Table 1 – Effect of dietary inclusion of live yeast (Yea-Sacc®, Alltech Inc.) on milk yield and composition in Holstein dairy cows (adapted from Tristant and Moran, 2015).


significant drops in rumen pH that are detrimental to the sen- sitive cellulolytic bacteria. These cellulolytic bacteria then thrive and generate volatile fatty acids (VFA), which are sub- sequently absorbed through the wall. This creates a more effi- cient and complete digestion of the ration, particularly the fi- bre portion, leading to improved feed efficiency (Figure 1). Promotion of cellulolytic bacteria results in a shift in VFA pro- duction towards acetate, which drives milk fat production. As well as scavenging oxygen, Yea-Sacc produces small peptides and co-factors that stimulate bacterial growth. There is much variation in the range of stimulatory compounds or metabo- lites generated by different yeast strains (Kondo et al., 2014), and this contributes to some of the varied responses across individual strains. This overall stimulatory effect promotes utilisation and diges- tion of nutrients, driving greater intake, in which increases are often seen following dietary inclusion of yeast.


Optimising selenium status Oxidative stress has also been cited as a negative influence on immune function. It is well accepted that selenium (Se) is a cru- cial antioxidant that works in tandem with vitamin E and has a significant impact on oxidative stress. Se-containing enzymes, such as glutathione peroxidase, scavenge compounds that en- hance oxidative stress and also have a vitamin E–sparing effect. Levels of selenoenzymes are known to decrease during the very late dry period but rapidly increase again in early lacta- tion. Selenium inadequacy is linked with adverse immune function effects, such as impaired antibody production. Ade- quate selenium status has a positive impact on the animal’s ability to resist infection, as well as on fertility parameters. Optimising Se status in cows is essential for supporting im- mune function, and this refers to both amount and form of Se supplied. It is well established that organic forms of minerals


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


generally have higher bioavailability and retention in the body compared with their inorganic counterparts. This retention in body tissues allows the animal to build a reserve of the mineral to use in times of increased physiological stress (e.g. transi- tion). As Se cannot be chelated, selenium-enriched yeast, such as Sel-Plex®, is often used to deliver Se in an organic form. Positive effects have been noted with feeding an organic form of other minerals, such as zinc and copper. Zinc deficien- cy can significantly increase susceptibility to infections. How- ever, an excess of zinc has also been shown to impair immune function. Zinc is also crucial for maintaining the integrity, nor- mal function and development of the primary barrier, the skin and cells involved in the inflammatory response. Similar- ly, copper affects the second, more specific, response by the immune system. Thus, copper-deficient animals often show immunosuppression. Essentially, immune function is critical to dairy cow perfor- mance, and transition animals are at greatest risk of impaired immune function. Nutrition plays a crucial role in dairy cow health, not least through direct effects on the immune sys- tem, and rations should be formulated to enhance immunity through optimal mineral supply, as well as to maximise rumen function.


References available on request. ▶DAIRY GLOBAL | Volume 7, No. 3, 2020 43


P value 0.003


<0.0001 0.0002 0.009


<0.0001 <0.0001 0.004


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