Marksmen from North Wales Police have shot dead a cow on the A55 expressway near Abergele.
Officers were called to the busy dual carriageway at 0700 BST on Wednesday after 17 cattle wandered across lanes to the central reservation.
A spokesperson for the force said one animal had to be "humanely destroyed" by armed officers.
The road has reopened after it was closed during the operation, leading to severe disruption.
"The police control room was flooded with 999 calls from concerned members of the public after the cows wandered across both carriageways and on to the central reservation," said a spokesman.
"The road was closed as officers and farmers attempted to herd the animals into farm vehicles.
"One cow broke away from the rest of the animals as it ran across the A55.
"It was humanely destroyed by armed officers as it was posing a danger to the public," the spokesman added.