Determinants of the annual pattern of reproduction in mature male Merino and Suffolk sheep: modification of endogenous rhythms by photoperiod

Abstract

In mediterranean environments, pastures arevery poor during the autumn and consequently small ruminants, such as sheep,would have been losing body mass for many months so, during mating, gameteproduction would be depressed in both sexes. Effectively, the nutritiverequirements for a photo-period-driven, annual reproductive cycle are out ofphase with seasonal changes in food availability. The problem could beovercome through more flexible timing of reproduction, perhaps explainingvariations in seasonality between breeds that originate from differinglatitudes. To study these concepts and the mechanisms involved, the endogenousrhythms and responses to photoperiod were compared in rams of‘mediterranean origin’ (Merino) and ‘temperate origin’(Suffolk). Groups of 16 rams of each breed were given a constant food supplyand subjected to 16 months of constant equinoctial photoperiod (12L : 12D) orsimulated ‘mediterranean’ changes in daylength (from 14L : 10D to14D : 10L). With nutritional and photoperiodic inputs held constant, Merinoand Suffolk rams showed similar endogenous rhythms in reproductive activity.Under constant nutritional inputs and a mediterranean photoperiodic cycle, theendogenous rhythms were modified differently in the two breeds, with theMerinos starting and finishing their seasons about 2 months earlier than theSuffolks. These observations partially explain the patterns observed in ramskept under field conditions. It is now necessary to test whether the rhythmsof reproduction in these breeds are also modified by changes in nutrition andsocial cues.