Early age thermal conditioning immediately reduces body temperature of broiler chicks in a tropical environment

Early age thermal conditioning (TC) durably improves resistance of broilers to heat stress and reduces body temperature (Tb). Three experiments on broiler chicks were conducted to evaluate the effects of TC at 5 d of age on Tb variation measured by thermometer between 4 and 7 d of age, under a tropical environment. Because manipulation of chickens to measure Tb with a thermometer may increase Tb, a preliminary experiment on 13 3-to-4-wk-old male broilers compared Tb measured by telemetry to Tb measured in the terminal colon during three successive periods at 22, 33, and 22degreesC. During heat exposure, Tb rapidly increased by 0.9degreesC and plateaued over 24 h. During the last period, seven of the broilers rapidly reduced Th to a plateau lower than the initial Tb, although six broilers exhibited more variable Tb. Measurement by thermometer underestimated on average core Tb by 0.28degreesC at 22degreesC and by 0.57degreesC at 33degreesC, whereas Tb recorded by telemetry was not affected by manipulation of the chickens. TC reduced Tb 24 h later in the three experiments. Compared to unexposed control chicks (N), 12 h of TC at 40degreesC did not significantly reduce Tb at 7 d of age, although 24 h did. TC at 38 and 40degreesC over 24 h significantly reduced Tb variation from 4 to 7 d of age compared to N chicks, whereas 36degreesC did not. Withdrawing feed from the chicks for 2 h prior to measurement did not significantly reduce Tb at 4 and 7 d of age, but Th reduction due to TC was greater in fed chicks (0.28degreesC) than in chicks without feed (0.05degreesC). Early age thermal conditioning at 38 to 40degreesC at 5 d of age for 24 h reduced body temperature of 7-d-old male broilers.