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Effect of dietary phosphorus on intestinal P absorption in growing Holstein steers
The effect of dietary phosphorus (P) intake on intestinal P absorption was evaluated in growing steers. Diets varying in P content (0.15%, 0.27%, 0.36% and 0.45%, DM basis) were fed to 8 growing Holstein steers (174 ± 10kg BW) fitted with permanent duodenal and ileal cannulas in a replicated 4×4 Latin square with 14 d periods.Ytterbium-labeled corn silage and cobalt-EDTA were used as particulate and liquid phase markers, respectively, to measure digesta flow. Duodenal and ileal samples and spot samples of urine were collected every 9 h from d 11 to 14. Total fecal collection was conducted on d 11 to 14with fecal bags. Blood samples were collected from the coccygeal vessel on d 14. Feed, digesta, and fecal samples were analyzed for total P and Pi using the molybdovanadate yellow method and blue method, respectively. Data was analyzed using PROC GLIMMIX in SAS with a model including treatment, square, period and interaction of treatment and square. Preplanned contrasts were used to evaluate linear and quadratic treatment effects. Results are reported as least square means. Dry matter intake (mean = 4.90 kg/d, 2.8% of BW) and apparent DM digestibility (mean =78.1%) were unaffected by treatment. Duodenal and ileal flow of total P increased linearly with increasing P intake (13.4, 18.5, 23.0 and 27.4 g/d, P < 0.01; 6.80, 7.87, 8.42, and 10.4 g/d, P < 0.05). Increasing P intake linearly increased the quantity of P absorbed from the small intestine (6.96, 11.1, 14.6 and 17.2 g/d, P < 0.01) but absorption efficiency was unchanged (mean = 59.6%). Phosphorus was absorbed on a net basis from the large intestine, but this was not affected by treatment and was a small percentage of total P absorption. Blood Pi increased linearly with increased dietary P (4.36, 6.31, 7.68, and 8.5 mg/dL, P < 0.01) and salivary P secretion was unchanged (mean = 5.79 g/d) suggesting that rumen function was prioritized during short-term P deficiency. The absence of change in absorption efficiency and salivary P secretion in the face of short term P deficiency may be used to improve published models of P digestion, absorption, and metabolism.
Keywords: phosphorus, absorption, growing steers