Effect of photoperiod on heterophil to lymphocyte ratio and tonic immobility duration of chickens

The experimental objectives of the current study were to analyze the effects of photoperiod on heterophil to lymphocyte ratio and tonic immobility duration of laying hens. There were two different trials. The first trial measured heterophil to lymphocyte ratio in 36-wk-old hens of three Spanish breeds of chickens (Castellana, Prat, and Vasca) exposed to lighting programs of 23L:1D, 14L:10D, or 18.5L:5.5D. There were no significant differences among lighting regimes. Results were consistent across the breeds. The second trial measured duration of tonic immobility in 36-wk-old hens of a synthetic breed (originated from a cross between the Castellana and Prat breeds) exposed to lighting programs of 23L:1D or 14L:10D. There was a significant difference between the two lighting regimens (P < 0.05). Hens housed under lighting of 23L:1D showed longer tonic immobility than did hens housed under lighting of 14L:10D. There was no evidence of an increased stress response as judged by heterophil to lymphocyte ratio, but there was evidence of an increased fear response as judged by tonic immobility duration. The results of the present study suggest that a near continuous light pattern can have negative consequences in reducing welfare in egg-laying hens.