Control of sheep and goat reproduction: Use of light and melatonin

Abstract Breeds of sheep and goats from temperate latitudes exhibit seasonal variations of breeding activity which are controlled by annual photoperiodic changes. Short days (SD) stimulate sexual activity, but prolonged exposure results in refractoriness to short days and subsequent cessation of reproductive activity. Refractoriness can be broken by exposing the animals to long days (LD), thus alternations between LD and SD are essential for the photoperiodic control of seasonal reproduction. Light pulses during the dark period can mimic a long day (‘LD’) and melatonin treatments can mimic a short day. For out-of-season control of sexual activity, treatments using the succession ‘LD’-decreasing days or ‘LD’-melatonin were very effective in advancing puberty in young rams in which sperm production was increased, permitting these animals to be submitted earlier for progeny testing, using artificial insemination (AI). In adult rams, such treatments also caused an important increase in testicular weight and sperm production in the spring. In the female goat, the succession ‘LD’-melatonin treatment efficiently induces and maintains oestrous and ovulatory activities in spring, leading to high fecundity after natural mating. This treatment so far appears to be less effective in the seasonal sheep breeds of Northern Europe than in goats. However, melatonin alone can be used after the end of May to advance the breeding season and to increase fecundity. Induction of permanent reproductive activity in rams and bucks was made possible by the observation that monthly alternations between LD and SD (short light cycles) abolished seasonality of behavioural and spermatogenic activities. These males could be used all the year round to produce a high number of AI doses without variations in sperm quality and with no variation in fertility. Short light cycles can be used in open barns by alternating ‘LD’ and melatonin. In contrast, in the ewe, such short light cycles were unable to abolish seasonality of ovulatory activity. Knowledge of the different effects of photoperiod on neuroendocrine pathways and the reproductive activity in sheep and goats has therefore allowed us to successfully apply light treatments to control seasonal reproductive activity in field conditions and in males raised in AI centres.