The fate of N in dairy cows: from mouth to milk

奶牛养殖业必须有利可图，但又要对环境问题敏感。如果饲料得到更好的平衡，以提供合成乳蛋白所需的氨基酸(AA：蛋白质的组成部分)的正确混合物，则可以通过较低的膳食蛋白质输入来实现这两个目标。我们的项目将考察AA代谢的两个主要问题，这两个问题可以纳入到牛奶蛋白质产量的预测模型中，例如NRC(2001)。首先，目前使用固定的将吸收的AA转移到牛奶蛋白的方法，导致在低供应时低估牛奶蛋白产量，而在高供应时高估牛奶蛋白产量。使用复杂的实验方法，我们将确定肠道、肝脏和乳腺中AA损失的关键调节因素。这将量化AA与供应有关的损失的大小，并使我们能够得出AA从吸收转移到牛奶的可变效率。这将更好地代表‘农场’的情况，以及对中间代谢的更好理解。其次，我们将评估能量供应对AA代谢的影响，即我们能否通过更好的能量供应来减少AA的分解代谢，进而减少蛋白质的摄入？除了补充我们关于AA代谢的知识外，这些成果还可以迅速转移到乳制品行业，并对农场实践产生直接影响，对环境和农场经济都有好处。

Dairy farming has to be profitable and yet sensitive to environmental concerns. Both can be achieved through a lower input of dietary protein if feeds were better balanced to provide the correct mixture of amino acids (AA: the building blocks of proteins) needed to synthesize milk protein. Our project will examine two major issues of AA metabolism that could be incorporated in the predictive models of milk protein yield, such as NRC (2001). First current use of a fixed transfer of absorbed AA to milk protein leads to underestimation of milk protein yield at low supply and overestimation at high supply. Using a sophisticated experimental approach, we will identify key regulators of the losses of AA across the gut, liver and mammary gland. This will quantify the magnitude of the losses of AA in relation to supply and enable us to derive the variable efficiency of transfer of AA from absorption to milk. This will better represent the situation 'on farm' bringing as well a better understanding of intermediary metabolism. Second, we will assess how AA metabolism is affected by energy supply, i.e. could we decrease AA catabolism and then protein intake with a better supply of energy? In addition to complementing our knowledge on AA metabolism, these results can be rapidly transferred to the dairy industry and have an immediate impact on farm practice, with benefits to both the environment and farm economics.