Agriculture Reference
In-Depth Information
fight less than sows in smaller groups (Broom
et al.
, 1995; Mendl 1994). In contrast,
Hemsworth
et al.
(2013) compared skin lesions in groups of 10, 30 and 80 animals and
found skin injuries in groups of 10 to be consistently the lowest on several observation
days. Intuitively this is more logical, as smaller groups mean less diverse social interactions
and potentially less aggression.
In practice, sows are kept in groups of up to 300 individuals with no obvious detrimental
effect to the social organisation, as assessed through aggression levels. It is possible that
pigs alter their strategy of negotiating with social encounters as group sizes increase
and they fail to recognise all individuals: they are less likely to engage in aggressive
interactions and show more avoidance behaviour (Turner and Edwards, 2000). This was
supported in a later study by Turner
et al.
(2001) where it was found that pigs from large
groups were less aggressive towards unfamiliar other animals in the group, compared
with pigs from small groups.
Separate groups of gilts or not?
Hodgkiss
et al.
(1998) studied large groups of 100-110 sows on one farm and found parity
1 and 2 sows to be significantly more injured compared with sows of parity 3 or more.
Similarly, Kirkwood and Zanella (2005) found that parity 0 sows fight less compared with
parity 1 and 2 sows. This obvious relationship between physical strength and the likelihood
to dominate over resources raises the question as to whether sows should be housed in
single parity groups or at least in groups were young and old animals are not mixed.
Hoy
et al.
(2009a) showed that primiparous sows grouped together with young sows
had a significantly higher social rank index compared with primiparous sows mixed
with older group-mates: they were more often the dominant animal. Potentially, this
means that they could be subjected to lower levels of aggression and stress during their
pregnancy. This was confirmed by Li
et al.
(2012) who found that first-parity sows housed
with gilts fought more frequently and for longer periods, but won more fights and had
fewer injuries after mixing compared with first-parity sows in pens with multiparous
sows. Their results suggest that sorting by parity shielded first-parity sows from severe
injuries caused by mixing-induced aggression so that their welfare and performance can
be improved in group housing systems.
A (vasectomised) boar in the group to reduce aggression?
It has been suggested that boar presence decreases aggression among weaned sows and
gilts (Luescher
et al.
, 1990). The idea is that a 'super dominant' individual in the group will
supress latent aspirations of others to dominate the group. Kirkwood and Zanella (2005)
confirmed this hypothesis when they investigated the effects of boar presence following
mixing, and so did Docking
et al.
(2000) who scored sow aggression in specialized mixing
pens with or without a boar present. Grandin and Bruning (1992) tested this boar effect
by mixing fifty 100 kg finishing pigs in pens with or without the presence of 3 mature
boars. Again, boar presence reduced the level of lesions, as well as the intensity and the
frequency of fighting. Seguin
et al.
(2006) found only a marginal effect on aggression
