Schwab Consulting LLC, 105 Doc Mac Drive, Boscobel, WI 53805.
Broderick Nutrition & Research LLC, 221 Glen Hollow Road, Madison, WI 53705.
J Dairy Sci. 2017 Dec;100(12):10094-10112. doi: 10.3168/jds.2017-13320.
Considerable progress has been made in understanding the protein and amino acid (AA) nutrition of dairy cows. The chemistry of feed crude protein (CP) appears to be well understood, as is the mechanism of ruminal protein degradation by rumen bacteria and protozoa. It has been shown that ammonia released from AA degradation in the rumen is used for bacterial protein formation and that urea can be a useful N supplement when lower protein diets are fed. It is now well documented that adequate rumen ammonia levels must be maintained for maximal synthesis of microbial protein and that a deficiency of rumen-degradable protein can decrease microbial protein synthesis, fiber digestibility, and feed intake. Rumen-synthesized microbial protein accounts for most of the CP flowing to the small intestine and is considered a high-quality protein for dairy cows because of apparent high digestibility and good AA composition. Much attention has been given to evaluating different methods to quantify ruminal protein degradation and escape and for measuring ruminal outflows of microbial protein and rumen-undegraded feed protein. The methods and accompanying results are used to determine the nutritional value of protein supplements and to develop nutritional models and evaluate their predictive ability. Lysine, methionine, and histidine have been identified most often as the most-limiting amino acids, with rumen-protected forms of lysine and methionine available for ration supplementation. Guidelines for protein feeding have evolved from simple feeding standards for dietary CP to more complex nutrition models that are designed to predict supplies and requirements for rumen ammonia and peptides and intestinally absorbable AA. The industry awaits more robust and mechanistic models for predicting supplies and requirements of rumen-available N and absorbed AA. Such models will be useful in allowing for feeding lower protein diets and increased efficiency of microbial protein synthesis.
在理解奶牛的蛋白质和氨基酸（AA）营养方面已经取得了相当大的进展。饲料粗蛋白（CP）的化学性质似乎已经得到很好的理解，瘤胃细菌和原生动物对蛋白质的降解机制也是如此。已经表明，从 AA 降解中释放的氨用于细菌蛋白质的形成，当饲喂低蛋白日粮时，尿素可以作为有用的 N 补充物。现在有充分的文献记载，为了最大限度地合成微生物蛋白，必须维持足够的瘤胃氨水平，并且瘤胃可降解蛋白的缺乏会降低微生物蛋白合成、纤维消化率和采食量。瘤胃合成的微生物蛋白占进入小肠的 CP 的大部分，并且由于表观高消化率和良好的 AA 组成，被认为是奶牛的优质蛋白质。人们非常关注评估不同方法来量化瘤胃蛋白质降解和逃逸，并测量微生物蛋白和瘤胃未降解饲料蛋白的瘤胃流出量。这些方法和伴随的结果用于确定蛋白质补充剂的营养价值，并开发营养模型并评估其预测能力。赖氨酸、蛋氨酸和组氨酸最常被认为是最限制的氨基酸，可利用赖氨酸和蛋氨酸的瘤胃保护形式来补充日粮。蛋白质饲养指南已经从简单的日粮 CP 饲养标准演变为更复杂的营养模型，旨在预测瘤胃氨和肽以及肠内可吸收 AA 的供应和需求。该行业正在等待更强大和更具机制性的模型来预测瘤胃中可用 N 和吸收 AA 的供应和需求。这样的模型将有助于饲喂低蛋白日粮和提高微生物蛋白合成效率。
