The effects of œstradiol benzoate (OB) and gonadotrophin releasing hormone (GnRH) on reproductive activity in beef cows at different intervals post partum

œstradiol gonadotrophin releasing Summary. Three experiments were carried out to determine the effects of OB and GnRH on beef suckler cows at various stages post partum. In experiment 1, the first post partum oestrus occurred at 36 ! 17 (mean ± SD), 44 ! 25 and 43 ! 22 days post partum for animals injected with 400 fL9 OB or 100 fL9 GnRH at days 15 and 30 post partum and controls, respectively. The first post partum ovulation, based on elevated plasma progesterone levels, occurred at 37 ! 17 (mean ± SD), 31 ! 18 and 34 ! 10 days post partum for these three groups, respectively. Neither OB nor GnRH increased the numbers of animals which exhibited LH peaks at day 15 or 30 post partum or which showed a subsequent rise in plasma progesterone. There was a high proportion of silent ovulations early in the post partum period. In experiment 2, the time from parturition to the first subsequent oestrus was 93 j- 18 days for animals given 20 fL9 of an LHRH analogue (Hoe 766) between days 37 and 77 post partum, and 86 ! 28 days for controls. Significantly more (P < 0.01) treated cows than controls had elevated plasma progesterone levels following the time of treatment. In experiment 3, animals given two injections of 100 mcg progesterone and one injection 20 fL9 Hoe 766 at two day intervals between days 26 and 87 post partum had an interval between calving and first subsequent cestrus of 81 ! 22 days (mean -L SD) and this interval for controls was 83 ! 13 days. Five of seven treated animals exhibited cestrus 21-22 days after treatment suggesting occurrence of a silent ovulation after treatment.