The experiment was conducted as a split-plot with 24 Holstein cows (mean ± standard deviation; 669 ± 55 kg of body weight; 87 ± 36 d in milk; 8 ruminally cannulated) randomly assigned to a diet of [dry matter (DM) basis] 15.3 or 16.6% CP (whole plot) and 0, 0.45, 0.90, or 1.80% of a tannin mixture in three 4 × 4 Latin squares within each level of CP (sub-plot).