Comparison of methods to determine relative bioavailability of magnesium in magnesium oxides for ruminants

Two Mg balance trials were conducted with wethers to compare relative bioavailability of Mg in several Mg oxides with that of reagent grade sulfate as determined by different methods. In Experiment 1,600 ppm Mg as sulfate or four feed grade oxides varying in origin and particle size were added to a semi-purified basal diet (200 ppm Mg). Diets were fed at 800 g/d to 30 crossbred wethers during the 14-d trial and fecal and urinary collections were made during the last 7 d. In Experiment 2, the basal corn-soybean meal-cottonseed hull diet (1387 ppm Mg) was supplemented with 0, 700, 1400, or 2100 ppm added Mg as reagent grade sulfate or 1400 ppm Mg as three of the oxides from Experiment 1 and fed at 1000 g/d to 35 wethers. Urine was collected daily for 10 d and feces were collected on d 7 through 10. In Experiments 1 and 2, absorption of Mg from the oxide of brine origin and larger particle size distribution was lower than that from sulfate, but there was no difference in absorption for sheep fed oxides derived from sea water or calcined magnesite. In Experiment 2, urinary Mg excretion on d 4 and 5 of the collection was lower for sheep fed the brine oxide than for those fed sulfate or oxide from calcined magnesite. Urinary Mg excretion on d 4 and 5 following addition of 1400 ppm Mg to practical diets could be used to predict bioavailability of Mg.