A trainer has been suspended and fined over the dumping of dead horses at a conservation park south of Adelaide.

Thoroughbred Racing SA says trainer Trevor Day engaged in conduct prejudicial to the image of racing after disposing of 13 horses in the Mount Magnificent Conservation Park.

Stewards determined the horses were in Day's care before their deaths and were disposed of over a nine-year period.

In a statement on Wednesday Thoroughbred Racing SA said an inquiry revealed no evidence of ill treatment or welfare-related issues with any of the horses and had taken Day's guilty plea and clean record into account.

He was fined $2,500 and suspended for 27 months but has a right to appeal the penalties.

The horses were found in April and some were identified as unraced thoroughbreds.