The objectiveof this work was to evaluate the effects of different chemical forms of nitrogenouscompounds (protein and non-protein) and carbohydrates (starch and soluble fiber)on intake, digestibility, and ruminal synthesis of microbial protein in cattlesupplemented during rainy season. It was used five crossbred Holstein ×Zebu steers, with average initial body weight of 211 ± 35 kg and fistulatedin the rumen and abomasum. The treatments were: control (only pasture), andsupplements based on corn + soybean meal; corn + urea, citrus pulp + soybeanmeal, and citrus pulp + urea. The supplements were balanced to present 30% ofcrude protein (CP), on dry matter (DM) basis, and provided at 3 g/kg BW. Theexperiment was carried out according to a 5 × 5 Latin square design ina 2 × 2 + 1 factorial arrangement, composed of two sources of nitrogenouscompounds, two sources of carbohydrates and a control treatment. Pasture intakewas reduced when supplements were fed to animals, with average substitutioncoefficient of 2.11 g of DM of pasture/g of DM of supplement. Supplementationdid not alter the total and ruminal digestibility coefficients of DM and thedietary content of total digestible nutrients (TDN) neither. Supplemented animalspresented higher ruminal and total digestibility coefficients of CP than non-supplementedanimals. The efficiency of microbial protein synthesis (EMPS), average of 123.1gof microbial CP/kg, was not changed by supplementation. However, animals supplementedwith corn presented higher EMPS in comparison to animals supplemented with citruspulp (137.6 and 106.1 g microbial CP/kg of TDN, respectively). Protein-energysupplementation for cattle grazing in tropical pastures during rainy seasondoes not cause nutritional benefits, which reflects the high coefficient ofsubstitution of forage by the supplement.
