Effects of dietary imbalance of magnesium, calcium and potassium on plasma insulin in sheep and cattle

Different levels of magnesium, calcium and potassium were fed to 12 mature wethers and 30 Angus cows to investigate the effects of these cations on plasma concentrations on plasma immunoreactive insulin. Experiment I consisted of two phases each repeated in two trials. In the first phase, four groups of three wethers each were assigned to two dietary levels of magnesium (0.04 and 0.24%) and calcium (0.06 and 0.30%) in a 2X2 factorial arrangement of rations.

Dietary variables in the second phase were magnesium (0.04 and 0.27%) and potassium (2.1 and 4.7%). In experiment II, three groups of 10 cows each were assigned either to normal hay, tetany prone hay or tetany prone pasture as the only feed.

Plasma magnesium concentrations were reduced by low dietary magnesium in wethers. Cows on tetany prone hay and pasture heavily fertilized with nitrogen and potassium had lower plasma magnesium concentrations than cows on normal hay. Plasma calcium was not affected by ration in wethers but was significantly higher (P<.05) in cows fed normal hay than in those fed tetany prone hay. Plasma potassium of wethers was not affected by dietary magnesium and calcium levels when dietary potassium was held constant, but was increased (P<.05) when the wethers were fed the higher level of potassium regardless of magnesium intake. Plasma potassium of the cows was not influenced (P<.05) by ration. Plasma immunoreactive insulin of both sheep and cows was lower (P<.05) than levels reported as normal and was not affected by experimental ration.

Previous reports have shown that hyperkalemia can stimulate insulin secretion and that insulin can reach higher levels in hypomagnesemic animals. However, these changes were not detected in wethers fed semipurified diets containing different levels of magnesium or in cows fed hay or pasture which induced hypomagnesemia.