Five lighthorse mares were actively immunized against gonadotropin releasing hormone (GnRH) to determine the relative importance of this hypothalamic hormone in the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Five mares immunized against the conjugation protein served as controls. Mares were initially immunized in November and received secondary immunizations 4 wk later, and then at 6-wk intervals until ovariectomy in June. All mares immunized against GnRH exhibited an increase (p less than 0.01) in the binding of tritiated GnRH by plasma, an indication that antibodies against this hormone had been elicited. Concentrations of LH, FSH and progesterone in weekly blood samples were lower (p less than 0.05) in GnRH-immunized mares than in controls after approximately 4 mo of immunization. However, the LH concentrations were affected to a greater degree than were FSH concentrations. All five control mares exhibited normal cycles of estrus and diestrus in spring, whereas no GnRH-immunized mare exhibited cyclic displays of estrus up to ovariectomy. All mares were injected intravenously with a GnRH analog (which cross-reacted less than 0.1% with the anti-GnRH antibodies) in May, after all control mares had displayed normal estrous cycles, to characterize the response of LH and FSH in these mares; two days later, the mares were injected with GnRH. The LH response to the analog, which was assessed by net area under the curve, was lower (p less than 0.01) by approximately 99% in mares immunized against GnRH than in control mares. In contrast, the FSH response to the analog was similar for both groups.(ABSTRACT TRUNCATED AT 250 WORDS)

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Summary A total of 77 embryo collection attempts from 28 donor mares resulted in 50 embryos (64.9 per cent recovery rate) ranging from Day‐6 morulae to Day‐7 expanded blastocysts. Thirty‐five of these embryos, classified as quality grades 1 and 2, were allotted randomly by developmental stage (Stage‐I, precapsulate and Stage‐II, postcapsulate) to a control group transferred non‐surgically within 1 h to synchronised recipients, and a treated group, cultured on a monolayer of foetal uterine fibroblast cells for 24 h prior to non‐surgical transfer to recipient mares. There was no significant difference in pregnancy rates between Stage‐I and Stage‐II embryos, nor between the control group (53.3 per cent) and the 24 h co‐cultured group (42.1 per cent). The use of a co‐culture system may provide a viable alternative to cryopreservation for the transport of equine embryos from one station to another.