The objective ofthis study was to evaluate the effect of different true protein:non-proteinnitrogen ratios in supplements on digestibility, microbial protein synthesisand nitrogen balance in grazing cattle during rainy season. Five crossbred Holstein× Zebu steers, averaging 335±35 kg of body weight and fitted withrumen and abomasum canullae were used. The treatments were: control (only pasture),and supplements based on urea, 2/3 of nitrogenous compounds from urea and 1/3of nitrogenous compounds from albumin, 1/3 of nitrogenous compounds from ureaand 2/3 of nitrogenous compounds from albumin, and albumin. Two hundred gramsof crude protein (CP) were daily supplied from supplements. The experiment wascarried out according to a 5 × 5 Latin square design, with five 15-dayexperimental periods. Supplementation did not affect the total digestibilitycoefficients or the diet content of total digestible nutrients. Supplementationincreased ruminal apparent digestibility coefficient of CP, which moved fromnegative for the control treatment to positive, but not different from zero,for treatments involving supplementation. Supplementation increased the rumenammonia 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 efficiencyof microbial protein synthesis was not influenced by supplementation or supplementcomposition. Supplementation with ruminal degradable nitrogenous compounds forgrazing cattle during the rainy season increases the efficiency of use of pasturemainly by improving the body nitrogenous compounds retention.
