A wagon has tipped over on the York-bound carriageway of the A64 at Huttons Ambo, near Malton.
Delays are expected.
A witness said: “It looks like it has gone over going round a corner.
“It was just lying on its side.”
A North Yorkshire police spokesman added: “Police were called at around 11.25am, after reports that a Land Rover had overturned on the A64 at Huttons Ambo.
“The Land Rover was carrying a trailer containing four cows.
“Following the collision the animals were secured safely in a nearby field. One of the cows sustained cuts to its mouth and leg in the incident.
“The driver of the Land Rover suffered minor injuries in the collision and did not require medical treatment.
“The westbound carriageway is blocked. However, recovery services are on their way to the scene to move the vehicles.”