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Our objective was to evaluate the lactational responses of dairy cows to methionine provided from 2 ruminally protected sources of methionine activity. Twenty-one Holstein dairy cows [11 primiparous (634 kg of body weight, 140 d in milk) and 10 second-parity (670 kg of body weight, 142 d in milk)] were assigned to a treatment sequence in 4 replicated 5 × 5 Latin squares plus 1 cow, with 14-d periods. Treatments were as follows: control; 7.5 or 15 g/d of a ruminally protected product of 2-hydoxy-4-methylthio-butyric acid (NTP-1401; Novus International Inc., St. Charles, MO); or 7.5 or 15 g/d of a ruminally protected dl -methionine product (Smartamine M; Adisseo, Alpharetta, GA). The diet was predicted to meet metabolizable protein and energy requirements. Diets contained 16.1% crude protein, and the control diet was predicted to be deficient in metabolizable methionine (1.85% of metabolizable protein) but sufficient in lysine (6.8% of metabolizable protein). Feed intake and milk yield were measured on d 11 to 14. Blood was collected on d 14. Dry matter intake, milk yield, energy-corrected milk, milk fat yield and percentage, and efficiencies of milk and energy-corrected milk yield were not affected by treatment. Milk protein percentage and milk protein yield increased linearly with supplementation, without differences between methionine sources or interactions between source and level. Linear regressions of milk protein percentage and milk protein yield against supplement amount within source led to slope ratios (NTP-1401:Smartamine M) of 95% for protein percentage and 84% for protein yield, with no differences between sources for increasing milk protein. Plasma methionine concentrations were increased linearly by methionine supplementation; the increase was greater for Smartamine M than for NTP-1401. Plasma d -methionine was increased only by Smartamine M. Plasma 2-hydoxy-4-methylthio-butyric acid was increased only by NTP-1401. Our data demonstrated that supplementation with these methionine sources can improve milk protein percentage and yield, and the 2 methionine sources did not differ in their effect on lactation performance or milk composition. [ABSTRACT FROM AUTHOR]