The hypothesis tested in this field trial was that 1 mg oestradiol benzoate (ODB) could be equally effective where administered to anoestrous animals either by intramuscular injection or in a gelatin capsule per vaginum. The ODB treatment was administered to 412 cows in 8 dairy herds 24 h after the end of a 6-day priming treatment with progesterone, also administered per vaginum. By 48 h after ODB administration, 87% of injected animals had been detected in oestrus and inseminated compared to only 44% of herdmates receiving the gelatin capsule (P<0.01). Re-examination of those animals not detected in oestrus by 13 days after ODB treatment showed that 30% of capsule-treated animals were still anoestrus and another 10% had ovulated without being detected in oestrus. Comparable percentages for the injected animals were 3% and 2% respectively (P<0.01). The vaginal treatment was also associated with a reduced pregnancy rate to first insemination (31.7% vs 47.7%; P<0.05). The reduced efficacy associated with the vaginal administration of ODB to anoestrous animals most probably involved a prolonged rate of absorption which did not stimulate behavioural oestrus or an ovulatory surge of LH but did reduce fertility in those animals which were inseminated.

KL, Macmillan, VK Taufa, and AM Day

Proceedings of the New Zealand Society of Animal Production, Volume 56, , 350-352, 1996
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