Effect of moderate forced physical activity on behaviour, lameness and osteochondrosis in growing pigs from two divergent lines selected for feed efficiency

In pig farming, physical constraints and genetic selection for high production are risk factors for the development of leg disorders, such as degraded locomotor activity. Interactions between both factors need to be explored. The study was carried out on two replicates of 80 pure-bred Large White growing-finishing pigs from the 8th generation of two divergent lines selected for low and high residual feed intake (LRFI, HRFI). Each replicate included 40 LRFI pigs and 40 HRFI pigs, housed on partly slatted flooring in a room equipped with a sorter allowing access to electronic self-feeders during two replicates. Ear tags determined the side of the room to which the pigs were oriented after the sorter exit and the distance back to the sorter (short: spontaneous activity, long: forced activity (FA)). Lameness was assessed individually weekly using visual gait scoring. At slaughter (weight of 100 kg), postmortem quantification of osteochondrosis (OC) lesions was performed on both the proximal and distal extremities of the humerus and femur. Low RFI pigs showed a lower feed conversion ratio (P < 0.001). They also showed lower individual numbers of sorter crossings per day and a lower proportion of standing pigs, which confirmed their lower physical activity. Forced activity clearly increased the number of sorter crossings/d/pig (P < 0.001), and the magnitude of the effect of FA was clearly lower in LRFI pigs than in HRFI pigs. The occurrence of gait was low (less than 9% of recorded scores). The proportion of scores classified as stiffness was higher for LRFI pigs than in HRFI pigs (P < 0.0001). The average lameness score was also higher for LRFI pigs and lower with FA (P < 0.05). The pigs of the LRFI line showed higher OC scores on both the proximal humerus and femur (P < 0.001) and lower OC scores on the distal humerus with surface evaluation (P < 0.05). The carcasses of LRFI pigs were heavier with a higher lean meat percentage (P < 0.001). Most OC scores were unaffected by FA. Only the OC scores of the distal femur (slice method) were higher with increased activity in LRFI pigs, whereas they were lower in HRFI pigs (P < 0.05). Seric biomarkers of cartilage synthesis and degradation were higher for pigs from the LRFI line, but no correlation could be observed between individual OC scores and cartilage biomarker contents.