Comparative effects of multiple sources of rumenprotected methionine on milk production and serum amino acid levels in midlactation dairy cows

Methionine (Met) and lysine (Lys) are limiting AA in dairy cow diets. Supplementation of rumen-protected (RP) Met and Lys can improve milk yield as well as yield and content of milk protein. Currently, multiple sources of RP-Met are available for supplementation; however, the comparative efficacy of these supplements to improve performance requires evaluation. Therefore, our objective was to characterize the production response of three RP-Met supplements in midlactation cows. Twelve multiparous Holstein cows (602 ± 46 kg BW and 174 ± 18 DIM) were used in a replicated 4 x 4 Latin square design with 21-d treatment periods. Dietary treatments included a corn silage and alfalfa haylage diet (control; no added Met) supplemented with one of three RP-Met sources (Novimet [Innovad], Smartamine M [Adisseo], and Mepron M85 [Evonik]). Treatments were designed to maintain a Lys:Met ratio of 2.9:1. For the control, Lys (RP-Lys; AjiPro) was added at 0.02% ration DM. For RP-Met supplementation, Met (RP-Met) was added at 0.03% ration DM. Cows fed RP-Met were provided Lys (RP-Lys) at 0.20% ration DM. Milk yields were recorded, and samples were collected during each period (d 19 to 21). Blood samples were collected on d 21 at 2, 4, and 6 h following feeding, and serum was pooled. Following solid-phase extraction, 21 serum AA were quantified using gas chromatography tandem mass spectrometry. Data were analyzed using a mixed model with repeated measures (fixed effects of treatment and period). Treatments had no effect on DMI or milk yield. Treatment did not modify milk fat, protein, or lactose yield; however, milk protein content was elevated with Smartamine M relative to control or Novimet (3.30 vs. 3.24 and 3.24% respectively; P < 0.05). Milk fat and lactose concentration were not modified with treatment. Treatment with RP-Met tended to increase milk urea nitrogen (P = 0.12). Smartamine M increased serum Met concentration (27.3 μM) compared with the control (21.2 μM), Novimet (22.7 μM), or Mepron M85 (23.3 μM) (P < 0.001). In a similar manner, Smartamine M lowered the serum Lys:Met ratio (4.5:1) compared with the control (5.2:1), Novimet (5.2:1), or Mepron M85 (5.1:1) (P < 0.05). Smartamine M was also able to enhance circulating glutamine relative to the control (320.8 vs. 289.5; P = 0.15). Treatment did not modify serum levels of all other AA, including Lys. We conclude that Smartamine M increased circulating Met and milk protein content more effectively than Novimet or M