Thyroid hormones are required for the neuroendocrine processes that cause the onset of the non-reproductive state each spring in red deer. To evaluate the feasibility of achieving out-of-season breeding using thyroidectomized (THX) hinds and a THX stag, oestrous behaviour and ovulation were compared in THX (n=9) and thyroid-intact control hinds (n=6) following synchronization of oestrous cycles with CIDRs and joining with a THX, vasectomized stag in the breeding and non-breeding seasons. In the control group, oestrus and ovulation were recorded in all (100%) and none (0%) of hinds during May (breeding season) and October (non-breeding season) respectively (P<0.05), however in THX hinds oestrus and ovulation occurred in a proportion of the animals (33-55%) and this incidence was not affected by season (P>0.05). Time of onset after synchronization and duration of oestrus were not significantly affected by thyroidectomy or season (P>0.05). To determine if pregnancy could be established in THX hinds in the non-breeding season, all 15 hinds were synchronized for oestrus using CIDRs plus 200 I.U. PMSG and joined with a fertile, THX stag in early December. Any hinds not mated on this occasion were resynchronized using CIDRs plus 400 I.U. PMSG and again joined with a fertile, THX stag. While the lower dose of PMSG was generally ineffective in inducing oestrus (i.e. only 2 THX hinds mated), 50% of control and 71% of THX hinds which received 400 I.U. PMSG exhibited overt oestrus. In all, seven out-of-season calvings (3 from thyroid intact and 4 from THX hinds) were obtained from 10 matings (all 3 unsuccessful matings occurred in THX hinds). These results imply that although thyroid gland secretions switch off reproductive activity at the end of the breeding season, they do have a supportive role in the expression of overt oestrus and occurrence of ovulation.
Proceedings of the New Zealand Society of Animal Production, Volume 58, , 20-24, 1998
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