A 45-year-old woman was left with serious injuries after being thrown from a horse in West Keal.
The woman was riding her horse along the A155 from Church Lane towards Hall Lane at approximately 2pm on Tuesday, January 3.
A green Claas tractor with red writing was travelling behind the horse and as the driver overtook it, the horse threw the rider.
The tractor driver did stop but then left the scene.
The rider was taken to Boston Pilgrim Hospital.
Lincolnshire Police are keen to trace the tractor driver.
Anyone with information should contact PC Lee Gennery, from the roads policing team, on 101, quoting incident number 84 of January 4.