Four experiments are described in which attempts were made to increase the number of live lambs born per ewe per unit time. In the first experiment 22 Scottish Half-bred ewes were subjected to an artificial photoperiod. This resulted in 95 % of the ewes being mated 128 days after their previous lambing. It was estimated that artificial photoperiod brought the breeding season forward by about 5 weeks. The use of 1500 i.u. Pregnant Mare Serum had no significant effect on mean litter size. In the second experiment three artificial photoperiods were compared with natural daylength. In Group 2 photoperiod was decreased rapidly on 2 April from 18 to 10 h, in Group 3 photoperiod was decreased gradually on 2 April from 24 h and in Group 4 photoperiod was decreased rapidly on 28 May from 18 to 10 h. Group 5 was the control. Median intervals from parturition to next conception for Groups 2–5 were 70, 138, 68 and 131–5 days respectively; median intervals from onset of decreasing daylength to conception were 129, 200, 83 and 118–5 days respectively. Treatments 2 and 4 significantly reduced the interval from parturition to next conception but treatment 3 was unsuccessful. The interval from onset of decreasing daylength to conception was significantly shorter for treatment 4 than for treatment 2.
In Exp. 3 an unsuccessful attempt was made to extend the natural breeding season by preventing photoperiod from increasing for 3 months after 22 December.
In Exp. 4 a similar photoperiodic pattern was used for Groups 9 and 10 (16 h photoperiod decreasing rapidly to 8 h on 18 February). The contrast was that in Group 9 most of the ‘light’ was natural, in Group 10 all the ‘light’ was artificial. This pattern was compared with the natural daylength pattern (Group 8), an equinoctial treatment (Group 11) and a short-day treatment, 8h light, 16 h dark (Group 12). The median intervals from parturition to next conception for Groups 8–12 were 194, 102, 99, 156, 155 days, respectively. There was no significant difference in this interval between Groups 9 or 10. Type of ‘light’ therefore had no influence on the median interval from parturition to next conception; but photoperiodic pattern did. A rapid increase to 16 h light followed by a rapid decrease to 8 h (Groups 9, 10) was more effective than either the equinoctial treatment (Group 11) or the short-day treatment (Group 12). The treatments imposed on Groups 11 and 12 were more effective in shortening the median interval from parturition to next conception than the natural daylength pattern (Group 8). Time of year of mating had no effect on lambing percentage.
It was concluded that the sensitivity of sheep to photo-stimulation varied directly, with time from the onset of the next natural breeding season and with the speed of photoperiodic decrease.