The use of estradiol benzoate to manage lambing period in ewes bred at synchronized estrus

The induction and management of lambing was studied in a commercial flock of ewes. These ewes were bred at synchronized estrus and managed to lamb three times in 2 yr, with lambings occurring in the sequence of February, September and June. Ewes treated with 2 mg of estradiol benzoate in oil (i.m.) at approximately 17 d prior to term did not lamb in response to treatment. When 2 mg estradiol benzoate was given at 0800 on d 142 of gestation, lambing occurred an average of 37 +/- 5 h later in ewes treated in June or September and at 70 +/- 10 h later in ewes treated in February. The comparable figures for oil-treated ewes (control) were 70 +/- 5 h and 116 +/- 9 h, respectively. Altering the time of day of injection or using two injections 24 h apart did not alter the time for injection to lambing. Increasing the dose of estradiol benzoate to 15 mg decreased the time from injection to lambing (43 +/- 7 h, ewes treated in February) but also increased the incidence of dystocia (50% incidence). The incidence of dystocia averaged 8% for all other treatments and controls. Death loss of lambs to 1 wk of age was 12% and was not affected by treatment. The length of the lambing period was reduced from 9.0 +/- 3.3 d in control ewes to 3.6 +/- 1.1 d in induced ewes. A single injection of 2 mg estradiol benzoate given at 0800 on d 142 of gestation or a double treatment, with injections at 2000 on d 142 and 143, resulted in 53% of ewes lambing between 0800 and 1600 on each day of the lambing period, compared with 33% in control ewes.