NOTE : The mineral supplements used as feed supplements are not chemically pure compounds , and the composition may vary substantially among sources . The supplier ’' s analysis should be used if it is available . For example , feed - grade dicalcium phosphate contains some monocalcium phosphate and feed - grade monocalcium phosphate contains some dicalcium phosphate . Dashes indicate that no data were available . a Estimates suggest 90 to 100 % bioavailability of calcium in most sources of monocalcium phosphate , dicalcium phosphate , tricalcium phosphate , defluorinated phosphate , calcium carbonate , calcium sulfate , and calcitic limestone . The calcium in high - magnesium limestone or dolomitic limestone is less bioavailable ( 50 to 80 %). b Bioavailability estimates are generally expressed as a percentage of monosodium phosphate or monocalcium phosphate . c Most calcitic limestones will contain 38 % or more calcium and less magnesium than shown . d Iron in defluorinated phosphate is about 65 % as available as the iron in ferrous sulfate . ( NRC , 1998 )