Synchronisation of Ewes at the Second Oestrus after Progestagen Treatment

To determine the change in distribution of ewes exhibiting oestrus at the first and second oestrous cycles after progestagen treatment, data were examined from six experiments in which 430 ewes received progestagens but experienced no fertile mating at the first post-treatment oestrus. Three progestagens, three breeds and two seasons were involved. In all experiments there were more silent heats at the first as compared with the second post-treatment cycle; hence more ewes were available for mating at the second cycle. Among 326 ewes which exhibited oestrus at both cycles, standard deviations about the mean day of oestrus ranged in the six groups from * 0.47 to t 1.13 days for the first oestrus and from t 0.85 to t 1.44 days for the second oestrus. Because of the increased dispersion at the second cycle, a higher percentage of ewes exhibited oestrus during the peak three days of the first cycle than was the case with the second. The number of heats falling within three days at the first cycle approximately equalled the numbers falling within four to five days at the second cycle. It is concluded that in some circumstances it may be advantageous to delay mating until the second cycle.