Parametric excitation of magnetoelastic waves in the easy-plane antiferromagnet FeBO3 is studied at room temperature. Measurements were performed at X-band microwave with a perpendicular pumping configuration. As increasing the pumping power, the microwave absorption by a quasimagnon mode is saturated above the threshold level. Above this threshold, the number of quasiphonons with a half pumping frequency grows exponentially. The microwave radiation from these quasiphonons is simultaneously observed. Further increase of microwave power causes auto-oscillations in the magnetoelastic system. The radiation spectrum shows side band peaks around a half pumping frequency.