Group sizes and resting locations of free range pigs when grazing in a natural environment

Abstract

A herd of 86 Iberian pigs aged 13 ± 0.5 months and with an average live weight of 110 ± 1.3 kg at the start of the study was followed to monitor their social organisation (spontaneous group size: GS) and resting preferences when free ranging on natural pastures in a dehesa of evergreen oaks ( Quercus ilex rotundifolia) of 111 ha (average of 27.4 adult oaks/ha) with 2 adjacent shelters for resting. Oaks were considered as activity centres and resource patches (characterized by unpredictability in the distribution of resources). 60 complete “pig grazing days”, distributed over 6 different observation days, were studied (10 pigs per day). Every observation session was carried out for 10.5 h, covering >90% of the daylight period. Results show that pigs split into small subgroups to forage and maintained this division during daylight rests, however animals regrouped to rest during the night in a common area and behaved as a united flock. The mean daylight GS (from 9:00 to 18:00) was 8.6 ± 0.3 pigs; however GS was larger when pigs were resting than when they were grazing (13.8 ± 0.6 vs. 5.6 ± 0.3 pigs, P < 0.001). GSs were significantly different when pigs were grazing in open pasture or under an oak crown (5.0 vs. 5.8, P < 0.001). The most frequent GS when grazing under an oak were ≤4 pigs, whereas GS of pigs eating acorns was ≤2 pigs for 26.4 ± 3.0% of observations. The behaviour of these pigs could be called “Chase Optimal Foraging”, to refer to those animals which are exploiting the remains of a preferred patch (oak mast) whilst paying attention to the discovery of another close preferred patch by their group mates. When eating acorns, subgrouping would be conditioned by the space requirements to avoid competition when foraging, hence the mean grazing surface when foraging acorns could be considered an indication of the space required to avoid social stress (mean was 8.9 m 2 and median was 5.2 m 2 per pig). GSs of resting pigs increased as the day progressed. In spite of the temperature at sunset (19:00) (8.4 ± 0.1 °C outside vs. 11.9 ± 0.1 °C inside, P < 0.001), 28.6 ± 3.3% of pigs rested outside the shelters in smaller subgroups (9.5 ± 0.9 pigs outside vs. 28.0 ± 0.7 pigs inside, P < 0.001). Pigs inside each shelter constituted a single subgroup. There was no significant difference in the body weight or sex of pigs resting inside or outside the shelters at sunset. The mean surface for pigs resting inside the shelter at sunset was 6.8 ± 0.1 m 2 per pig. The results suggest a reconsideration of the lower critical temperature for rustic and non-lean porcine breeds, in terms of comfort and the minimum space allowance per pig for preferred resting distribution.