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