This study was conducted in order to evaluate the effects of replacing dicalcium phosphate by rock phosphate in diets of growing steers. It was determined the following aspects: apparent digestibility of diets; phosphorus (P), calcium and fluorine apparent absorption; ruminal pH; ruminal ammonia concentration; microbial efficiency and plasma P. Five rumen fistulated Holstein steers weighting 275 - 283 kg were used. Experimental design was a 5 × 5 Latin square and diets consisted of 0, 25, 50, 75 and 100% of dicalcium phosphate replacement by rock phosphate in the mineral supplement. Addition of rock phosphate in the diets produced a linear increase on intake, rumen outflow and fecal output and total disappearance of fluorine. Diets did not differ to apparent absorption of calcium as well as dry matter, organic matter, crude protein, neutral detergent fiber and non-fiber carbohydrates intake, excretion, digestion and partial and total apparent digestibility. Plasma phosphorus was not influenced by diets and the average was 5.93 mg/dL. There was no difference for ruminal pH and ruminal ammonia concentration. Dicalcium phosphate replacement did not affect apparent and true protein microbial synthesis. Total replacement of dicalcium phosphate by rock phosphate in growing steer mineral supplement did not affect ruminal environment and protein synthesis in rumen. Considering this, dicalcium phosphate replacement in growing steer diets decreases P absorption and depending on its requirements it should be carefully seen.
