Two other victims were reported in serious condition.
A farm employee who was not struck by the lightning said it hit the victims Thursday afternoon, KUSA-TV, Denver, reported.
"It was all big blue as soon as it hit the ground. It started scattering all around. The people were moaning and everything. They were in pain," Guadalupe Jaquez said.
Ambulances transported the workers to a hospital, KUSA-TV said.
Fire officials said there were no visible injures but the victims may have been injured internally.
Lightning bolts will stop a person's heart or damage the neurological system, doctors said.