There is considerable interest in the effect of dietary supplementation with probiotics or yeast cultures on diary cow performance. Yeast cultures such as Diamond V 'XP' are produced by growing selected yeast strains on a semi-solid medium under stressed conditions, which are then dried. Yeast cultures are now used by a majority of the high yielding herds in North America (McCullough 1995). The objective of this work was to evaluate the effect of feeding 'XP' Yeast to a moderate-high yielding (X, 6800kg) herd of dairy cows fed a typical UK diet based on grass silage.

72 recently calved Holstein-Friesian dairy cows were allocated to one of two dietary treatments on which they remained throughout 150 days of winter feeding. Both groups received ad libitum grass silage which had a Dry Matter of 224 g/Kg, an estimated ME of 10.8 MJ/Kg DM and a Crude Protein (CP) of 156 g/Kg DM.