There were some effects of temperature setting on behavioural variables,
with latencies generally decreased at setting 4, applied when
temperatures are colder (setting 3 was applied between 13 and 18 °C,
and setting 4 was applied between 5 and 12 °C). This does not agree
with previous findings where behavioural latencies increased at temperature
settings applied at colder temperatures [30], possibly due to
the fact that cold air has greater density and lower humidity (with
consequently increased oxygen availability). The differences between
the studies may be due to the exact temperature and pressure curve applied
at the time of LAPS (each setting relates to a range) and resulting
under or overcompensation. They may also relate to the underlying physiological state of the birds when they underwent LAPS in relation
to food withdrawal, metabolic status, previous activity, presence of
any sub-clinical pathology and body temperature. In Mackie and
McKeegan, food withdrawal time was standardised at 8 h, but in the
current trial, because ambient conditions (and thus temperature setting)
and time of day were related, these effects are confounded [30].
The baseline heart rate of birds killed at temperature setting 4 was
higher than those killed at setting 3 — these birds were killed earlier
in the day and underwent a holding period in crate in cold conditions.
The effect of temperature on heart rate is well established; thermoregulatory
effort increases heart rate and oxygen consumption [21,51] and there is evidence that modern broilers have compromised thermoregulatory
capacity compared to laying hens [28]. These factors suggest
that responses to LAPS will be related to ambient temperature in a nonlinear
way, since thermoregulatory effort and potential alterations in
metabolic rate and respiratory control outside the thermally neutral
zone will be altered in both hot and cold conditions. This could explain
the apparent paradox in the current results of decreased behavioural
latencies in cold conditions, despite increased air density, and is supported
by the baseline heart rate data. Substantial thermoregulatory effort
and associated changes in acid–base and electrolyte status [23,24]
in hot conditions may also explain reduced estimates of time to loss of
consciousness (as measured by EEG) in earlier work [35]. Effects of temperature
setting on EEG responses were much less apparent, with only
one variable (time to isoelectric EEG) affected, being reduced at the
colder setting