Blood plasma magnesium, potassium, glucose, and immunoreactive insulin changes in cows moved abruptly from barn feeding to early spring pasture.

Cations and immunoreactive insulin in plasma were measured in 35 lactating cows moved abruptly to early spring pasture. After change of cows from grass-clover hay to fescue-bluegrass pasture containing 22 to 31 g potassium/kg dry matter, immunoreactive insulin of 5 Holstein cows increased 30% in 5 days and averaged 45% above prepasture concentrations for 40 days. Magnesium averaged 44% below prepasture content of plasma during this period and was correlated negatively with potassium -.17 and immunoreactive insulin -.37. Thirty Herford cows were changed from corn silage and grass-clover hay to wheat-rye pasture containing 3.06% potassium in the dry matter. Each day on pasture, 10 cows each were fed 2.3 kg cornmeal, 10 were given 30 g magnesium oxide by capsule, and 10 were given no supplement. After unsupplemented cows were moved to pasture, immunoreactive insulin rose 51% in 8 days and plasma magnesium fell 24%. Both supplements reduced immunoreactive insulin, but magnesium was maintained higher by magnesium oxide than by cornmeal. Injection of two Holstein cows with insulin (2 IU/kg body weight) reduced plasma concentrations of both potassium and magnesium 20% below that of two cows injected with only physiological saline. Whether elevated plasma insulin may accelerate development of hypomagnesemia in cattle on spring pasture with relatively high potassium content has not been established.