The objective of the present study was to determinate the nitrogenous compounds fractions present in the extrusa of grazing cattle maintained in a natural grassland in the Zona da Mata region, MG. The samples were obtained by using four esophageal fistulated steers in two experimental periods during the rainy season of 1996, and three of these animals were used to take samples in two experimental periods during the dry season of the same year. In order to obtain the nitrogen fractionation (fractions A, B1, B2, B3 e C), the crude protein (CP), non-protein nitrogen (NPN), borate-phosphate buffer insoluble nitrogen, and neutral detergent and acid detergent insoluble nitrogen were determined. The in vitro degradation profiles of the protein fractions were obtained by incubating the pasture samples with Streptomyces griseus proteases. With these techniques it was possible to detect reductions in the CP content and in the percentage (% CP) of B3 fraction and increases in the percentages of the A, B1 and C fractions from EC to ES; no differences were found for the percentage of the fraction B2 between seasons. The degradation rates of fractions B2 and B3 were extremely variable: its average estimates were 2.427 and 0.057 h-1, respectively, during the rainy season, and 2.149and 0.034 h-1, during the dry season.
nitrogenous compounds; tropical pastures; ruminants
