There is considerable interest in the effect of dietary supplementation with probiotics or yeast cultures on dairy 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. They are now used by a majority of the high yielding herds in North America (McCullough, 1995) and have been shown to have a significant effect on butterfat and protein yield with grass silage fed UK dairy cows (Rowlinson et al., 1995). The objective of this work was to evaluate the effect of feeding a yeast culture to a medium-high yielding (8,100 kg) herd of dairy cows fed a diet based on grass and maize silage.