Effects of magnesium supplements on the digestion of forages and milk production of cows with hypomagnesaemia

Abstract

ABSTRACT1. Ground forages were digested in vitro in rumen liquor supplemented with four concentrations of magnesium as MgSO4-7H2O (5, 20, 80, 320mg Mg/1) to determine the effects of magnesium on organic matter digestibility. A medium concentration of magnesium (generally 80 mg/1) resulted in optimum digestion although there was a large interaction between forage type and the response to magnesium.2. Supplements of MgSO4-7H2O (2g Mg/day) increased the apparent digestibility of two types of hay fed to sheep, the difference being significant for poor quality hay (P<0·05).3. The milk production of hypomagnesaemic cows given supplementary magnesium either orally, by rectal infusion, or by subcutaneous injection was compared. Plasma magnesium concentrations were increased to a similar and significant extent (P<0·01 ) but a significant increase in milk and fat production occurred only in the orally treated cows (P<0·05).It was concluded that a dietary magnesium deficiency results in reduced rumen fermentation and hence poor production, as well as hypomagnesaemia. It was suggested that the effects on performance of other mineral deficiencies may also be mediated in part through depressions in rumen digestion.