Comparative effects of multiple sources of rumen-protected methionine on milk production and serum amino acid levels in mid-lactation dairy cows

The dairy industry demand for rumen-protected methionine (RP-Met) supplements has been competitive because of the constant emergence of new products. To evaluate performances, our study was designed to characterize the production response of 3 RP-Met supplements in mid-lactation dairy cows. Twelve multiparous Holstein cows were used in a replicated 4 × 4 Latin square design with 21-d treatment period. Treatments included control [(basal) diet based on corn silage and alfalfa haylage, supplemented with 0.025% of ration DM of lysine (Lys; Ajipro)] or 1 of 3 RP-Met supplements [Smartamine M (SMM), Mepron M85 (MM85), or Novimet (NVM)]. For RP-Met groups, Met and Lys were supplemented to the basal diet at 0.03 and 0.20% of ration DM, respectively. Treatments had no effect on DMI or milk yield. Treatment did not modify milk fat or lactose concentration; however, milk protein content was elevated with SMM, relative to control or NVM (3.30% vs. 3.24 or 3.24%, respectively; P < 0.05). Milk fat, protein, and lactose yield were not modified by treatments. Treatments tended (P = 0.12) to affect milk urea nitrogen. Serum Met concentration increased for SMM compared with control, MM85, or NVM (27.3 μM vs. 21.2, 23.3, or 22.7 μM, respectively; P < 0.001). Similarly, supplementation of SMM reduced the serum Lys:Met ratio (4.5:1) compared with control (5.2:1), MM85 (5.1:1), or NVM (5.2:1) (P < 0.05). Treatment did not modify the serum levels of all other EAA. We conclude that SMM increased circulating Met and milk protein content more effectively than NVM or MM85.