Criteria for evaluating husbandry practices to alleviate heat stress in broilers
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This study was undertaken to investigate the effects of management techniques used under heat stress on growth as well as developmental instability, duration of tonic immobility (TI), and heterophil-to-lymphocyte ratios (H:L) of broilers. Three-hundred sixty cockerels obtained from a commercial hatchery were randomly assigned to 12 floor pens. When the cockerels reached 21 d of age, three pens were kept as controls. Broilers in the other nine pens were moved to a room heated from 32 to 35degreesC between 1000 and 1700 h each day from 21 to 42 d of age. Broilers in the heated room were randomized into three treatment groups consisting of 1) conditioned (chicks exposed to 36degreesC for 24 h at 5 d of age), 2) feed-restricted (during the heat stress, feed was withdrawn 2 h before the hot period, and chicks were fed between 1700 and 0800 h), 3) and heat-stressed only. Conditioned and feed-restricted broilers gained 3.2 and 2.8%, respectively, more BW than heat-stressed broilers. Relative asymmetry (RA) averaged across several bilateral traits for the prolonged heat stressor was more informative than the RA for a single bilateral trait. Corrrelations suggest that RA were not closely associated with duration of TI and H:L under the conditions of this experiment.