Responses of market lambs and Suffolk rams to a stimulatory skeleton photoperiod.

Young crossbred lambs and Suffolk rams were exposed to photoperiods stimulatory to growth or reproduction respectively. Whereas long days are known to facilitate rate of gain, feed efficiency and carcass yield in young market lambs, exposure to decreasing daylengths or short days is necessary for successful reproduction of the species. Successful substitution of a "skeleton" long photoperiod (7L:9D:1L:7D) for long photoperiod (16L:8D) exposure to growing lambs to improve performance provides strong support for the conclusion that long-day stimulation is not the result of the interval of day being illuminated but rather the time interval between two periods within a day which is illuminated. The same "skeleton" long photoperiod (7L:9D:1:7D) is shown to substitute effectively for a long (16L:8D) photoperiod when used alternately with a short (8L:16D) photoperiod in eight-week light cycle exposure of mature Suffolk rams. Light cycles of this duration entrain LH, testosterone and testicular weight and stimulate reproductive activity to a level comparable to that observed in rams during the normal breeding season. Evidence is presented that the relatively short eight-week light cycles prevent photorefractoriness from occurring and thereby can maintain indefinitely the behavioral and gametogenic activities required of the breeding ram. Implementation of short days combined with alternating "skeleton" long days should be energy efficient and may prove useful for application to sire management.