Stocking density, sex and season on tonic immobility in broiler chickens

Objective. To assess the effect of stocking density, season of the year, and sex on latency time and number of attempts required to induce tonic immobility in broiler chickens in an industrial production system. Material and methods. A total of 216.000 commercial hybrid Cobb500 chickens were used, reared in mixed batches, 54.000 in each of the four seasons of the year, in dark houses with forced tunnel ventilation, under two stocking densities: standard (14 chickens/m2) and reduced (12 chickens/m2). On day 35 of the productive cycle, 90 birds of each sex, of each density and in each season of the year were sampled and the tonic immobility test was performed, recording the latency time in seconds (s) and the number of induction attempts. Results. Stocking density significantly affected (p<0.005) latency time with values (median and interquartile range) of 72 s (38-143) for standard density and 63 s (36-127) for reduced density. The difference was marginally significant for the number of attempts. Non-significant differences were observed between sexes and among seasons in any of the two variables. Conclusions. The duration of tonic immobility is a useful tool as an indicator of fear in specific situations. Decreasing the stocking density decreases the latency period and more induction attempts are required in chickens, while sex and season do not produce similar effects.