INFLUENCE OF MANUFACTURING CONDITIONS ON THE BIOAVAILABILITY OF MAGNESIUM IN CALCINED MAGNESITES MEASURED IN-VIVO AND IN-VITRO

Dietary calcined magnesite supplements of different particle sizes and temperatures of calcination were examined at Glasgow University Veter inary School in 1981. Balance experiments with wether sheep revealed t hat particle sizes < 75, 75-150, 150-250 and 500-1000 mu m diameter of a feed-grade calcined magnesite all increased urinary output of magne sium to a similar extent but the apparent magnesium availability coeff icient for the 500-1000 mu m diameter fraction (0.03) was significantl y less than for fractions of smaller diameter (0.17-0.23) (P < 0.01). A 1000-2000 mu m fraction of fertilizer-grade material had an apparent availability of 0.18 but had the least effect on urinary magnesium ou tput. Calcination temperatures of 800, 900 and 1100 degrees C signific antly increased apparent availability (c. 0.46) compared with that for temperatures of 500 and 650 degrees C (c. 0.12) (P < 0.01), and signi ficantly increased urinary output of magnesium. Losses of magnesium fr om these supplements incubated in the rumen of cows at grass in 24 mu m mesh nylon bags showed some correlations with their apparent availab ility and urinary magnesium output. Solubility in molar ammonium nitra te showed some good correlations with urinary magnesium output but not with magnesium availability. Supplementation of a low magnesium diet given to lactating ewes with fine particle (< 75 mu m) calcined magnes ite resulted in significantly greater increases in plasma magnesium co ncentration than when coarse particle material (500-1000 mu m) was giv en (P < 0.05), but magnesites calcined at 650 and 800 degrees C induce d similar changes in plasma magnesium.