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The seasonal nature of fertility of sheep has been accepted for many years (Hafez, 1952). It has also placed major restraints on farmers wishing to maximize farm output Because sheep have a 5-month pregnancy and a 3-month lactation period it should be possible to produce 3 sets of lambs every 2 years. A major restraint on such a programme is the very strong seasonal photoperiodic influence upon fertility. In the farming systems of Europe there is a need to maximize the efficiency of labour-intensive management while in Australia there is a need to minimize labour-extensive farming systems. Both management strategies require better control of the onset of breeding activity In many areas of Australia the late autumn start of breeding of Suffolk. Romney. Border Leicester and Perindale ewes means that the lambs are often weaned at a time of deteriorating pasture quality in late spring/summer with subsequent poor weight gain. An earlier season would allow better growth of the lambs before the summer burn off. This review covers recent studies on the photoperiodic control of reproduction in ewes. We concentrate on evidence for the role of light, the pineal gland and melatonin on seasonal breeding in the ewe and discuss the ways this information has been used to gain control of sheep fertility and fecundity.

Photoperiodic control of the onset of breeding activity and fecundity in ewes