The growth parameters exhibited by seven Thoroughbred (Tb) foals that had experienced a 'restricted' in utero existence following transfer as embryos to the uteri of smaller Pony (P) mares (Tb-in-P) and, conversely, six P foals that experienced a 'luxurious' in utero existence after transfer to larger Tb mares (P-in-Tb), were compared from birth to 3 years of age with those exhibited by six normal Tb-in-Tb and six P-in-P foals conceived by within-breed artificial insemination. Bodyweight, height at the withers, girth, poll-to-nose length, crown -rump length and three foreleg longbone measurements were made at regular intervals. At birth, an approximate 15% reduction or increase in parameters was observed in the Tb-in-P and P-in-Tb respectively, which declined to 5% by 3 years of age. Growth post partum was affected by restricted or enhanced growth in utero. In the first 6 months post partum, growth rate was enhanced in the previously restricted Tb-in-P foals and curbed in the previously enhanced P-in-Tb foals compared with their respective controls. Overall, the similarity of the responses of the offspring to both 'restriction' and 'luxury' in utero ensured that no major changes to conformation resulted from either treatment. Thus, the Thoroughbreds carried by the Pony mares were merely scaled down versions of the Tb-in-Tb controls while the Ponies carried by the Thoroughbred mares were scaled up versions of the P-in-P controls.

Summary Reasons for performing study: To compensate for the wide variation in the freezability of stallion spermatozoa, it has become common veterinary practice to carry out repeated ultrasonography of the ovaries of oestrous mares in order to be able to inseminate them within 6–12 h of ovulation with a minimum of 300–500 × 106 frozen‐thawed spermatozoa. Furthermore, in order to achieve satisfactory fertility, this requirement for relatively high numbers of spermatozoa currently limits our ability to exploit recently available artificial breeding technologies, such as sex‐sorted semen, for which only 5–20 × 106 spermatozoa are available for insemination. Objectives: This study was designed to evaluate and compare the efficacy of hysteroscopic vs. conventional insemination when low numbers of spermatozoa are used at a single fixed time after administration of an ovulation‐inducing agent. Methods: In the present study, pregnancy rates were compared in 86 mares inseminated once only with low numbers of frozen‐thawed spermatozoa (3–14 × 106) at 32 h after treatment with human chorionic gonadotrophin (hCG), either conventionally into the body of the uterus or hysteroscopically by depositing a small volume of the inseminate directly onto the uterotubal papilla ipsilateral to the ovary containing the pre‐ovulatory follicle. Results: Pregnancy rates were similarly high in mares inseminated conventionally or hysteroscopically with 14 × 106 motile frozen‐thawed spermatozoa (67% vs. 64%). However, when the insemination dose was reduced to 3 × 106 spermatozoa, the pregnancy rate was significantly higher in the mares inseminated hysteroscopically onto the uterotubal junction compared to those inseminated into the uterine body (47 vs. 15%, P<0.05). Conclusions: When inseminating mares with <10 × 106 frozen‐thawed stallion spermatozoa, hysteroscopic uterotubal junction deposition of the inseminate is the preferred method. Potential clinical relevance: Satisfactory pregnancy rates are achievable after insemination of mares with frozen‐thawed semen from fertile stallions 32 h after administration of human chorionic gonadotrophin (Chorulon)1. Furthermore, these results were obtained when mares were inseminated with 14 × 106 progressively motile frozen‐thawed spermatozoa from 2 stallions of proven fertility.

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