The natural chemical compounds released by some forage species or cover crops have been considered as important alternatives to minimize the use of synthetic herbicides in agriculture . This property depends on the ability of these natural compounds in inhibiting the germination and growth of the weed species. This work was planned to investigate this possibility for the cover crop named GMP29*. Dried aerial parts of GMP29 (5.0 kg) were powered and exhaustively extracted with methanol at room temperature. Vacuum concentration yielded the crude methanol extract (205.6 g), which was suspended in H2O:MeOH (1:1) and partitioned into hexane, dichloromethane, ethyl acetate and 1-butanol. The phytotoxic potential of the fractions was tested on Urochloa decumbens and the most active fractions (hexanic, dichloromethane and ethyl acetate) were submitted to successive chromatographic columns over silica gel and Sephadex LH-20 to afford three compounds. The isolated compounds were identified, by comparison of their spectroscopic data with those reported in the literature, as stigmasterol (1) isolated from the hexane fraction, tricin (2) from the dichloromethane fraction and p-cumaric acid (3) from the ethyl acetate fraction. The evaluation of GMP29 phytotoxic potential was carried in U. decumbens. The compounds were evaluated against seed germination, primary root and aerial parts length and root and aerial parts fresh and dry weight. The compound 1 was solubilized in chloroform and 2, 3 in methanol, and placed on double sheets of germination paper in plastic germination boxes (110 × 110 mm). The control received only methanol. The boxes were maintained for 24 h at rest to evaporate the methanol. The final concentration of the fractions was 500 µg mL−1. After 24 h, each germination box received 10 mL of distilled water and seeds of U. decumbens. Each treatment used five plates (replicates), and each replicate consisted of 50 seeds distributed among the germination boxes. These boxes were placed in a growth chamber with a photoperiod specific (8 h light (photon flux density of approximately 230 mol m−2 s−1) at 30 °C and 16 h dark at 20 °C). The aerial parts and primary roots were excised for measurement of their length and fresh weight. Dry weight was evaluated after incubation of the fresh materials at 60 °C for 48 h. The phytotoxic activity revealed significant effects only for substance 3, which inhibited all analyzed parameters. There was a reduction of 80.95% in seed germination and 76.08% in aerial parts length, in comparison to control. The fresh and dry weight of the aerial part was also reduced in 93.9 and 84.85%, in relation to the control, respectively. There was no root development. Although substances 1 and 2 promoted a tendency to reduce root and aerial part length, there was no significant difference in comparison to controls. This work provided important data for the use of straws of GMP29 in weed suppression in the field, an important property for choosing this forage for the reduction of herbicide use.