Ionic milieu of bovine and ovine rumen as affected by diet.

Postprandial changes in osmolality, mineral, and volatile fatty acid concentrations in rumen fluid were examined in three cattle fed six widely differing diets. Mineral concentrations varied with diet and postprandial time. However, net changes in postprandial mineral concentrations did not contribute significantly to changes in osmotic pressure. There was an effect of diet and time after feeding on concentrations of volatile fatty acids. Postprandial changes in osmotic pressure were primarily due to changes in concentrations of volatile fatty acids. Magnesium oxide and sodium bicarbonate were fed to sheep to increase the osmotic pressure in rumen fluid. The salt supplement decreased acetate, propionate, heat, and methane production. However, rumen water flux was unaffected by the salt supplementation. Diets which produce hypertonic rumen fluid due to high mineral and/or concentrations of volatile fatty acids may reduce fermentation in rumen.