The effects of oxalate in some tropical grasses on the availability to horses of calcium, phosphorus and magnesium

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Abstract
SUMMARY: Horses fed various species of tropical grass hays containing more than 0·5% total oxalate were in negative calcium balance of up to 41 mg/kg live weight/day in mineral balance experiments. In most cases, a concurrent negative phosphorus balance of up to 22 mg/kg live weight/day was measured. These results were in contrast to those obtained when grass hays low in oxalate were fed. Magnesium balance was little affected by oxalate. The degree of negative calcium balance could not be reconciled with total calcium and total oxalate concentrations, but could be explained if a major fraction of calcium in the grasses was in the form of calcium oxalate crystals and if calcium in this form was unavailable for absorption. It is suggested that soluble oxalates exert a variable but smaller effect on absorption of the remaining calcium fraction of the grasses. In general, nutritional secondary hyperparathyroidism may occur in horses grazed solely on pastures in which calcium to oxalate ratios are less than 0·5, such ratios usually being found in grasses containing more than 0·5% total oxalate.