Table 2 Ž
. ADG
kgrpigrday of focal pigs and, considering all members of the pen, pigs of lower and upper inter-quartile start weight in the four combinations of group size and drinker allocation
Treatment S.E.M.
Significance 60 pigs,
20 pigs, 60 pigs,
20 pigs, Drinkers
Group Drinkers=
3 drinkers 1 drinker
6 drinkers 2 drinkers
size group size
Pooled focal pigs 0.70
0.70 0.65
0.67 0.024
U
Lower quartile 0.53
0.68 0.68
0.58 0.037
UU
Upper quartile 0.63
0.80 0.70
0.64 0.037
pattern only reached significance after 2 weeks on trial. The lower quartile animals Ž
. gained most slowly in a group of 60 with three drinkers P - 0.05 . This pattern was
Ž .
repeated by the animals with an upper inter-quartile start weight P - 0.01 , and the
interaction between start weight and treatment was not significant.
4. Discussion
Quantification of the drinking behaviour showed the existence of large individual variation in drinking patterns and total daily drinking time. The extent to which this
precisely correlated with similar variability in water intake could not be ascertained in the current experiment, but it indicates much higher variability than would be expected
from differences in individual metabolic requirement alone and appeared unrelated to pig liveweight. Differences in the rate of water ingestion between individuals may
contribute to this variability and may need examination in the future. Such data support
Ž .
the contention of Brooks and Carpenter 1993 that no single value for water require- ment can be defined with our present state of knowledge.
The diurnal pattern of drinking behaviour showed a clear increase in the late afternoon and a low overnight. Most previous reports of diurnal patterns of drinking
Ž .
Ž .
Bigelow and Houpt, 1988; Gill and Barber, 1993 or water use Hepherd et al., 1983 have come from pigs fed in restricted daily meals. These indicate that the majority of
drinking is associated with the daily meals, during both the pre- and post-prandial periods, with little drinking occurring overnight. Reports on drinking patterns of pigs fed
ad libitum are scarce, but in one study of pigs in groups of 42 in straw courts, where
Ž different allowances of open water trough space were compared McDonald et al.,
. 1996 , a diurnal pattern which was very similar to that in the present study was
recorded. Since pigs fed ad libitum do not fully synchronise their feeding behaviour with the other pen mates, a more uniform diurnal water intake pattern for the group would be
expected. When fed ad libitum pigs generally show two peaks in feeding activity, one in
Ž .
the morning and one in the afternoon e.g., Schouten, 1986; de Haer and Merkst, 1992 . The single very prominent peak in drinking is therefore difficult to explain by simple
association with feeding patterns as repeatably reported in the literature, although these were not recorded in the present experiment.
The drinking behaviour parameters recorded in detail for individual pigs closely Ž
. mirror those reported previously from sampling 30 minrh drinking behaviour of the
Ž .
group as a whole Turner et al., 1999 . In this study, where water was provided via bite drinkers, the number of daily drinking bouts and their duration were both greater than
Ž .
those recorded in the study by McDonald et al. 1996 , where water was consumed from an open trough and greater ingestion rates would have been possible. However, in
Ž .
neither the study by McDonald et al. 1996 nor the current experiment were significant differences in drinking behaviour of pigs of different liveweight seen for any treatment.
The assumption that a high liveweight automatically assures a high social rank in Ž
. growing pigs is, at best, tenuous. Rasmussen et al. 1962 and Meese and Ewbank
Ž .
Ž .
1973 failed to find such a relationship in pigs, while Tennessen and Gonyou 1981 Ž
. reported a similar finding in cattle. In contrast, McBride et al. 1964 , working with
young pigs, described a positive correlation between weight and social rank; a finding Ž
. supported by Stricklin 1983 in cattle. The magnitude of weight difference within a
group may be influential in this context; the difference in weight between the heavy and Ž
. light weight focal pigs in the present study was substantial 26 .
It is also debatable whether a high social rank confers preferential access to Ž
. resources. Syme 1974 found lower correlations between dominance and competitive
Ž .
orders than predicted from this assumption. McGlone 1986 observed that the most aggressive pigs spent the greatest time drinking, but that generally, dominance was not
Ž .
an advantage in gaining access to resources. Similarly, Stricklin and Gonyou 1981 , found that dominance was not a good predictor of access to a single space feeding stall
in cattle, although even slight amounts of physical contact were included in their study as replacements.
Consequently, it is difficult to unequivocally support a hypothesis that heavy pigs would, by virtue of their higher rank, obtain preferential access to drinking points.
However, even assuming that social rank plays a negligible role in allocating resource access in many situations, the comparison of drinker use by different weight categories
Ž of pig is still a valid one as the critical interest was how liveweight and by inference,
. competitive ability itself influenced resource access.
An alternative explanation for the apparent absence of a weight effect on drinker access is that the resource was not sufficiently limited to encourage the necessary degree
of competition. This may account for the similarity in diurnal use of the drinkers for Ž
heavy, medium and light weight animals. From the results of various workers Morrow and Walker, 1994, and Botermans et al., 1997 in pigs; Gonyou and Stricklin, 1981, and
. Stricklin and Gonyou, 1981, in beef cattle , sub-optimal feeder allocation caused
animals, particularly subordinate individuals, to make frequent visits to the feeder at night. In the present experiment, there was no interaction between liveweight and
treatment on diurnal pattern of drinker use, indicating that competition was not sufficient in any treatment to encourage preferential resource access. Indeed, although the pattern
did not reach statistical significance, the lightest weight pigs actually performed a slightly greater percentage of their total daily drinking time during the peak drinking
period of 1700–2059 h than either heavy or medium weight pigs. In addition, it was the lightest weight pigs which performed the lowest percentage of their daily drinking time
Ž .
during the period 0500–0859 h. However, Turner et al. 1999
reported that the percentage of drinking bouts which ended while another pig was queuing was signifi-
cantly greater when drinkers were offered at a ratio of one per 20 animals. Despite this,
it was noted that pigs very often expressed a preference to queue for an occupied drinker rather than to use an unoccupied one. It is plausible that small differences in drinking
behaviour resulting from weight category or treatment effects may be masked by large inter-individual differences in drinking behaviour between pigs. The dramatic range of
7–98 visits to the drinkers in 1 day, displayed in Fig. 1b, highlights this possibility.
The higher lesion score of the heavier animals may be indicative of their attempt to Ž
. attain a high social rank. Beilharz and Zeeb 1982 reported that dominant cattle had
probably achieved their rank by being aggressive in the past, although this is in contrast Ž
. Ž
. to earlier work by Rasmussen et al. 1962 and Beilharz and Mylrea 1963 . The low
lesion score of the light weight animals may reflect their avoidance of aggressive situations. The lack of an interaction between weight and treatment on lesion score and
the lack of an effect of treatment on pooled focal pig lesion score, as reported by Turner
Ž .
et al. 1999 , further suggests that the intensity of competition was similar in each treatment. Aggression also did not appear to influence the daily number of visits a pig
made to the drinkers, the mean drinking bout length or the total time spent drinking per day as indicated by the lack of a strong correlation between mean lesion score and these
parameters of drinking behaviour. However, the percentage of drinking bouts terminated by aggression was significantly greater in the groups of 60 pigs provided with three
Ž .
drinkers in the report by Turner et al. 1999 . Despite this, it was also observed that the total number of aggressive acts per pig, including those specifically at the drinkers, was
similar for each treatment. The ADG of the focal pigs was not significantly influenced by treatment, weight
category or their interaction. Regarding the mean performance of the pen as a whole, Ž
. Turner et al. 1999 found no significant effect of treatment on ADG. However, in the
present experiment, pigs of lower inter-quartile start weight displayed a poorer perfor- mance when in a large group size of 60 with only three drinkers. This pattern was
repeated in those pigs of an upper-interquartile start weight, indicating that low weight animals were not being selected against specifically. The similarity between the pen
mean ADG for each treatment indicates that the medium weight pigs performed well in a group of 60 pigs with three drinkers. Once again, the absence of an interaction
between weight category and treatment on performance in our experiment suggests that the competition pressure was not sufficient to highlight a rank effect, should one exist.
5. Conclusion
