Arriola Apelo S I, Knapp J R, Hanigan M D
Department of Dairy Science, Virginia Tech, Blacksburg 24060.
Fox Hollow Consulting LLC, Columbus, OH 43201.
J Dairy Sci. 2014 Jul;97(7):4000-17. doi: 10.3168/jds.2013-7392. Epub 2014 Apr 24.
In current dairy production systems, an average of 25% of dietary N is captured in milk, with the remainder being excreted in urine and feces. About 60% of total N losses occur postabsorption. Splanchnic tissues extract a fixed proportion of total inflow of each essential AA (EAA). Those EAA removed by splanchnic tissues and not incorporated into protein are subjected to catabolism, with the resulting N converted to urea. Splanchnic affinity varies among individual EAA, from several fold lower than mammary glands' affinity for the branched-chain AA to similar or higher affinity for Phe, Met, His, and Arg. On average, 85% of absorbed EAA appear in peripheral circulation, indicating that first-pass removal is not the main source of loss. Essential AA in excess of the needs of the mammary glands return to general circulation. High splanchnic blood flow dictates that a large proportion of EAA that return to general circulation flow through splanchnic tissues. In association with this constant recycling, EAA are removed and catabolized by splanchnic tissues. This results in splanchnic catabolism equaling or surpassing the use of many EAA for milk protein synthesis. Recent studies have demonstrated that EAA, energy substrates, and hormones activate signaling pathways that in turn regulate local blood flow, tissue extraction of EAA, and rates of milk protein synthesis. These recent findings would allow manipulation of dairy diets to maximize mammary uptake of EAA and reduce catabolism by splanchnic tissues. Dairy cattle nutrient requirement systems consider EAA requirements in aggregate as metabolizable protein (MP) and assume a fixed efficiency of MP use for milk protein. Lysine and Met sufficiency is only considered after MP requirements have been met. By doing so, requirement systems limit the scope of diet manipulation to achieve improved gross N efficiency. Therefore, this review focuses on understanding the dynamics of EAA metabolism in mammary and splanchnic tissues that would lead to improved requirement prediction systems. Inclusion of variable individual EAA efficiencies derived from splanchnic and mammary responses to nutrient and hormonal signals should help reduce dietary protein levels. Supplementing reduced crude protein diets with individual EAA should increase gross N efficiency to more than 30%, reducing N excretion by the US dairy industry by 92,000 t annually.
在当前的奶牛生产体系中，日粮中平均25%的氮被用于合成牛奶，其余的则通过尿液和粪便排出体外。大约60%的氮损失发生在吸收后。内脏组织会摄取每种必需氨基酸（EAA）总流入量的固定比例。那些被内脏组织摄取但未被用于合成蛋白质的必需氨基酸会被分解代谢，产生的氮会转化为尿素。不同必需氨基酸的内脏摄取率各不相同，从比乳腺对支链氨基酸的摄取率低几倍到对苯丙氨酸、蛋氨酸、组氨酸和精氨酸的摄取率相似或更高。平均而言，85%被吸收的必需氨基酸会出现在外周循环中，这表明首过清除并非损失的主要来源。超过乳腺需求的必需氨基酸会返回体循环。高内脏血流量意味着返回体循环的大部分必需氨基酸会流经内脏组织。伴随着这种持续的循环利用，必需氨基酸会被内脏组织摄取并分解代谢。这导致内脏分解代谢等于或超过许多必需氨基酸用于合成牛奶蛋白的量。最近的研究表明，必需氨基酸、能量底物和激素会激活信号通路，进而调节局部血流量、必需氨基酸的组织摄取量以及牛奶蛋白的合成速率。这些最新发现有助于调整奶牛日粮，以最大限度地提高乳腺对必需氨基酸的摄取量，并减少内脏组织的分解代谢。奶牛营养需求体系将必需氨基酸的需求总量视为可代谢蛋白质（MP），并假定MP用于合成牛奶蛋白的效率是固定的。只有在满足MP需求后才会考虑赖氨酸和蛋氨酸的充足性。通过这样做，需求体系限制了日粮调整的范围，以实现更高的总氮效率。因此，本综述着重于了解乳腺和内脏组织中必需氨基酸代谢的动态过程，这将有助于改进需求预测体系。纳入源自内脏和乳腺对营养和激素信号反应的可变个体必需氨基酸效率，应有助于降低日粮蛋白质水平。用单个必需氨基酸补充低粗蛋白日粮，应能将总氮效率提高到30%以上，每年减少美国奶牛业的氮排泄量92000吨。
