The capacity of the mature cow to lose and recover nitrogen and the significance of protein reserves.

I . Six experiments were undertaken with mature, dry, non-pregnant cows to determine the capacity to lose and recover nitrogen and to study the significance of the labile and total protein reserves. 2. It was concluded that, without altering its ability to reach N equilibrium, the mature cow is able to store and lose large amounts of body proteins (certainly more than 15 kg), when its N and energy intakes are greatly varied. The repletion or depletion of protein reserves can occur over a long period of time (sometimes more than 5 months). Total protein reserves include a very labile part. This part is more rapidly lost during fasting than during feeding on low-protein diets; it can be used to meet temporarily the energy requirements of the animal. 3. N balance and body-weight gains were generally very well correlated.