The objective of this study was to evaluate the effect of different true protein:non-protein nitrogen ratios in supplements on digestibility, microbial protein synthesis and nitrogen balance in grazing cattle during rainy season. Five crossbred Holstein × Zebu steers, averaging 335±35 kg of body weight and fitted with rumen and abomasum canullae were used. The treatments were: control (only pasture), and supplements based on urea, 2/3 of nitrogenous compounds from urea and 1/3 of nitrogenous compounds from albumin, 1/3 of nitrogenous compounds from urea and 2/3 of nitrogenous compounds from albumin, and albumin. Two hundred grams of crude protein (CP) were daily supplied from supplements. The experiment was carried out according to a 5 × 5 Latin square design, with five 15-day experimental periods. Supplementation did not affect the total digestibility coefficients or the diet content of total digestible nutrients. Supplementation increased ruminal apparent digestibility coefficient of CP, which moved from negative for the control treatment to positive, but not different from zero, for treatments involving supplementation. Supplementation increased the rumen ammonia nitrogen concentration. Nitrogen balance was enhanced by supplementation. The replacement of urea by albumin caused a cubic effect on the nitrogen balance, with the higher value obtained at 1/3 of supplemental CP as urea. The efficiency of microbial protein synthesis was not influenced by supplementation or supplement composition. Supplementation with ruminal degradable nitrogenous compounds for grazing cattle during the rainy season increases the efficiency of use of pasture mainly by improving the body nitrogenous compounds retention.
albumin; ammonia nitrogen; partial digestibility; supplementation; urea
