Milk production, estimated phosphorus excretion, and bone characteristics of dairy cows fed different amounts of phosphorus for two or three years.

Diets containing 0.31, 0.39, or 0.47% P on a DM basis were fed to 10, 14, and 13 multiparous Holstein cows, respectively, for a full lactation. Most of the cows (33) were fed similar amounts of P in the previous one or two lactations. The objective was to obtain information on bone integrity after prolonged feeding of different amounts of P. At the end of the experiment, a section (-20 cm) of the 12th rib was surgically removed. The bone was tested for strength and analyzed for ash and P content. The shear strength and the energy required to deform the bone to the point of fracture did not differ among treatments. Bone specific gravities were 1.50, 1.57, and 1.55 for the three treatments. Ash and P content of the bone, measured in wet weight, dry weight, and wet bone volume, were similar for the 0.39 and 0.47% P treatments, but tended to be lower with the 0.31% P treatment. Milk production of cows in all groups was high, averaging >11,900 kg for the 308-d lactation. Feeding P at 0.31% of dietary DM over two to three lactations appeared to decrease P concentration of bone, but the decrease was not severe enough to affect bone strength. Dietary P at 0.39% did not affect bone P content or strength. Dietary P content of 0.31% appears to be borderline deficient for cows producing >11,900 kg/308 d.