Vitalmiro Bastos de Moura has been tried three times and sentenced to between 29 and 30 years in prison, but his lawyers appealed and the Supreme Court annulled Moura's latest conviction, saying he wasn't given enough time to prepare his defense during the 2010 trial.
Prosecutors contend that Moura and another rancher hired gunmen to kill Stang, who was born in Dayton, Ohio. The defense contends there isn't enough evidence linking Moura to the crime.
Thursday's proceedings are taking place in Belem, the capital of the violence-wracked Amazonian state of Para. A verdict is expected by the end of the day.