Corn silage, an important forage fed to dairy cows in the United States, is energy rich but protein poor. The objectives of this experiment were to investigate the effects on production of milk and milk components of feeding corn silage-based diets with 4 levels of dietary crude protein (CP) plus rumen-protected methionine (RPM). Thirty-six cows were blocked by days in milk into 9 squares and randomly assigned to 9 balanced 4 × 4 Latin squares with four 4-wk periods. All diets were formulated to contain, as a percent of dry matter (DM), 50% corn silage, 10% alfalfa silage, 4% soyhulls, 2.4% mineral-vitamin supplement, and 30% neutral detergent fiber. Supplemental RPM (Mepron, Evonik Corp., Kennesaw, GA) was added to all diets to maintain a Lys:Met ratio of 3.1 in digested AA. Ground high-moisture corn was reduced and soybean meal (SBM) plus RPM increased to give diets containing, on average, 11% CP (28% corn, 31% starch, 6% SBM, 4 g of RPM/d), 13% CP (23% corn, 29% starch, 10% SBM, 8 g of RPM/d), 15% CP (19% corn, 26% starch, 15% SBM, 10 g of RPM/d), and 17% CP (14% corn, 24% starch, 19% SBM, 12 g of RPM/d). Data from the last 14 d of each period were analyzed using the mixed procedures in SAS (SAS Institute Inc., Cary, NC). With the exception of milk fat and milk lactose content, we found no significant effects of diet on all production traits. We did note linear responses to dietary CP concentration for intake, production of milk and milk components, and MUN. Cows fed the 11% CP diet had reduced DM intake, lost weight, and yielded less milk and milk components. Mean separation indicated that only true protein yield was lower on 13% CP than on 17% dietary CP, but not different between 15 and 17% CP. This indicated no improvement in production of milk and milk components above 15% CP. Quadratic trends for yield of milk, energy-corrected milk, and true protein suggested that a dietary CP concentration greater than 15% may be necessary to maximize production or, alternately, that a plateau was reached and no further CP was required. Although diet influenced apparent digestibility of DM, organic matter, and neutral detergent fiber, digestibility did not increase linearly with dietary CP. However, we observed linear and quadratic effects of dietary CP on acid detergent fiber digestibility. As expected, we found a linear effect of dietary CP on apparent N digestibility and on fecal and urinary N excretion, but no effect of diet on estimated true N digestibility. Ruminal concentrations of ammonia, total AA, peptides, and branched-chain volatile fatty acids also increased linearly with dietary CP. Quadratic responses indicated that 14.0 to 14.8% CP was necessary to optimize digestion and energy utilization. Overall results indicated that, when RPM was added to increase Lys:Met to 3.1, 15% CP was adequate for lactating dairy cows fed corn silage diets supplemented with SBM and secreting about 40 kg of milk/d; N excretion was lower than at 17% CP but with no reduction in yield of milk and milk components.

玉米青贮饲料是美国奶牛的重要饲料，能量丰富但蛋白质匮乏。本实验的目的是研究饲喂以玉米青贮饲料为主的日粮中添加 4 种粗蛋白 (CP) 和过瘤胃保护蛋氨酸 (RPM) 的饲料对牛奶产量和牛奶成分的影响。将 36 头奶牛按牛奶天数分为 9 个方格，并随机分配到 9 个平衡的 4 × 4 拉丁方格，每个方格有 4 个 4 周的周期。所有日粮均按干物质 (DM) 的百分比配制，含有 50% 玉米青贮饲料、10% 苜蓿青贮饲料、4% 豆皮、2.4% 矿物质维生素补充剂和 30% 中性洗涤纤维。所有日粮中均添加补充 RPM（Mepron，Evonik Corp.，Kennesaw，GA），以维持消化 AA 中的 Lys:Met 比例为 3.1。减少磨碎的高水分玉米，增加豆粕 (SBM) 加 RPM，使日粮平均含有 11% CP（28% 玉米、31% 淀粉、6% SBM、4 克 RPM/天）、13% CP（23% 玉米、29% 淀粉、10% SBM、8 g RPM/d）、15% CP（19% 玉米、26% 淀粉、15% SBM、10 g RPM/d）和 17% CP （14% 玉米、24% 淀粉、19% SBM、12 克 RPM/d）。使用 SAS（SAS Institute Inc.，卡里，北卡罗来纳州）中的混合程序对每个周期最后 14 天的数据进行分析。除乳脂和乳糖含量外，我们发现日粮对所有生产性状均无显着影响。我们确实注意到饮食CP浓度对摄入量、牛奶产量和牛奶成分以及MUN的线性响应。饲喂 11% CP 日粮的奶牛 DM 摄入量减少，体重减轻，产奶量和乳成分减少。平均分离表明，仅 13% CP 的真实蛋白质产量低于 17% 日粮 CP 的真实蛋白质产量，但 15% CP 和 17% CP 之间没有差异。 这表明牛奶产量和牛奶成分含量高于 15% CP 没有改善。牛奶产量、能量校正牛奶和真正蛋白质的二次趋势表明，日粮 CP 浓度可能需要大于 15%，才能最大限度地提高产量，或者达到稳定水平，不再需要进一步的 CP。尽管饮食影响DM、有机物和中性洗涤纤维的表观消化率，但消化率并不随饮食CP线性增加。然而，我们观察到日粮 CP 对酸性洗涤纤维消化率的线性和二次影响。正如预期的那样，我们发现膳食CP对表观氮消化率以及粪便和尿液氮排泄量具有线性影响，但饮食对估计的真实氮消化率没有影响。瘤胃中氨、总 AA、肽和支链挥发性脂肪酸的浓度也随日粮 CP 线性增加。二次响应表明，14.0% 至 14.8% 的 CP 对于优化消化和能量利用是必要的。总体结果表明，当添加RPM将Lys:Met提高至3.1时，对于饲喂添加SBM的玉米青贮日粮且每天产奶约40kg的泌乳奶牛来说，15%CP是足够的；氮排泄量低于 17% CP 时的水平，但乳汁和乳汁成分的产量没有减少。