Photoperiodic regulation of growth: a photosensitive phase during light-dark cycle.

Growth and performance of 72 young ram lambs were evaluated for susceptibility to exposure to three different photoperiods. Lambs were penned in groups of two and exposed to either short (8L:16D), long (16L:8D), or split (7L:9D:1L:7D) photoperiods. Lambs exposed to 16L:8D and 7L:9D:1L:7D photoperiods ate more feed (P less than 0.01), gained weight more rapidly (P less than 0.01), and required less feed per unit gain (P less than 0.01) than did lambs exposed to the 8L:16D photoperiod. Final weight and carcass weight were increased (P less than 0.01) as a result of exposure to 16L:8D and 7L:9D:1L:7D photoperiods; however, carcass attributes were not affected. Lambs exposed to the 7L:9D:1L:7D photoperiod had elevated prolactin concentrations that generally paralleled the prolactin profiles of lambs exposed to the 16L:8D photoperiod; however, concentrations of luteinizing hormone and testosterone were not affected by photoperiod. Lambs exhibited a daily feed-intake rhythm that was closely associated with daily photoperiod. These rhythms could account in part for the increased feed intake and performance of lambs exposed to 16L:8D and 7L:9D:1L:7D photoperiods. From these results, we conclude that 1) light is an important Zeitgeber for the regulation of feed intake, prolactin secretion, and growth rate in young lambs, and 2) a photosensitive phase exists for these functions during the light-dark cycle in sheep.