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, a nd 13 multiparous Holstein cows, respectively, for a full lactation. Most o f the cows (33) were fed similar amounts of P in the previous one or two la ctations. The objective was to obtain information on bone integrity after p rolonged feeding of different amounts of P. At the end of the experiment, a section (similar to 20 cm) of the 12th rib was surgically removed. The bon e was tested for strength and analyzed for ash and P content. The sheer str ength and the energy required to deform the bone to the point of fracture d id not differ among treatments. Bone specific gravities were 1.50, 1.57, an d 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. Mil k 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 lactati ons 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 bo ne P content or strength. Dietary P content of 0.31% appears to be borderli ne deficient for cows producing > 11,900 kg/308 d.