153 SEASON DOES NOT INFLUENCE EARLY CONCEPTUS GROWTH AND RECOVERY IN THE HORSE

Abstract

Although the horse is a seasonal breeding species, a considerable number of mares continues to cycle throughout autumn and winter. This allows for embryo collection outside the breeding season. However, slower equine embryo growth during the non-breeding season has been hypothesised. Because in this species, smaller embryo size (<0.3 mm diameter) is beneficial for conceptus cryopreservation, embryo collection outside the breeding season could be an interesting approach for the production of frozen horse embryos. In the present study we have therefore analysed embryo recovery rates in a herd of research mares (Haflinger breed, n = 30) that are used for embryo production throughout the year. Oestrous mares were inseminated (500 Mio progressively motile sperm) at 48-h intervals and ovulation was detected by transrectal ultrasound. Diameter of the conceptus at collection was measured with a microscope scale (<day 11) or by electronic calipers of the ultrasound machine (>day 10). Statistical analysis was done with the SPSS Statistics 21 software. The influence of collection day (7 to 14 after ovulation) and season (SPR: March – May; SUM: June – August; AUT: September – November; WIN: December – February) on conceptus size was analysed by GLM univariate analysis. Influence of season on recovery rate was compared by Chi-squared analysis. Values for conceptus diameter are given as means ± s.e.M. A total of 352 embryo collection attempts were performed. Mean recovery was 64.2%. It was not affected by season (number of embryos collected: SPR: 51/72, SUM 67/105, AUT 71/114, WIN 37/61 flushes). The conceptus diameter was evaluated in a total of 165 embryos. Conceptus diameter significantly increased (P < 0.001) with day of pregnancy (e.g. day 7: 0.5 ± 0.05, day 8: 0.76 ± 0.2, day 10: 3.8 ± 0.2, day 12: 9.8 ± 0.6, day 14: 17.5 ± 1.0 mm), but was not influenced (P = 0.957) by season (e.g. day 10: SPR 4.2 ± 0.6, SUM 3.6 ± 0.4, AUT 3.9 ± 0.4, WIN 3.8 ± 0.6 mm). These results demonstrate that embryo collection in horses can be performed successfully throughout the year if mares continue to ovulate during the non-breeding season. Because conceptus growth is similar with regard to season, production of cryopreserved equine embryos is not facilitated at a specific time of the year.