Effect of dietary phytate on phosphorus digestibility in dairy cows1

The objective was to evaluate the effect of dietary phytate P (Pp) supply on ruminal and postruminal Pp digestion and net disappearance of P from the lower digestive tract of lactating cows. Six ruminally and ileally cannulated crossbred lactating cows were used in 2 incomplete Latin squares with four 21-d periods (17 d of diet adaptation, 4 d of total collection). Dietary treatments were low Pp, medium Pp, and high Pp, and a high inorganic P (Pi) diet with the same total P content as the highest Pp diet but with P mostly from inorganic sources. The diets contained 0.10, 0.18, 0.29, and 0.11% Pp and 0.43, 0.48, 0.54, and 0.53% total P on a dry matter basis, with cottonseed meal used to increase Pp content. Ytterbium-labeled corn silage and Co-EDTA were used as particulate and liquid phase markers to measure omasal and ileal digesta flow. Omasal and ileal digesta were collected every 6 h on d 20 and 21 and rumen contents were collected on d 21. Samples were analyzed for total P (molybdovanadate yellow method), Pi (blue method), and Pp (high performance ion chromatography). Phytate P and total P intake increased linearly with increasing dietary Pp. Ruminal Pp disappearance also increased linearly with dietary Pp but the magnitude of change was small. Small intestinal net disappearance of Pi was not affected by dietary Pp. Phytate P was hydrolyzed in the large intestine but its hydrolysis was not influenced by dietary Pp. Net disappearance of Pi form the large intestine did not vary with dietary Pp. Dry matter digestibility decreased linearly with increasing dietary Pp, as did apparent digestion of P, and fecal P increased linearly. Dry matter digestibility was higher for high Pi than for high Pp, likely due to the effect of cottonseed meal in the latter diet. Replacing a portion of Pp with Pi resulted in decreased P excretion but this effect was confounded with increased fecal dry matter for the high-Pp (high-cottonseed meal) diet. In lactating cows Pp digestibility was not negatively influenced by dietary Pp and fecal P excretion was regulated by dietary total P rather than by form of dietary P.