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

奶牛养殖必须有利可图，但对环境问题敏感。如果饲料能够更好地平衡，提供合成乳蛋白所需的氨基酸（AA：蛋白质的构建单元）的正确混合物，则可以通过较低的膳食蛋白质投入来实现这两个目标。我们的项目将研究AA代谢的两个主要问题，这两个问题可以纳入乳蛋白产量的预测模型，如NRC（2001）。首先，目前使用的固定转移吸收的AA到牛奶蛋白质导致低估牛奶蛋白质产量在低供应和高估在高供应。使用复杂的实验方法，我们将确定AA在肠道，肝脏和乳腺中损失的关键调节因子。这将量化与供应相关的AA损失的大小，并使我们能够推导出AA从吸收到牛奶的转移效率。这将更好地代表"农场“的情况，并带来对中间代谢的更好理解。其次，我们将评估AA代谢如何受到能量供应的影响，即我们能否通过更好的能量供应来减少AA代谢，然后减少蛋白质摄入量？除了补充我们对AA代谢的知识外，这些结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.