In a series of 10 trials involving over 5000 lactating dairy cows, animals inseminated when detected in oestrus from 3 days after an injection of a prostaglandin F2a (PGF) had an average pregnancy rate of 68% compared to 59% in untreated herdmates. The variation in the post injection interval to oestrus necessitated detecting oestrus before the PGF-related fertility effect could be repeatedly demonstrated. When a synthetic analogue of gonadotrophin releasing hormone (GnRH) was injected before PGF in an attempt to reduce the variation in this injection-to-oestrus interval, the opposite effect occurred, the average interval was increased by 3 days (4.13 v 7.03) and the PGF-related fertility effect was eliminated. A second trial with GnRH confirmed that an injection of this hormone had an effect on the corpus luteum which prevented complete luteolysis by PGF. This interaction between GnRH and the corpus luteum was studied in 3 further trials in which GnRH was injected once at from 1 to 13 days post insemination. Positive fertility effects were observed in each trial, but they were not consistent. Nonetheless, the strategic use of these 2 types of hormones offer prospects for significantly increasing pregnancy rates to first insemination in lactating dairy cows.

JC, McEwan, C Mathieson, and H Hawker

Proceedings of the New Zealand Society of Animal Production, Volume 43, , 45-48, 1983
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