Effect of the timing of insemination (pre- or post-ovulatory) and type of semen (refrigerated or frozen) on the sex ratio of equine offspring

Abstract

The normal sex ratio of equine newborns is close to 1:1, with a slight increase of females over males, particularly when the mare is in poor physical condition (Cameron et al., Behavioral Ecology 1999 10(5):472-475). Many other factors like the age of the mare and stallion (Machado Santos et al., Theriogenology 2015 Oct 15;84(7):1238-45), maternal lineage, and uterine status of the mare (Kuhl et al., PLoS ONE 2015, Oct 5;10(10):e0139358) can influence the sex ratio as well. There are some reports describing the impact of insemination timing on the sex of newborns in humans, pigs, and cows (Roelofs et al, Ani Repro Sci. 2006 Jul;93(3-4):366-71; Martin, Hum Reprod. 1996 Aug;11(8):1812-3; Soede et al., Theriogenology 2000 Mar 1;53(4):1003-11). The present work aimed to study the effect of both the timing of insemination and the type of semen (refrigerated versus frozen) on the sex ratio in horse offspring. All the analyzed data were obtained from mares enrolled in a commercial program. In the presence of a preovulatory follicle (>35 mm) and endometrial edema, ovulation was induced by administering 2,500 IU of hCG (IM). Mares intended to be inseminated with refrigerated semen were inseminated after 24 hours from hCG injection, and if they haven't ovulated by then, theywere checked 1 or 2 times a day until ovulation was confirmed. Mares enrolled for insemination with frozen semen were controlled 3 to 4 times a day on the expected day of ovulation. The sex ratio of newborns was not influenced either by the timing of insemination or by the type of semen used (Table 1). Considering our findings, we can conclude that the insemination timing in relation to ovulation and the use of refrigerated or frozen semen, do not affect the sex of offspring in horses.