12 Artificial insemination and embryo transfer results in ewes during a long daylength period

Abstract

Sheep are polyestrous, short-day breeders (∼11 h of daylength), and exhibit oestrus approximately every 16–17 days during the breeding season, usually in late September to late December in the Northern Hemisphere. Progressive sheep producers often use assisted reproductive techniques such as laparoscopic AI and ovarian hyper-stimulation, embryo collection, and embryo transfer (ET) to increase genetic gain, and strive to have early December lambs to target specific show markets, which dictates AI or ET during the late summer. This field trial compared pregnancy rates following AI or ET in July and August (∼14 h of daylength) in southwest Virginia (36–38′12″ N). Ewes (AI, n = 83; ET recipients, n = 33) were synchronized using a modified Ovsynch protocol involving intravaginal progesterone implants for 14 days, prostaglandin F_2α (intramuscular) 48 h before expected oestrus, and PG600 (IM) and gonadotrophin-releasing hormone (intramuscular) 52 to 54 h and 16 h before AI, respectively. Ewes were subjected to AI (frozen/thawed semen) regardless of whether they displayed signs of oestrus, and ewes selected as embryo recipients were subjected to a similar protocol but instead received 1 or 2 embryos (based upon the number of viable embryos produced per embryo donor) 6 days following the AI of the embryo donors. Ovarian hyper-stimulation of the embryo donors (n = 13) was enabled by twice-daily FSH injections [totalling 350–455 IU of Folltropin V (10–13 mL)] for the 4 days before AI. Six days following AI, embryos were recovered surgically from the embryo donors (n = 13) and yielded an average (± s.e.m.) of 6.6 ± 1.2 total ova, 4.7 ± 1.1 transferable quality embryos, and 1.9 ± 0.8 unfertilized ova per collection. Pregnancy was detected using transrectal ultrasonography at ∼30 days of gestation and the pregnancy rates were analysed using Chi-squared. There was a tendency (P = 0.092) for more pregnancies to be established following ET (22/33; 66%) compared with AI (41/83; 49%). There was no statistical relationship between AI ewes or ET recipient ewes that became pregnant relative to whether they displayed signs of oestrus or not. Embryo transfer was a more successful approach to produce pregnancies in ewes compared with AI during long daylength periods in this field trial. Further, ova from hyper-stimulated embryo donor ewes experienced a very high fertilization rate. Future studies will evaluate the ova capability directly via laparoscopic aspiration of ovarian follicles and IVF and evaluation of hyper- and non-hyper-stimulated follicular waves (using transrectal ultrasonography) and endocrine dynamics during different long and short daylength periods. Extending the opportunity to generate embryos and offspring regardless of daylength will assist aggressive sheep producers in reaching their reproductive, financial, and genetic goals.