Experiments were conducted with the aim of determining the influence of melatonin administration on vigilance and tonic immobility (TI) responses of Marshall Broiler chickens, reared on different lighting regimens during the hot dry season in the Northern Guinea Savannah zone of Nigeria.
Simple random sampling was used to assign 300 broiler chicks into three groups, comprising 100 chicks each. Group I (12L:12D cycle) was raised under natural photoperiod of 12 h light and 12 h darkness, without melatonin supplementation. Group II (LL) was kept under 24-h continuous lighting, without melatonin administration. Group III (LL + MEL) was raised under 24-h continuous lighting; with melatonin supplementation at 0.5 mg/kg per os, via drinking water. Beginning from day-old, birds in group III were individually administered with melatonin once daily for 56 consecutive days at 17:00 h. Tonic immobility (TI), induced by manual restraint, and vigilance, by self-righting, were graded for three days, 14 days apart in 15 labeled broiler chickens from each of the three groups at 06:00 h, 13:00 h and 18:00 h starting from day 28 up to day 56. The dry-bulb temperature (DBT) and relative humidity (RH) were recorded at the experimental site and concurrently during the vigilance and TI tests. The weekly mean temperature-humidity index (THI) was calculated inside the poultry house using the DBT and RH values.
The THI was lowest on day 28 and highest on day 56 of the study, with the values of 48.00 ± 0.08 oC and 51.57 ± 0.98 oC, respectively (P < 0.05). At day 28, the relationship between the THI and the TI induction attempts was stronger in 12L:12D cycle (r = 0.589, P < 0.001) than LL (r = 0.264, P > 0.05) and LL + MEL (r = 0.096, P > 0.05) broilers, indicating that the broiler chickens on 12L:12D cycle were more active compared to their melatonin-treated counterparts, apparently due to the adverse effects of high DBT and RH on the broilers during the hot-dry season. The results showed that the number of TI induction trial attempts fluctuated insignificantly as the hours of the day increased, especially in 12L:12D cycle birds (r = -0.130, P > 0.05). The highest numbers of TI induction trial attempts were 2.13 ± 0.34 and 2.15 ± 0.22, and the values were both recorded at 13:00 h in 12L:12D cycle and LL groups, respectively, when the birds were 56 days' old. The overall mean values of induction trial attempts differed significantly (P < 0.0001) between the groups, with the lowest mean values of 1.22 ± 0.4 recorded in Group III birds, administered with melatonin. At day 42, the lowest mean TI duration of 101.87 ± 10.24 s in the LL group recorded at 06:00 h rose (P < 0.001) to 184.07 ± 23.69 s at 13:00 h. Again, the overall mean values of TI duration differed significantly (P < 0.0001) between the groups, with the highest mean duration of 167.82 ± 8.35 s, recorded in group III broiler chickens administered with melatonin. Unlike the result obtained on TI duration, similar to that recorded in the number of induction attempts, the values of mean vigilance behaviour rankings at the different hours of the day did not differ (P > 0.05). The overall mean vigilance behavioural ranking values of 1.85 ± 0.07 and 1.70 ± 0.08, recorded in 12L:12D cycle and LL broiler chickens, respectively were higher (Kruskal-Wallis test = 20.87; P < 0.0001) than the value of 1.44 ± 0.05 recorded in melatonin-treated birds. The results indicated that the broiler chickens belonging to both 12L:12D cycle and LL groups were more emotionally unstable, fearful or anxious compared to those in group III that were treated with exogenous melatonin.
It is concluded that melatonin elicits boldness and confidence by suppressing freezing behaviour in broiler chickens.
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