Four forage treatments (45% corn silage, 33.75% corn silage plus 11.25% alfalfa hay, 11.25% bermudagrass hay, or 11.25% cottonseed hulls on a DM basis) were arranged factorially with no added fat, 12.5% whole cottonseed, or 2.5% tallow. Different diets were fed during three 28-d periods to 20 control Holstein cows and to 20 cows receiving yeast continuously in a split-plot design. Milk yield of cows fed cottonseed hulls with corn silage was 2.4 kg/d higher than with corn silage plus bermudagrass hay and .7 kg/d higher than with corn silage only or corn silage plus alfalfa hay. Whole cottonseed depressed milk yield by 1 kg/d. Cows fed yeast had increased DMI, and yeast interacted with forage so that more milk was produced by cows fed alfalfa diets. Yeast depressed milk protein percentage. Holstein cows in a commercial Florida dairy were fed no yeast or 10 g/d continuously for 60 d; milk fat percentage was greater (3.51 vs. 3.37%) with yeast. In summary, effects on milk and SCM were positive when cottonseed hulls were utilized with corn silage, negative with whole cottonseed, and neutral with supplemental tallow. Yeast effects on SCM, although not significant for either experiment, tended to be positive for both (mean +1.2 kg/d per cow).