Phosphorus: ruminal availability and effects on digestion.

The availability of P for ruminal digestion in vivo from a mono-dicalcium phosphate containing 21% P (mono-dical), a mono-dicalcium phosphate containing 18.5% P (dical), and defluorinated rock phosphate was compared with sodium phosphate (Na2HPO4 X 7H2O). Mono-dical, dical and defluorinated rock phosphate were found to be 88, 62 and 40% as available as sodium phosphate in the rumen. Compared with sodium phosphate, P from mono-dical, dical and defluorinated rock phosphate was 46.4, 28.8 and 2.5% as soluble in an in vitro ruminal buffer. In vitro P solubility in abomasal fluid increased with incubation time up to 1 h. Relative solubilities of the P sources at 1 h were 100, 71.6, 41.3 and 29.7% for sodium phosphate, mono-dical, dical and defluorinated rock phosphate, respectively. These sources can be solubilized in the abomasum and become available postruminally despite low solubility in the rumen. To determine the effect of P on ruminal and total tract digestion, diets low (.12%) and adequate (.23%) in P were fed to ruminally cannulated steers (700 kg) in a crossover design. Although higher ruminal P concentrations were detected with the high P diet than with the low P diet (398 vs 208 mg/liter), dry matter disappearance rate from nylon bags of ground corn, cotton duck or cottonseed hulls was unchanged. Estimated retention of P was higher (P less than .01) with the high P diet (8.3 g/d) than with the low P diet (1.0 g/d), but total tract digestibility was not enhanced significantly by added P. It appears that increasing ruminal P concentration from 208 to 398 mg/liter did not increase microbial cellulose digestion, but the low level was inadequate for maintaining the adult ruminant animal's P stores.