Influence of Variations in Dietary Calcium: Phosphorus Ratio on Performance and Blood Constituents of Calves

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Abstract
Forty-five Hereford calves were used in a factorial experiment with three levels each of calcium (0.27, 0.81 and 2.43% of the diet) and of phosphorus (0.17, 0.34 and 0.68% of the diet). The 9 resulting calcium: phosphorus ratios ranged from 0.4:1 to 14.3:1. Levels and ratios of Ca and P were obtained by addition of varying amounts of calcium carbonate, defluorinated rock phosphate and dibasic sodium phosphate to a semi-purified diet based on degerminated corn meal, starch, glucose, urea, blood meal, beet pulp, corn oil, minerals and vitamins. The calves were individually fed, ad libitum, over a period of 98 days for the first two replications and 112 days for the last three replications. Performance and nutrient conversion were markedly decreased with Ca:P ratios lower than 1:1. Ratios between 1:1 and 7:1 gave similar and satisfactory results. Ca:P ratios above 7:1 resulted in decreased performance and nutrient conversion values, but adverse effects were not as marked as with the ratios below 1:1. Several significant trends were evidenced in serum inorganic phosphorus, magnesium and phosphatase activity attributable to levels and ratios of dietary calcium and phosphorus. Notable among these observations was that serum magnesium levels were decreased with high levels of dietary phosphorus when dietary calcium levels were low. Diets with high calcium levels resulted in a “normal” serum magnesium level even at the highest phosphorus level.