Evaluation of a rumen-protected methionine product to improve amino acid balance in lactating cows.

The study objective was to evaluate a rumen-protected methionine product (RP-Met, Milk Specialties Global, Eden Prairie, MN) designed to help meet the Met requirements for energy-corrected milk yield of dairy cattle. The study was conducted for 84 d with 144 lactating Hol- stein cows in replicated 16-cow pens, with 3 pens per treatment. Three diets were formulated, using the Cornell Net Carbohydrate and Protein System v6.5.5, to supply 0.96 g/Mcal ME (no RP-Met), 1.05 g/Mcal ME, and 1.19 g/Mcal ME, corresponding to 80%, 88%, and 100% of requirements, respectively. Cows were fed isocaloric diets, formulated for 5% sugar, 28.5% starch, 28.5% aNDFom, 4.3% EE (DM basis), and metabolizable Lys and His at 3.2 and 1.19 g/Mcal ME, respectively. Cattle were milked 3 times daily, where milk was sampled weekly in 3 contiguous milkings and analyzed for fatty acids (FA), true protein, MUN, and somatic cell count. Milk yield and pen dry matter intake were recorded daily by the farm software. Data were analyzed using a mixed model in SAS, with the fixed effect of diet and random effects of pen nested within diet and cow nested within pen and diet. Energy- corrected milk was greater in cattle fed diets 2 and 3 (50.1 and 50.4 kg/d, respectively) versus diet 1 (48.7 kg/d; SEM 0.31; P = 0.02). As supplemental Met increased, milk true protein percentage increased from 3.09% to 3.24% and 3.34% (SEM 0.01; P < 0.01), across diets 1, 2, and 3, respectively. Milk fat percentage was greater in diet 3 (4.36%; SEM 0.03; P < 0.01) compared with cattle fed diets 1 and 2 (4.21% and 4.25%, respectively). De novo FA concentration increased (SEM 0.01; P < 0.01) with increasing Met supply, consistent with the increase in milk protein, and mixed FA were greater in cattle fed diet 3 compared with diet 1 (1.70 vs. 1.65 g/100 g; SEM 0.015; P < 0.01). In summary, supplementation of this product improved the supply of metabolizable Met across diets, resulting in significant increases in milk protein per- centage and yield and fat percentage, when metabolizable Lys and His were not predicted to be deficient.