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The effects of treatment with intravaginal progesterone (PRID) oestradiol benzoate and cloprostenol at various stages of the oestrous cycle were examined in 2 experiments. In Exp. 1, PRIDs were inserted for 14 days commencing on Day 3, 10 or 17 of the oestrous cycle and half the animals in each group received 5 mg oestradiol benzoate at PRID insertion. Plasma samples were collected daily from the time of PRID insertion for 16 days, then every 4 days for a further 24 days. Samples were analysed for plasma progesterone concentration. In Exp. 2, heifers were treated on Day 10 as in Exp. 1, but half of each treatment group received 750 micrograms cloprostenol at PRID insertion. In Exp. 1, treatment with PRID alone appeared to inhibit endogenous progesterone production when treatment began on Day 3, but not when treatment began on Days 10 or 17. Treatment with oestradiol benzoate at the time of PRID insertion depressed progesterone levels, from about 2-5 days after injection. When treatment began on Day 10, the fall in progesterone levels after oestradiol benzoate was preceded by a marked rise in levels. In Exp. 2, treatment with cloprostenol markedly reduced peripheral concentrations of progesterone and also removed the effect of oestradiol. We suggest that oestradiol benzoate had a luteolytic effect, depressing luteal progesterone production when given on Days 3, 10 or 17 and that when given on Day 10 there was a transitory luteotrophic effect which preceded its luteolytic action.

Effects of progesterone, oestradiol benzoate and cloprostenol on luteal function in the heifer