Short light cycles induce persistent reproductive activity in Ile-de-France rams.

European breeds of rams appear to be responsive to photoperiodic changes even though there are large differences between breeds in the timing and amplitude of endocrine (LH and testosterone) and gametogenetic variations before the sexual season. Light regimens such as 6-month light cycles or alternations of constant short and long days every 12-16 weeks are able to entrain the parameters of sexual activity. In these regimens in which the period of the light cycle is shortened, LH release is markedly stimulated during decreasing daylength and evidence is presented, from the relationship of LH and testosterone patterns, that the dampening of LH stimulation could simply result from the effect of steroid feedback. However, there is a gap of several weeks between the maximum LH and testosterone concentrations during which testis growth occurs. Experiments were conducted with Ile-de-France rams, markedly seasonal breeders, in which the period of the light cycle was decreased, in different groups of animals, from 6 to 4, 3, 2 or 1 month. Rams submitted to the three light regimens with the longest periods presented testicular variations which paralleled those of the photoperiod, but those kept in the two regimens with the shortest periods had a progressive increase in testicular weight up to the maximum value (300-350 g) with no further major changes. Therefore, in rams kept in 2-month light cycles, testicular weight remained constant for twelve successive cycles (2 years). LH and testosterone plasma measurements indicated that LH was sufficiently stimulated to maintain testicular development during each decreasing daylength phase but that the stimulation was shifted before testosterone could reach levels at which feedback effects could be exerted. However, all the measures of sperm production were at values characteristic of the sexual season. Similar testicular weight maintenance was also obtained in rams submitted to a regimen in which short days (8L:16D) alternated every month with a split photoperiod interpreted as a long day (7L:8D:1L:8D). It is concluded that short light cycles are able to induce persistent reproductive activity in Ile-de-France rams, which may have practical applications in sheep production systems.