The nutritive value of wet potato processing waste for dairy cattle was determined in two experiments. In Experiment 1, rations contained, on a dry matter basis, 0, 10, 15, and 20% potato waste and were substituted for high moisture corn in diets for 32 lactating Holstein cows for 12 wk. Substituting potato waste for corn did not significantly affect milk yield, milk composition, milk production persistency, or dry matter intake. Cows fed 20% potato waste tended to decrease in milk fat percent and to shift molar proportions of rumen volatile fatty acids toward a decrease in acetate: propionate ratio. In Experiment 2, six steers were used in a 3 X 3 Latin square design to test digestibility and nitrogen utilization of potato waste substituted for high moisture corn at 0, 10, and 20% of the ration dry matter. A second group of four steers with rumen fistulas were used in a 4 X 4 Latin square to test rumen fermentation parameters. Diets contained 0, 10, 20, and 30% potato wastes and were similar to Experiment 1. Potato waste did not significantly affect digestibility of crude protein or dry matter, but at 20% substitution digestibility of acid detergent fiber decreased. Rumen ammonia, acetate, acetate to propionate ratios, and total volatile fatty acids were lower at high intakes of potato waste and pH was increased. The shift in rumen fermentation when large amounts of potatoes were fed explains the depressed butter fat on these rations.