Abstract
In areas with intensive livestock production, grass and grassilage are generally rich in K which, after feeding, causes a low efficiency of Mg absorption in cows and enhances the risk of hypomagnesemia. Concentrates generally are relatively low in K. Thus, we hypothesized that iso-energetic substitution of concentrate for dried grass would raise Mg absorption. We fed nonpregnant, nonlactating cows on rations containing artificially dried grass (39.4 g K/kg dry matter) and increasing amounts of concentrate (13.0 g K/kg dry matter) while keeping Mg intake constant (17.4 g Mg/cow per day). Four cows fitted with a rumen fistula were subjected to a Latin square design with dietary treatments consisting of the following roughage/concentrate ratios expressed as energy percentages: 100:0, 80:20, 60:40 and 40:60. Increasing the amount of concentrate from zero to 60% raised apparent Mg absorption from 12.1 to 1.5% of intake. Magnesium absorption, expressed as % of intake, rose by 0.56 percentage units when the dietary K content fell by 1 g/kg dry matter. Ruminal K concentrations, rather than ruminal pH, ruminal Mg concentration and rumen volume, contributed significantly to the observed variance in Mg absorption. This study provides information which does not invariably support the common practice of supplementing with Mg the concentrates to be used in concentrate-rich rations for high-producing dairy cows. Calculations are presented that indicate that at least under specified conditions, supplementation with Mg would be unnecessary. (C) 2002 Elsevier Science B.V. All rights reserved.