Abstract
Twenty-four Holstein bull calves, 12 Holstein heifer calves, and 6 Jersey bull calves were assigned to a 2 × 3 factorial experiment at 8 to 9 wk of age. Three concentrations (.26, .34, and .41%) and two sources (feed grades monoammonium phosphate and dicalcium phosphate) of dietary P were fed during a 10-wk trial. The biological availability of monoammonium phosphate was equal to dicalcium phosphate as a supplemental P source. Increasing dietary P from .26 to .34% increased feed intake, body weight gain, plasma inorganic P, plasma Mg, bending moment of the tibia and ninth rib, percentage ash, and ash content (P and Mg) of the 7th and 10th ribs and lowered plasma Ca. Only plasma inorganic P increased from .34 to .41%. Treatment means for .26, .34, and .41% P, respectively, were feed intake (kg.d−1) 2.98, 3.48, and 3.34; daily body weight gain (kg.d−1) .83, .94, and .92; wk 9 and 10 plasma inorganic P (mg-100ml−1) 6.08, 8.38, and 9.05; bending moment (kg.cm) of the 9th rib 31.6, 47.6, and 40.7 and tibia 1314.84, 1783.94, and 1751.00. These results indicate a requirement for calves greater than the .26% P currently recommended.
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