Embryo transfer, used for obtaining foals from elite mares without interrupting their competition careers, is not necessarily a straightforward route to producing a foal, fresh research suggests.
The findings, published in the journal, Equine Veterinary Education, were the result of a review of research by Dr Madeleine Campbell, of the Royal Veterinary College in England.
Embryo transfer (ET) into a surrogate mare was an accepted and successful breeding technique, Campbell said.
However, her research suggests that the potential of factors – including heat, exercise, repeated embryo flushing, and repeated manipulation of the reproductive cycle using introduced hormones – to negatively impact fertility may have been underestimated.
Her paper reviewed the evidence for involvement of these factors in repeated failures to recover embryos from competition mares of breeding age without obvious signs of reproductive abnormalities.
It concludes that, for some mares at least, the stopping of exercise around the time of ovulation (the periovulatory period) and the time between ovulation and embryo flushing, combined with careful management of flushing-induced inflammation of the inner lining or the uterus, and minimal hormonal manipulation of the reproductive cycle, may be necessary to optimise embryo recovery rates.
“Mare owners may have been encouraged to request embryo transfer for their mares following high-profile examples in the media of elite mares that have produced foals by ET whilst competing,” Campbell wrote.
“The veterinarian should educate mare owners about the multiple factors that may affect the chances of recovering an embryo from their mares, and should manage the expectations of mare owners so that they do not approach ET programmes in the expectation that there will be no disruption to their training and competition plans.”
The technique has long been promoted as a means of breeding from competition mares before they undergo an age-associated reduction in fertility, without interrupting their athletic careers, Campbell noted.
“This has been particularly beneficial in mares competing in sports such as dressage and eventing, in which many years of training are necessary before horses reach elite levels of competition.
“High-profile examples of competition mares producing foals by ET have helped to increase the uptake of ET technology amongst mare owners, and to persuade them that they can have the best of both worlds by reaping the simultaneous benefits of their mares’ competitive and reproductive success.
Industry-wide, the technique has undoubtedly proved a commercial success, she said, providing a useful tool for breeding from competition mares.
What was, in the early days, a surgical technique with associated risks for the recipient mare has been refined, over the years, into a nonsurgical technique.
“Embryo transfer is probably the most widely used artificial reproductive technique in mares other than artificial insemination. Yet its widespread practice is increasingly tempered by research into management factors affecting fertility, which suggests that breeding from competition mares may not be as straightforward as early advocates of ET suggested.”
She said her review of recent papers suggested words of caution might be justified when managing competition mare owners’ expectations.
“Disruption to training and competition schedules, for some mares at least, may be necessary to optimise embryo recovery rates,” she said.
“The research papers reviewed … suggest that mare management can affect embryo recovery rates.
“It follows that, ideally, mares would not be exercised, nor exposed to events likely to induce heat-stress, psychological stress or physical stress, either during the periovulatory period (which seems to be most crucial), or during the time between insemination and embryo flushing.
“Equally, mares that are bad travellers would not be transported (for example to and from a clinic) in the periovulatory period, or during dioestrus post insemination.
“Embryo transfer procedures would probably ideally be scheduled during the physiological breeding season.
“In those mares in which repeated embryo recovery attempts induce endometritis, uterine inflammation would be controlled between ET cycles to optimise embryo recovery and recipient pregnancy rates.
“Furthermore, since a negative effect of repeated manipulation of the reproductive cycle of donors and recipients on embryo recovery rates and survival rates post transfer cannot be ruled out, hormonal manipulation of the reproductive cycle would be minimised (which means that the number of potential recipients per donor would be high).”
Campbell concluded that embryo transfer was a useful and efficient means of breeding from mares that are still training and competing.
“The science on the effects of exercise and heat on conception is equivocal, and good embryo recovery rates industry-wide suggest that in many cases mares manage to conceive repeatedly and to donate embryos despite the stresses of training, competition and transportation.
“This is fortunate, since the ideal management conditions specified above would be difficult to meet in the situation where large herds of recipient mares are few and far between, and donor mares need to keep competing.
“Nonetheless, mare owners should not approach ET programmes in the expectation that there will be no disruption to their training and competition plans.
“It is in the interests of the owners as well as the veterinarians involved for everyone to understand at the outset that … ET has the potential to deliver disappointment as well as success.
The research has spelt out the potential for exercise, heat and stress to disrupt ovarian function, she said. “In mares from which there is a repeated failure of embryo recovery in the absence of clinically detectable reproductive abnormality, these factors should be considered.
“For some mares at least, owners may have to accept that alterations to management are necessary, and that training as well as competition may have to cease entirely during the periovulatory period and the gap between insemination and flushing.
“Furthermore, to minimise the possible negative effect of repeated hormonal manipulations of the reproductive cycle on pregnancy rates, even those owners who are prepared to give their mares an ‘easy week’ around the time of insemination and flushing may need to be flexible about when that week occurs, rather than to expect to be able to dictate it based solely around competition schedules.”
Campbell, M. L. H. (2014), Embryo transfer in competition horses: Managing mares and expectations. Equine Veterinary Education. doi: 10.1111/eve.12182
The full study can be read here.