CHANGES IN SODIUM REQUIREMENT OF THE SHEEP ASSOCIATED WITH CHANGES OF DIET

Observations were made on 3 sheep while their rations were changed from dried grass cubes to fresh frozen grass, then to a dry diet of hay and meals and finally back to fresh frozen grass. In spite of the low sodium intake on the fresh grass, the sheep were neither losing nor gaining sodium up to the time the change to hay and meals was made. When hay and meals were given, the excretion of sodium in the urine fell to very low levels, and the composition of the saliva changed as if aldosterone secretion were increased. Since the hay and meals ration had the highest sodium content, a net retention of sodium was observed. This retention, accompanied by an increase in liveweight, appeared to be due to a sequestration of sodium and water within the rumen. On returning to the grass ration the sodium retained during the period when hay and meals were given was rapidly lost in the urine with a corresponding fall in the liveweight. The transient changes in sodium requirement associated with certain dietary changes thus appeared to reflect changes in the amount of sodium in the rumen. Less potassium tended to be retained during the period when hay and meals were given, but more was retained when the final ration of grass was begun; these changes were smaller than the changes in sodium balance. A transient mild hypomagnesaemia accompanied an initial period of magnesium loss both times that grass feeding began, with recovery of the concentration of magnesium in the plasma during the diet of hay and meals. We wish to thank Professor A. T. Phillipson and Dr. K. B. Blaxter for advice and criticism of the manuscript. Appreciation is also offered to Mr. J. Ingram and Mr. W. Fettes for skilled technical assistance during the course of these experiments. We also wish to thank Dr. F. Raymond, Grassland Research Institute, Hurley for advice on the cutting and storing of the frozen herbage.