A semitrailer slammed into a small herd of cows Sunday morning, causing it to roll off the side of the road, killing its passenger and all five cows, the Colorado State Patrol said.

The tractor-trailer was hauling 205 barrels of crude oil westbound on Colorado Highway 66 in Weld County when five cows started to cross the road. The driver braked suddenly but he couldn't avoid the cows. The impact between the big rig and the herd sent the semitrailer rolling off the left side of the road.

The truck's passenger was pronounced dead at the scene. The driver, 43-year-old Arthur Anton, was taken to Northern Colorado Medical Center with moderate injuries. Both people were wearing seat belts.

The name of the passenger has not been released, pending notification of next of kin.

The 5:05 a.m. crash caused the truck to leak more than 200 gallons of oil into a ditch near the roadway. The Colorado State Patrol Hazardous Materials Unit was sent to the scene to clean and control the spill.

State troopers do not believe that excessive speed or alcohol played a role in the crash.