Effect of dietary mineral phosphorus and phytate on in situ ruminal phytate disappearance from different concentrates in dairy cows.

The first objective of this study was to determine the influence of dietary composition on the in situ disappearance of phytate (InsP6) from wheat, corn, soybean meal, and rapeseed meal [solvent-extracted, without (RSM) or with (hRSM) heat treatment] in the rumen of dairy cows. The second objective was to assess the primary degradation products of InsP6 in the rumen. Three diets differing in phosphorus and InsP6 concentration (basal diet = 0.38% P in dry matter; high-P diet = 0.56% P; high-InsP6 diet = 0.39% P) were fed to 3 ruminally fistulated lactating Jersey cows in a 3 × 3 Latin square. Ground concentrates (sieve size = 2 mm) were incubated in polyester bags in the rumen for 2, 4, 8, 16, and 24 h. The bag residues were analyzed for P, InsP6, isomers of lower inositol phosphates (InsP5, InsP4, InsP3), and crude protein. The InsP6 disappeared more rapidly from cereal grains than from oilseed meals; however, after 24 h of incubation ≥95% InsP6 had disappeared from all concentrates except hRSM (57%; diet average). Feeding the high-InsP6 diet increased InsP6 disappearance for oilseed meals, but not for corn and wheat. The predominant InsP5 isomer in all bag residues was Ins(1,2,4,5,6)P5 followed by Ins(1,2,3,4,5)P5 and Ins(1,2,3,4,6)P5. A further InsP5 isomer [Ins(1,3,4,5,6)P5] was detected in both rapeseed meal bag residues. Feeding the high-InsP6 diet led to lower concentrations of Ins(1,2,4,5,6)P5 and Ins(1,2,3,4,5)P5, whereas an interaction between diet, concentrate, and time occurred for Ins(1,2,3,4,6)P5 and Ins(1,3,4,5,6)P5. The results confirm the high potential of rumen microorganisms to hydrolyze InsP6; however, increasing the amount of InsP6 in the diet can further enhance InsP6 hydrolysis, which may be relevant when concentrates with slowly degradable InsP6, such as RSM or heat-treated concentrates, are fed to dairy cows. Based on the concentrations of InsP5 isomers, 3 and 6 phytases appear to play a major role in the rumen. Conversely, intrinsic plant phytase activity appears to be less relevant as the percentage of its primary hydrolysis product, Ins(1,2,3,4,5)P5, changed only slightly upon using wheat known for high intrinsic phytase activity instead of the other concentrates. Additional information regarding the factors influencing the extent of ruminal InsP6 disappearance will require further studies to determine the phytase activity of rumen microorganisms and the characteristics of their respective phytases.