Photoperiodic responses of broilers. I. Growth, feeding behaviour, breast meat yield, and testicular growth.

1. A total of 7960 Cobb and Ross broiler males were reared in two trials to 35 d on various photoperiods between 2 and 21 h or under continuous illumination; a total of 444 birds were randomly selected at 35 d and retained for subsequent determination of breast meat yield and testicular weight at 40 or 54 d of age. 2. In both strains, feed intake and growth were positively correlated with photoperiod during the first 21 d, but afterwards feed intake was not significantly affected by photoperiods longer than 6 h and growth was negatively correlated with photoperiod beyond 12 h. Overall, to 35 d, there were no significant photoperiodic influences on either feed intake or growth for >or=6-h photoperiods, but significant depressions in feed intake and growth for photoperiods shorter than 6 h. Feed conversion efficiency was maximised by 12-h photoperiods, with decreases in efficiency above and below 12 h. Mortality was unaffected by photoperiod <12 h, but increased proportionately with photoperiod >12 h. The incidence of Sudden Death Syndrome (SDS) had an inverse relationship with photoperiod <or=10 h, but was positively correlated with photoperiod >10 h. The European Efficiency Factor was curvilinearly related to photoperiod, with the highest efficiency occurring at 12 h. Ross birds had significantly greater feed intakes but poorer feed conversion efficiencies than Cobb; differences in growth, overall mortality and the incidence of SDS between the strains were not significantly different. 3. By 5 d, birds given <or=15 h illumination had learned to eat in the dark, with the amount of feed consumed being inversely proportional to photoperiod; further increases in the amount of nocturnal feeding occurred between 5 and 20 d for <or=12-h photoperiods. The mean hourly rate of nocturnal feeding was consistently lower than diurnal feeding, irrespective of photoperiod. Nocturnal feeding patterns were similar for both genotypes. 4. Breast meat yield at 40 d was unaffected by photoperiod in Cobb birds, but significantly higher in continuously illuminated Ross birds than <or=21 h. At 54 d, breast meat yield was significantly higher in both genotypes given 21 h or continuous illumination and, overall, higher than at 40 d. 5. Testicular weights at 40 and 54 d of age increased with photoperiod in both genotypes to 12 or 15 h. Thereafter, weights plateaued for Cobb but decreased for Ross as the photoperiod was further extended to continuous illumination. 6. New EU welfare regulations come into effect on 30 June 2010 and these state that meat-chickens must have at least 6 h of darkness in each 24-h period, i.e. a maximum photoperiod of 18 h; compliance with the regulations should have no adverse effect on either performance or profitability.