What determines the social rank of a sow in a group-housing system?
Freedom of movement and pain management are still the major challenges of animal welfare to swine production. One restriction to freedom of movement is housing pregnant sows in individual stalls. To address the issue, pork producers have been gradually and cautiously moving away from individual housing towards group-housing of gestating sows.
Currently, about 20-25% of gestating sows are kept in a variety of group-housing systems in the United States. However, when sows are housed in group pens, the welfare level of individual sows can vary greatly. This is because the welfare of a sow in a group depends on her social rank. Understanding the social rank of a sow will help producers identify sows that are less competitive and need special care to maintain sow welfare and performance.
In a recently completed study at the WCROC, we investigated what makes a sow high or low ranking and how social rank affects reproductive performance of the sow. The project was partially supported by the National Pork Board. A total of 150 sows were tested over a period of 2 years. After weaning their litters, sows were moved to and mixed in gestation pens. Each gestation pen housed 15 sows with individual feeding stalls. Fighting among sows was video-recorded during the first 24 h after mixing. Among the 15 sows in each pen, three groups of 5 sows were classified as high, middle, and low ranking based on outcomes (won, lost, and unsolved) of fights. Individual weight, body condition, and back-fat thickness of sows were recorded before mixing and before farrowing. Heart rate and fear response of sows were measured 5 to 6 week after mixing. Litter size and individual weight of piglets were recorded at farrowing and at weaning in a group-lactation system.
Results indicate that low ranking sows fought less frequently at mixing than high ranking sows, but sustained the same amount of skin lesions as high ranking sows, indicating that low ranking sows received more injuries per fight than high ranking sows. Low ranking sows were more fearful than high ranking sows, suggesting that low ranking sows may suffer from social stress in the group housing system. Compared to high ranking sows, low ranking sows were younger and lighter during gestation. During lactation, low ranking sows had lower percentage of pre-weaning mortality of piglets than high ranking sows, which was associated with smaller litter sizes farrowed by low ranking sows. High and low ranking sows were not different in heart rate, body condition, back-fat thickness and weight gain during gestation, litter size weaned, or body weight of piglets farrowed or weaned.
These results suggest that experience (age and parity), body weight, fear response, and fighting ability collectively determine the social rank of a sow. Low ranking sows had poor welfare compared to high ranking sows as indicated by skin lesions and fearfulness. But, social rank did not affect sow reproductive performance significantly in the group housing system studied.
Reviewed by Wayne Martin