IRVING, Texas -- The big winner at the first day of the Dallas Cowboys' rookie minicamp was the weather.

Several players had to leave early in the afternoon practice, including second-rounder Randy Gregory because of dehydration issues.

Third-round pick Chaz Green also limped off the field early as did undrafted wide receiver Lucky Whitehead. Undrafted receiver Deontay Greenberry appeared to have the wind knocked out of him after making a deep catch near the sideline.

After the morning session Gregory said he was "a little bit out of shape," which is not unusual for rookies coming into their first NFL practices after a drawn-out draft process. Dez Bryant struggled with his stamina in his first practice as well.

"The heat is what was getting me," Gregory said after the morning practice. "Actually kind of cramping up when we got to special teams. Just got to hydrate more. In college you have a chance to get on a knee, have a helmet off. Can't really do that here. Trying to get used to that. But I'm having fun. That's the main thing. I think I'm doing a lot of good things out there. I'd like to do some more good things."

The Cowboys have two more days of practice ahead for the rookies but they could move Saturday's session to AT&T Stadium because of the impending rainy weather.