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Effect of Source and Particle Size of Supplemental Phosphate on Rumen Function of Steers Fed High Concentrate Diets¹

Department of Dairy Science, University of Illinois, Urbana 61801

ABSTRACT

We examined effects of source and particle size of supplemental defluorinated rock phosphate, to meet phosphorus requirements, on rumen function of 195-kg Holstein steers fed high concentrate. Two sources and two particle sizes of each source were evaluated in a 5 x 5 Latin square with 14-day periods. There was no effect of source on ruminal mH [– log (mean (H⁺)]; however, ruminal mH was higher in animals fed supplements of larger particle size. This effect was also evident when rumen pH versus time curves were integrated below pH 6. Animals fed supplements of larger particle size had less area below pH 6 than those fed supplements of smaller size. Ruminal buffering capacity at pH 7 was affected by diet; however, orthogonal comparisons between treatment means were not significant. Neither source nor particle size of the supplement affected ruminal fluid osmolality, total volatile fatty acid concentration, or fecal starch. Water intake and ruminal fluid outflow were greater as was ruminal dry matter on HyCal supplemented diets; however, there was also a trend toward increasing rumen fluid volume. The net effect was little change of dilution rate of ruminal fluid. This may explain why rumen fermentation was not affected greatly. Conventional phosphate supplements may have potential as rumen buffering agents, but higher levels of feeding should be studied.

¹ Supported in part by the University of Illinois Agricultural Experiment Station Hatch Project 35-360 and a gift from Amax Chemical Corporation, Greenwhich, CT.