Ruminal Availability of Phosphorus and Its Effect on Digestion

Story in Brief The phosphorus concentration in ruminal fluid was determined for steers limit-fed rations with phosphorus concentrations ranging from .06 to .17 percent. Ruminal phosphorus concentration was increased with phosphorus intake at dietary concentrations below .12 percent for steers. This relationship was employed to estimate the ruminal availability of phosphorus from four phosphorus sources. Compared with sodium phosphate, assumed to have a ruminal phosphorus a\'ailability of I00 percent, a 21-percent phosphorus mono-dical, an 18.5-percent phosphorus mono-dical, and a defluorinated rock phosphate were found to be 59, 42 and 28 percent as available in the rumen. Phosphorus solubility in solutions butTered at pH 5, 6 and 7 paralleled the apparent availability in the rumen. Low ruminal dietary phosphorus did not reduce total tract or ruminal digestibility greatly, howe\'er, suggesting that ruminal concentrations above 210 mg per liter are adequate for microbial digestion in the rumen.