Criteria to evaluate bone mineralization in cattle. I. Effect of dietary phosphorus on chemical, physical, and mechanical properties

Fourteen Angus heifers (210 +/- 6 kg initial BW) were allotted randomly to either a low P (LP: .12% P, DM basis) or an adequate P (AP: .20% P, DM basis) diet fed for 14 to 16 mo under drylot conditions on concrete floors to determine the influence of dietary P on chemical, physical, and mechanical properties of bone. Three weeks postpartum, after 14 to 16 mo on their diets, heifers were slaughtered and the right and left third metacarpals (McIII) were excised, soft tissue was removed and metacarpals were frozen in .9% saline. Metacarpals were subjected to a three-point flexure test using an Instron Testing Machine with a crossload speed of 50 mm/min to determine mechanical properties. Broken McIII were reassembled and a 2-cm section was removed at point of loading for determination of chemical and physical properties. Breaking load (BL) was greater (P < .05) for McIII from the AP than for those from LP heifers (1,348 vs 1,179 kg). Breaking strength (BS) was greater (P < .05) for AP than for LP heifers (202.5 vs 189.2 MPa). Animals receiving AP diets had greater (P < .01) bone mineral content (12.6 vs 11.2 g/2-cm slice) and percentage of bone ash (68.0 vs 67.2%) than did LP animals. No differences (P > .10) were observed between treatment groups in Ca, P, or Mg percentage in bone ash. Circular, elliptical, radiographic, and planimeter area indices all were greater (P < .05) in AP than in LP animals (1,048, 729, 1,069, and 570 vs 932, 660, 957, and 523 mm2, respectively). These data indicate that mechanical properties of bovine third metacarpals are sensitive to dietary P and reflect P status in the bovine. Mineral content of bone was highly correlated with its mechanical and physical properties.