Effect of change of diet on the mineral composition of rumen fluid, on magnesium metabolism and on water balance in sheep.

1. The effects of four diets on water intake, rumen fluid outflow-rate, rumen pH and mineral metabolism were studied in wether sheep. The diets were barley and hay, flaked maize and hay, dried grass and frozen grass. 2. Experimental periods were of 12 d duration, and plasma magnesium concentrations were lower at the end of treatment periods when the grass diets were given and were significantly different (P less than 0.05) at 11.00 and 20.45 hours. Also, the concentration was significantly lower with the dried-grass diet than with the frozen-grass diet (P less than 0.05). 3. The concentration of Mg in rumen fluid centrifuged at 30,000 g (ultracentrifuged) varied with the diet. Maximum concentrations (tmax) were reached 4 h later on the grass diets than on the hay and concentrate diets. In the latter case tmax coincided with that for calcium, potassium, chloride and ammonia. At this time sodium and phosphate were at a minimum. The concentration of Mg in ultracentrifuged rumen fluid was negatively correlated (r -0.89) with pH, which was significantly higher (P less than 0.01) at all times on the grass diets. This relation was also reflected in the apparent availability of Mg. 4. Total water intake on the frozen grass was about twice that on the barley and hay diet. The outflow rate of liquid from the rumen was higher on the frozen grass than on the other three diets. 5. The proportion of absorbed Mg excreted in urine was significantly influenced by diet.