The Effect of an Equatorial Light Environment on Reproduction in Sheep. I. The Breeding Season

Abstract

Various aspects of the breeding seasonality of Welsh Mountain (WM), Border Leicester (BL), North Country Cheviot (NCC) and Scottish Blackface (SBF) ewes were investigated during two breeding seasons in housed and field ewes in the local light (LL) environment of Potters Bar (lat. 51°43′N) and in ewes housed in a simulated equatorial light environment (EL) (13L: 11D). Significant advancement of the onset of oestrous periods was observed in all breeds maintained in the EL environment and resulted in a rephased and shorter breeding season. Advancement varied according to breed and was greater in WM (62 days) and BL (42 days). There was no significant difference between NCC (30 days) and SBF (28 days) ewes. The level of oestrous activity in a breed over a sequence of 17-day periods and the duration of oestrus tended to be reduced in EL ewes. In the EL environment NCC and SBF ewes had significantly lower mid-cycle progesterone concentrations than ewes of the same breed in the LL environment. There was also a higher incidence of ovulation without oestrus and oestrus without ovulation in the EL environment. The plasma progesterone concentration at various stages of pregnancy did not show a significant difference between light environments. Plasma progesterone concentration on day 17 after mating had an accuracy of 92·5% as a diagnosis of pregnancy. The value of plasma progesterone determinations in the study of seasonal reproductive activity is discussed.