An average of 60 Rambouillet and 60 Suffolk ewes was maintained under an intensive management system to lamb about every 7 months. Decreased daylength was used for oestrus induction in normally anoestrous periods. The experiment lasted 3·7 years, during which time six lamb crops were produced, three of them as a result of out-of-season mating. Ewes of each breed were subdivided into three experimental groups and each breed-group was maintained under one of three different light treatments before and during out-of-season mating. Imposed Treatment 1 consisted of a pre-mating period under natural daylight followed by a mating period of short daylength (8 h/day). Ewes under imposed Treatments 2 and 3 were exposed to 16 h of light during the pre-mating period and to 8 h of daylight during the mating period. This did not affect conception significantly in the Rambouillet but it did in the Suffolk. Different intensities and wavelengths of light during the long daylight period (16 h/day) were used in Treatments 2 and 3 but did not cause different responses.
In Rambouillet ewes the first mating (out-of-season) gave 73% conception. It was followed by two in-season matings with 72 and 85% conceptions. The last three matings (out-of-season, in-season, out-of-season) resulted in 57, 42, and 58% conceptions. In the Suffolk ewes, the conceptions were 30, 88, 77, 3, 77, and 10% in the same order as those for Rambouillet. The average production ofRambouil-lets on the three treatments was 166 lambs per 100 ewes per year (165, 177, 157) whereas Suffolks produced 132 lambs (120, 143, 135). Control Rambouillet and Suffolk ewes maintained under natural daylight year-round produced 139 and 117 lambs.