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饲养的重要饲料，能量丰富但蛋白质缺乏。该实验的目的是研究饲喂4种日粮粗蛋白（CP）加瘤胃保护的蛋氨酸（RPM）的玉米青贮饲料对牛奶和牛奶成分的生产的影响。三十六头奶牛被挤入9个方格的牛奶中，天数被随机分配给9个平衡的4×4拉丁方格，每个周期为4个4周。所有日粮均配制为含有50％的玉米青贮饲料，10％的苜蓿青贮饲料，4％的豆粕，2.4％的矿物质维生素补充剂和30％的中性清洁剂纤维，占干物质（DM）的百分比。在所有日粮中添加了补充RPM（Mepron，赢创公司，Evonik Corp.，肯尼索，乔治亚州），以保持AA消化后的Lys：Met比率为3.1。减少了地面高水分玉米，增加了豆粕（SBM）和RPM，使日粮平均含有11％CP（28％玉米，31％淀粉，6％SBM，4 g RPM / d），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 g RPM / d）。使用SAS（SAS Institute Inc.，Cary，NC）中的混合程序分析每个周期的最后14天的数据。除乳脂和乳乳糖含量外，我们发现饮食对所有生产性状均无显着影响。我们确实注意到饮食中CP浓度对摄入，牛奶和牛奶成分的生产以及MUN的线性响应。饲喂11％CP饲料的母牛减少了DM的摄入，减轻了体重，并且减少了牛奶和牛奶成分的摄入。平均分离表明，只有真正的蛋白质产量在13％的CP下比17％的日粮CP低，但在15％和17％的CP之间没有差异。这表明高于15％CP的牛奶和牛奶成分的生产没有改善。牛奶，经过能量校正的牛奶和纯蛋白质的产量呈二次趋势，这表明日粮中CP浓度可能超过15％，才能使产量最大化，或者达到稳定水平，不需要进一步的CP。尽管饮食会影响DM，有机物和中性清洁剂纤维的表观消化率，但消化率并不会随着日粮CP的增加而线性增加。但是，我们观察到日粮CP对酸性洗涤剂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N消化率以及粪便和尿N排泄有线性影响，但饮食对估计的真实N消化率没有影响。饲料中CP的氨，总AA，肽和支链挥发性脂肪酸的瘤胃浓度也呈线性增加。二次响应表明，要优化消化和能量利用，必须有14.0至14.8％的CP。总体结果表明，当添加RPM使Lys：Met增加到3.1时，饲喂补充了SBM的玉米青贮饲料的泌乳奶牛的CP含量为15％足够/ d；氮排泄量低于CP浓度为17％时的排泄量，但牛奶和牛奶成分的产量并未降低。为了优化消化和能量利用，必须使用8％的CP。总体结果表明，当添加RPM使Lys：Met增加到3.1时，饲喂补充了SBM的玉米青贮饲料的泌乳奶牛的CP含量为15％足够/ d；氮排泄量低于CP浓度为17％时的排泄量，但牛奶和牛奶成分的产量并未降低。为了优化消化和能量利用，必须使用8％的CP。总体结果表明，当添加RPM使Lys：Met增加到3.1时，饲喂补充了SBM的玉米青贮饲料的泌乳奶牛的CP含量为15％足够/ d；氮排泄量低于CP浓度为17％时的排泄量，但牛奶和牛奶成分的产量并未降低。