentrations across yeasts (Kondo et al., 2014). This overall stimulatory effect by live yeast not only in- creases feed utilisation and digestion but also drives dry mat- ter intake (DMI), increases of which are often seen following dietary inclusion of yeast. Earlier studies with a live yeast show the effects on milk yield and composition in Holstein dairy cows (Table 1).


Conclusions Ultimately, the rumen is a fairly unstable fermentation vessel that relies on an intimate relationship with various microorgan- isms. The majority of beneficial bacteria in the rumen rely on a stable, anaerobic environment in order to function, and as such, both oxygen and acid are threats. Microbes also rely on a supply of both energy and nitrogen, and by adequately balanc- ing the supply of these and avoiding excessive dietary crude protein levels, the detrimental impact of excess nitrogen can be mitigated. Live yeasts are known to scavenge oxygen and produce stimulatory compounds that have a beneficial impact on both the rumen ecosystem and subsequent animal perfor- mance, though there is much variation in their efficacy.


References available on request.


Table 1 - Effect of dietary inclusion of live yeast on milk yield and composition in Holstein dairy cows (adapted from Tristan and Moran, 2015)


Milk yield (kg/d)


ECM (4% fat, 3.4% CP) (kg/day) Fat (g/kg)


Protein (g/kg) SCC (1,000/ml) Lactose (g/kg) Urea (mg/l)


Control 35.9a 34.3a 39.8b 32.5a 1.95b 49.8a


0.248b


Yea-Sacc 36.7b 35.7b 38.7a 32.8b 1.79a 50.6b


0.239a SD


0.25 0.35 0.3 0.1 0.3


0.09 0.003 ▶ DAIRY GLOBAL | Volume 5, No. 4, 2018


P value 0.003


<0.0001 0.0002 0.009


<0.0001 <0.0001 0.004


31


Yeast cultures based on Saccha romyces cerevisiae have a positive influ- ence on animal performance, milk production and digestibility.


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