Mammary gland responses to supplementation of rumen-protected methionine products in dairy cows

Methionine can be a limiting nutrient in lactating dairy cows consuming diets with corn, corn-based, and soybean-based ingredients. Rumen-protected Met (RPM) supplementation improves productivity in high-producing cows, and the supplementation of RPM may improve productivity in high-producing cows fed moderate-concentration CP diets. The objective of this work was to compare an RPM prototype with a commercially available RPM product on milk protein concentration, plasma Met concentration, and mammary gland clearance of Met in dairy cows. In experiment 1, 10 multiparous cows (DIM = 76 ± 17 arithmetic mean ± SD) were used in a duplicated 5 × 5 Latin square with 18 d periods. A commercial product and an RPM prototype were used to formulate the treatments as follows: (1) control (CTL1; 0 g/d of RPM supplemented), (2) 22 g/d of Met supplemented as RPM using Smartamine M (SM, Adisseo, Alpharetta, GA), (3) 22 g/d of Met supplemented using RPM prototype 1 (PT1.0), (4) 27 g/d of Met supplemented using RPM prototype 1 (PT1.25), and (5) 33 g/d of Met supplemented using RPM prototype 1 (PT1.5). In experiment 2, 20 multiparous cows (DIM = 72 ± 43, arithmetic mean ± SD) were used in a quadruplicate 5 × 5 Latin square with 14 d periods. Cows were randomly assigned to the following treatments: (1) control (CTL2; 0 g of RPM supplemented), (2) 6 g/d metabolizable Met supplemented using Smartamine M (SM6), (3) 12 g/d metabolizable Met supplemented using Smartamine M (SM12), (4) 6 g/d metabolizable Met supplemented using RPM prototype 2 (PT6), and (5) 12 g/d of metabolizable Met supplemented using RPM prototype 2 (PT12). The CTL1 and CTL2 diets were predicted to be deficient in metabolizable Met. The CTL1 and CTL2 diets contained 16.0% and 16.7% CP, 2.03% and 2.02% of metabolizable Met, and 6.74% and 6.84% of metabolizable Lys. In experiments 1 and 2, feed ingredients, milk, and blood (coccygeal vessels) samples were collected and analyzed. In experiment 2, abdominal mammary vein blood samples were collected to determine mammary gland clearance. Data were analyzed using mixed models with animal as the experimental unit, treatment and time as fixed effects, and animal as a random effect. In experiment 1, and compared with CTL1, SM increased milk protein concentration. Compared with CTL1, treatment SM increased plasma Met concentrations. Compared with SM, the PT1.0, PT1.25, and PT1.5 treatments had lower plasma Met concentrations. Treatments PT1.5 and SM supported similar milk protein concentration response. In experiment 2, SM12 increased milk protein concentration compared with CTL2, and the prototype treatments sustained milk protein content similar to SM6 and SM12 treatments. Compared with CTL2, supplementation with SM12 increased plasma Met concentrations, and SM12 had greater concentrations than PT12 in the abdominal mammary vein. Compared with CTL2, SM12 decreased Met mammary clearance rate. Supplementation with RPM products may have increased protein synthesis in the udder of lactating cows. In conclusion, although supplementation with RPM products may have increased protein synthesis in the udder of lactating cows, it indeed increased plasma Met concentrations, confirming the ability of RPM products to increase plasma Met. Supplementation with RPM reduced mammary gland clearance of Met, suggesting the use of additional Met supply in other tissues.