Abstract

This study tested the hypothesis that oestrous cycles with two follicle waves created using progesterone, could be converted to oestrous cycles with three follicle waves using gonadotrophin releasing hormone (GnRH). Oestrus (Day 0) was synchronised in 24 Friesian cows. On Day 2, a progesterone containing device (CIDR) was inserted into the vagina of 16 cows for 10 days. Eight of these animals received 8µg of buserelin (GnRH) at device removal. Eight untreated animals formed the Control group. Daily transrectal ultrasonography showed that the oestrous cycles of the cows treated only with a CIDR device had two ovarian follicle waves compared to 1/8 in CIDR+GnRH and 3/8 in Control groups (p<0.01). The average length of the oestrous cycle was shorter in the CIDR-only group (19.3 ± 0.5 days), compared to the CIDR+GnRH (20.9 ± 0.2 days) and Control (22.3 ± 1.1 days) groups (p<0.05). These results demonstrate that strategic administration of GnRH can convert progesterone induced two-wave oestrous cycles into a three-wave pattern.

BA, Clark, FM Rhodes, CR Burke, SR Morgan, and KL Macmillan

Proceedings of the New Zealand Society of Animal Production, Volume 58, , 85-87, 1998