Over the past five years, the use of assisted reproductive technologies (ARTs) has increased in livestock species, including the horse. The ARTs used for equine breeding appear to follow those used in the livestock industry. These technologies enable increased progeny and genetic improvement with a reduction in waste animals born.
While technologies such as chilled and cryopreserved semen and the use of embryo transfer are now common in equine breeding, there has been a rise in the popularity of intracytoplasmic sperm injection (ICSI), ovum pick-up (OPU) and semen sexing technologies. Parallel to the rise of ARTs in equine breeding within the last five years in the United Kingdom, four equine semen collection centres, seven embryo collection teams and five germinal product storage centres have been added to the approved list under the Regulation (EU) 2016/429 and Delegated Regulation (EU) 2020/692.
The increase of new equine collection centres and the rise in the use of ARTs is set to continue. However, these technologies, such as ICSI, OPU and semen sexing, should be used responsibly in equine breeding programmes.
Intracytoplasmic sperm injection
ICSI was first developed to overcome sub/infertility of the stallion by directly injecting one spermatozoon directly into an oocyte – the spermatozoa do not even need to be motile for success. This technique can also be used following the death of either the stallion or the mare or where there are limited straw numbers left, thus increasing the production efficiency of offspring.
Twemlows Stud Farm, Shropshire, has recently (September 2023) announced a positive 42-day pregnancy scan from a recipient mare using a fresh ICSI embryo transfer completed on-site. As far as the author is aware, this is the only centre in the UK currently offering an ICSI service commercially. However, there is availability for ICSI services in the European Union and worldwide. Many veterinary centres and studs in the UK export frozen semen straws and embryos/oocytes to the EU, offering this service to UK clients. At one of the largest centres in the EU offering ICSI (Italy), the number of ICSI sessions completed doubled from approximately 1,000 in 2017 to 2,000 in 2019, and the rise in the use of this technique is predicted to continue worldwide (Lazzari et al., 2020).
Breeding sub- or infertile individuals is beneficial for the conservation of rare breed equids; however, equine breeders should consider the negative impacts of this as well
Currently in equine breeding, there seems to be a drive to modify and improve techniques to successfully breed from sub- or infertile mares and stallions. Breeding sub- or infertile individuals is beneficial for the conservation of rare breed equids; however, equine breeders should consider the negative impacts of this as well. There is the possibility of creating a genetic bottleneck in equine populations, where there are large numbers of horses with related genetics due to increased breeding from a small selection of animals. Not only this but there is evidence that fertility has a genetic component, thus breeding from subfertile individuals may pass this trait onto the offspring (Gonçalves et al., 2023). Therefore, with the new and rising use of ARTs comes the responsibility of equine breeders to continue to safeguard the genetics of the future equine population.
Ovum pick-up
OPU enables the production of multiple progenies per year from older mares, competition mares or sub/infertile mares. Oocytes are collected earlier in the oestrus cycle than for embryo transfer by aspirating the ovaries and collecting the oocytes before ovulation. Compared to embryo transfer, OUP reduces the time and frequency of veterinary visits, increases the potential number of offspring produced from one collection of oocytes and can be carried out when the mare is not in season – all of which are appealing to breeders (Claes and Stout, 2022). However, OPU is not a standalone ART: the oocytes are then transferred to a facility that can perform ICSI.
While many veterinary centres specialising in equine reproduction in the UK are now offering this technique, the majority of oocytes are currently being shipped to the EU for ICSI, which does require additional cost. Similarly to the rise in number of ICSI procedures, a prominent laboratory in the EU (Italy) has also seen an increase in OPU-ICSI procedures, with numbers rising from approximately 600 in 2017 to 1,100 in 2019, and expects the rise to continue (Lazzari et al., 2020).
[Ovum pick-up] is more invasive for the mare as the ovaries are aspirated for multiple oocytes, and success of pregnancy is not guaranteed
Although OPU may have increased costs compared to embryo transfer due to many oocytes being exported to the EU for ICSI, it seems to be a popular choice due to the many benefits for the equine breeder. However, it should be noted that this procedure is more invasive for the mare as the ovaries are aspirated for multiple oocytes, and success of pregnancy is not guaranteed.
Semen sexing
The ability to sex semen was first capitalised on by the dairy cattle industry to enable the production of replacement cows and reduce the waste of male calves that had no role in the herd. Today there is also a drive to use the same technology to choose the sex of the foal for the equine breeder. Following collection of the semen from a stallion, it is sorted using the difference between the X and Y chromosomes and packaged ready for insemination.
Success has been seen in the UK with two filly foals born (2020 and 2022) using fresh sexed semen. However, this ART is currently only available at one centre in the UK (Stallion AI Services Ltd in conjunction with Cogent UK), as far as the author is aware.
Research continues for sexed equine semen as the success rates are still low (Ducheyne et al., 2023). While there has been success, sexed equine semen is taking its time to reach commercial viability due to cost, stallion fertility and available expertise and equipment. This year (2023) has, however, seen the commercial production of one foal using frozen, sex-sorted semen via ICSI in conjunction with Stallion AI Services Ltd (UK), Cogent (UK) and Avantea (Italy) (Ducheyne et al., 2023). Another milestone has thus been reached for ARTs in equine reproduction for the UK, with the commercial progress set to continue.
Closing comments
There have been major milestones reached in terms of ARTs for equine breeding in the last few years, with an increasing trend of techniques such as OPU-ICSI and semen sexing being used. However, it is worth noting that responsible breeding should be at the forefront for equine breeders to promote genetic diversity within the equine population while using ARTs.
Responsible breeding should be at the forefront for equine breeders to promote genetic diversity within the equine population while using assisted reproductive technologies
