The Figures below present the efficiency of conversion of gross energy intake into milk energy and the efficiency of conversion of N intake into milk protein N (commonly referred to as Nitrogen Use Efficiency, or NUE).  The energy graph was constructed based on NRC (2001) prediction but the N graph was constructed with data of the literature. Each point is the treatment average (n~ 72) of approximately 12 studies.

Instructions:

1. Explain the increase in efficiency of utilization of dietary energy with increase multiple of maintenance.  


2.  Explain the NRC system predicts that efficiency of utilization of dietary energy no longer increases for cows producing more that approximately 45-50 kg of milk. 


3. Why doesn't the conversion of dietary N into milk N follow the same pattern as for the energy?