Past studies on small groups (10-40) of pigs have found a negative impact of crowding on productivity and welfare. Studies examining groups of greater than 40 pigs per pen have found setbacks in the growth rate of pigs soon after mixing. Research into the effects of crowding on grow-finish pigs housed in large groups is minimal, although it has been suggested that pigs housed in large groups may be able to use space more efficiently. This study was designed to assess the space requirements of both large and small groups, and the effects of space restriction on pig performance, behaviour, physiology, health and welfare.

For this study, space allowance was expressed using an allometric approach relating body weight (BW) to floor area, as determined by the equation: k = area(m2)/BW(kg)0.667. Past research has indicated that, above k = 0.035, growth is normal. Below k = 0.035, space becomes restrictive and growth depression begins. Due to previously set animal care guidelines, the crowding treatment in the current study was terminated at k = 0.025 (approximately 94 kg BW at 0.52 m2/pig).

Eight, 8-week blocks of 288 pigs were carried out. Group sizes were small (18 pigs) or large (108 pigs) and space allowances were crowded (0.52 m2/pig) or uncrowded (0.78 m2/pig), creating four treatment groups: small uncrowded, small crowded, large uncrowded and large crowded (Figure 1 a-d, respectively). Gains, feed intake, and feed efficiency were calculated on a weekly basis. Postural and feeding behaviour were assessed on a biweekly basis, as were injuries and salivary cortisol concentrations (indicative of acute stress). Adrenal gland (indicative of chronic stress) and carcass data were collected at slaughter. Pig morbidity and mortality were determined for all eight blocks. A 1:1 ratio of barrows and gilts were used in the first two blocks, and only production, injury, health, and carcass data were collected. The remaining six blocks used barrows only, and all data listed above were collected. One wet/dry ad-libitum feeder space was provided for every nine pigs. One environmental enrichment device was provided for every 18 pigs.

Barrows gained more than gilts (1.0644 vs. 1.0124 ± 0.0094 kg/d, P < 0.018) and had a higher fat depth at slaughter (20.57 vs. 18.022 ± 0.25 mm, P < 0.002). Gilts had a higher carcass index than barrows (114.01 vs. 111.95 ± 0.32, P = 0.011). There were no indications that one gender was more affected by large group housing or reduced space allowance than the other. Overall, crowded pigs had a lower growth rate and a lower final body weight than uncrowded pigs (Table 1). Growth rate was depressed by 9.8 % during the final week of the study. Pigs housed in large groups had a lower overall growth rate than pigs housed in small groups (Table 1). Among pigs housed in large groups, daily gain was most affected during the first two weeks, at which time it was depressed by 5.4 %. The difference in initial body weights (Table 1) of pigs housed in the large groups indicated that growth depression began within the first four days after group formation. The first sign of growth depression in response to crowding occurred much sooner for pigs in large groups compared with pigs in small groups. In the large groups, the critical point (k value) at which crowding and growth depression began was k = 0.042 (43 kg BW), while k = 0.035 (57 kg BW) was the critical point for pigs housed in the small groups. However, the rate of depression in gains was more gradual for pigs in large groups. Growth was depressed by 0.5 % for every 1 % reduction in space below the critical point in the small groups, but growth was only depressed by 0.2 % for every 1 % reduction in space below the critical point in the large groups. Thus, by the final week of the trial, pigs in both large and small crowded groups had similar gains. Overall, crowded pigs had a lower feed efficiency than uncrowded pigs (Table 1). Efficiency was depressed by 11 % during the final week of the study. Crowded pigs ate fewer meals and spent less time eating overall, but feed intake did not differ from that of uncrowded pigs. This suggests that they were consuming feed at a faster rate than uncrowded pigs. The level of crowding did not affect injury scores for the severity of lameness, flank bites, tail bites, or leg lesions. Similarly, it did not affect the number of animals requiring medical treatment (antibiotics) or removal from the trial, or the level of acute or chronic stress experienced by the pigs. Pigs housed in large groups ate fewer meals, but took longer to eat each meal, than pigs housed in small groups. Pigs housed in large groups also had a greater severity of lameness and leg injuries than pigs housed in small groups. Pigs housed in small groups spent more time sitting and lying on their sternum (chest), and less time lying on their side, than pigs housed in large groups. Group size did not affect stress levels, the number of animals requiring medical treatment, or the number of animals requiring removal from the trial. Pigs in uncrowded small groups had the highest carcass lean yield while pigs in uncrowded large groups had the highest fat depth. Pigs in crowded large groups had the highest lameness scores. The Bottom Line Both crowding and large group housing were found to negatively affect pig performance. Pigs housed in large groups were affected by space restriction sooner than pigs in small groups although, the depression in growth was much more gradual for pigs housed in large groups. There was limited evidence, none of which was related to productivity, that pigs in large groups were able to use space more efficiently than pigs in small groups