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

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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.