Severity of osteochondrosis in three genetic lines of Duroc swine divergently selected for front-leg weakness

The relationship of osteochondrosis to genetic differences in front-leg structure in Duroc swine was investigated by using offspring from low (poor front-leg structure), control (intermediate front-leg structure) and high (superior front-leg structure) lines from a 5-generation divergent selection experiment for front-leg structure. Ten pigs of each sex and line were chosen at random and visually scored for front- and rear-leg structure. After slaughter, synovial fluid was collected and cultured from 3 major joints of each front leg, and macroscopic, microradiographic and histologic osteochondral lesion scores were obtained for the articular cartilage and growth plates of several areas of both front legs and of the left rear leg. Lesion scores ranged from 0 to 3 or 0 to 5 with 0 being best. Osteochondrosis data were analyzed by using a mixed threshold model, which included the effects of line, sire/line, sex and side of the pig and a covariate for slaughter weight or age of slaughter. Means of front-leg structure scores on a scale of 1 (worst) to 9 (best) were 3.02 ± 0.17 (low line), 5.67 ± 0.21 (control line) and 7.91 ± 0.21 (high line) (P < 0.05). The high line had lower osteochondral lesion scores (P < 0.05) for the proximal radius/ulna than did the control or low lines, but severity of osteochondrosis at the other joints and growth plates was not different between lines. Boars had greater severity of lesions than did gilts (P < 0.01) for the growth plate of the distal ulna and articular cartilage of the distal femur. Mycoplasma hyosynoviae was isolated from joints of a few pigs, however, no relationship between isolation of the organisms and line was observed. These results suggest that, although osteochondrosis may play a role, it is not the primary cause of front-leg weakness in Duroc swine.