From the DDN:

The four-day, eight-man tandem system will become a five-man rotation with four other pitchers staying on a four-day rotation.
“We’ve taken some of the younger pitchers and put them on five-days,” Powell said. “Many of those guys basically pitched in high school last year, and putting them on a four-day is a tough workload.”
In other words, teenagers Homer Bailey, Philippe Valiquette and 22-year-old Ramon Ramirez were coming up with tired or sore arms.
The new rotation will feature five designated pitchers starting every time out rather than pitching the second half of games. Camilo Vazquez is expected to start Monday, followed by Valiquette, Bailey (held out of a second straight start Saturday as a precaution), Ramirez and Rafael Gonzalez.
Although Valiquette is with the Dragons, he is on the roster of the rookie Billings team to give his arm a rest. For now, he is expected to take the roster spot of Gonzalez, who is on the temporary inactive list after going home to New York to be with his ailing mother.
Four other pitchers — Josh Thigpen, Jon George, Brad Morenko and Joe Wilson, will continue on a four-day rotation, pitching relief behind the five starters.