Genetic parameters of juvenile osteochondral conditions (JOCC) in French Trotters.

Juvenile osteochondral conditions (JOCC) have been defined as lesions resulting from biomechanical influences (compressive, tensional or shear forces) on the developing and growing musculoskeletal system. They include different types of osteochondrosis, osteochondral fragmentation of the articular surface or of the periarticular margins, juvenile subchondral bone cysts, osteochondral collapse, avulsion fractures of epiphyseal (or metaphyseal) ossifying bone and 'physitis'. The aim of this study was to estimate heritability of JOCC in a sample of 2106 French Trotters from four different sources, comprising representative samples of the Trotter population, as well as material from auctions. Horses were aged 6-24months and were either not yet in training or just beginning training. Radiographs were taken of fore and hind feet, including proximal interphalangeal (pastern) joints, metacarpophalangeal and metatarsophalangeal (fetlock) joints, tarsocrural (hock) joints, carpi and femoropatellar (stifle) joints. The threshold model used included sex, age, region and month of birth, sampling group and sire (n=159) with all inter-sire relationships. The main results were a moderate heritability for findings in the hind fetlock (0.29) and the hock (0.19). There was a weak genetic correlation between findings in fetlocks and hocks (0.26). Higher heritability was found for findings in the hock (0.37 for findings in the distal row and 0.49 for the proximal row of tarsal bones) in that part of the data (699 horses) in which it was possible to integrate the grade, bilateral occurrence or not, and distal or proximal location of the lesions. It is possible to use these genetic parameters in breeding selection with more efficiency when detailed phenotypes are considered.