One hundred fifty multiparous cows were balanced to 1 of 6 treatment pens (experimental unit) according to previous lactation 305-d mature equivalent yield. Cows were housed in a freestall barn that contained 6 pens (70 stalls/pen), 3 on each side of a 6-row barn. Cows entered the groups at calving and remained through 14 wk postpartum. Treatments were randomly assigned to each pen per side of barn. Pens were identical in layout and each pen contained an exit alley such that cows from adjacent pens would not interfere with or have access to another pen’s treatment feed when moved for milking. Beds were sand-based, and water and feed space accessibility was equal in all pens. The 3 treatments in this study were as follows: (1) control: fresh cow diet; (2) yeast culture (YC): control diet with the inclusion of A-Max Yeast Culture Concentrate (Vi-COR, Mason City, IA) at 56 g/cow per day; and (3) yeast culture + enzymatically hydrolyzed yeast (YC+EHY): control diet with the inclusion of Celmanax (Vi-COR) at 28 g/cow per day. The basal diets were formulated for requirements of a lactating cow at 50 DIM, producing 45 kg of 3.5% FCM milk (NRC, 2001); ingredient and nutrient composition of the control diet are shown in Tables 1 and 2. As cows were assigned to each pen, they were housed with nontrial cows of similar DIM to accommodate a one cow-one stall environment. Trial animals were identified with treatment-specific color-coded ear tags. When the first cow entered each treatment pen, treatment diets commenced until the last treatment cow was removed.