BIOAVAILABILITY ASSESSMENTS OF GRANULAR CALCINED MAGNESITES DERIVED FROM MAGNESITE ROCKS AND OF MAGNESIUM-HYDROXIDE POWDER IN SHEEP

Granular calcined magnesites originating from Spain, China and Greece and magnesium hydroxide powder were evaluated in a series of balance e xperiments conducted with wether sheep (c. 42 kg liveweight) given a b asal ration of dried grass at Glasgow University Veterinary School bet ween 1989 and 1994. Four Spanish 'Agma' products and three Spanish 'Na varras' products contained less material (0-104g/kg)>1000 mu m diamete r than four Chinese and four Greek materials 178-483 g/kg). The Spanis h products had lower (18-45 g/kg) losses on ignition compared with the Chinese and Greek materials (53-112 g/kg). A fifth Greek product cont ained only 4 g/kg > 1000 mu m diameter. In three experiments comparing supplementation of the basal diet with 2.0 g Mg as either magnesium h ydroxide or as one of the three separate purchases of the commercial c alcined magnesite sources, the powdered magnesium hydroxide had the hi ghest mean apparent availability coefficient (0.30) (faecal increase m ethod). The corresponding values were 0.24 (Agma and Navarras), 0.20 ( Chinese) and 0.15 (Greek) (pooled S.E. 0.014, D.F. 10). Powdered magne sium hydroxide also had the highest (0.18) mean availability coefficie nt (urinary increase method) compared with 0.12 (Agma), 0.13 (Navarras ), 0.14 (Chinese), 0.10 (Greek) (pooled S.E. 0.005, D.F. 10). In two s ubsequent experiments, Agma had a superior apparent availability coeff icient (0.26) (faecal method) than either the Chinese (0.14) or the Gr eek materials (0.19). A finer grade of Greek calcined magnesite (0.25) was equivalent to Agma. For the 12 calcined magnesites evaluated in t he three main experiments, significant (P < 0.05) correlations (r) wer e found between the magnesium availability coefficient (faecal method) and loss on ignition (-0.65); particle size proportions (< 250 mu m, 0.59; and > 1000 mu m, -0.65) and tended towards significance for rume n solubility in 24 mu m nylon bags in hay-fed cows over two days (r = 0.54, P = 0.07) and rate of reaction with citric acid (0.57, P = 0.055 ). No correlations were found with magnesium availability coefficients (urinary increase method). This confirms the limitations of both in v itro predictions and in vivo (faecal) estimates of magnesium availabil ity for different calcined magnesites. The mean magnesium availability coefficient (faeces) of the 17 calcined magnesites was 0.21 (S.D. 0.0 5). If this were to be confirmed by other observations, it would have implications for the amounts of dietary calcined magnesite required to meet recommended dietary magnesium allowances for ruminants.