Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/21/2008
Publication Date: 3/23/2009
Citation: Esser, N.M., Hoffman, P.C., Coblentz, W.K., Orth, M.W., Weigel, K.A. 2009. The Effect of Dietary Phosphorus on Bone Development in Dairy Heifers. Journal of Dairy Science. 92:1741-1749. Interpretive Summary: When phosphorus is fed to cattle in amounts greater than what they can utilize, the excess is excreted in the manure. If phosphorus-rich manure is later spread on farm fields to provide nutrients for growing crops, it may cause an excess of phosphorus in runoff that may end up in lakes where it can promote the undesirable growth of algae. For growing dairy heifers, phosphorus is essential for bone growth and development. The amount of phosphorus that they need to consume in their diet is very similar to the amount that’s found naturally in their diets, suggesting supplemental phosphorus in heifer diets may be minimally required. However, no long-term studies addressing this issue exist; therefore, we conducted a study on heifers from 4 to 22 months of age. Heifers were fed diets with or without supplemental phosphorus resulting in phosphorus concentrations of 0.29 or 0.39 percent. We intensively evaluated bone growth, composition, and metabolism on a sub-group of these heifers. Our results show that supplementing phosphorus to heifers had no effect on the extent of frame development, bone density, or bone metabolism. This information will help dairy producers and their nutritionists formulate heifer rations that contain adequate, but not excessive, amounts of phosphorus so that the growth needs of the heifers are met without adding excess phosphorus to the environment.
Technical Abstract: The NRC (2001) P requirements for dairy heifers (0.20 to 0.35%) and endogenous levels (0.20 to 0.35%) of P in feeds are similar, suggesting supplemental P in heifer diets may be minimally required. Because long-term studies are unavailable, 183 Holstein heifers and 182 crossbred heifers were fed diets with (0.39%) and without (0.29%) supplemental P from 4 to 22 mo of age in a replicated pen design. Two sub-populations of heifers were selected mid-trial for intensive measurement of bone development and metabolism. Thirty-two heifers at 628 d (± 10.0 d) of age, and balanced by breed and diet were evaluated for bone development. External frame measurements included hip height, length, heart girth, hip width, cannon bone circumference, as well as pelvic length, height, and width. Tails of heifers were surgically amputated with the 13 and 14th coccygeal vertebrae retained. After tissue removal, the 13th coccygeal vertebrae were scanned using peripheral quantitative computed tomography with cortical, trabecular and total bone densities determined. A second sub-population (n=64) of heifers (375 d ± 33 d), balanced for breed and diet, were evaluated for serum pyridinoline and osteocalcin to assess bone metabolism. Data were analyzed as a completely randomized design with breed, treatment and their interaction in the model. External skeletal measurements revealed significant differences in hip height, hip width, heart girth, cannon bone circumference and pelvic length between Holstein and crossbred heifers. Supplementing P had no effect on external frame measurements, bone density, or bone metabolism markers. Bone P and Mg contents were lower (P < 0.08) and bone K content greater in heifers fed no supplemental P. Data suggest P supplementation within dairy-heifer diets modestly increased bone P content, but increased bone P was not reflected in frame growth, bone density, or bone metabolism.