Can We Improve The Fertility Of Horses?

Dr. Angus Mckinnon is one of the foremost experts in equine reproduction in the world. After graduating from the University of Melbourne in 1978, Angus spent 10 years in the USA, at Ontario Veterinary College in Canada and from 1984-88 at Colorado State University. Colorado State University has an international reputation for its excellence in reproduction research and from that base Dr. Mckinnon collaborated with Dr. James Voss to produce the outstanding textbook Equine Reproduction. This was published in the United States in 1993 by Lea and Febiger. Since returning to Australia 6 years ago, Dr. McKinnon has established a practice at Shepparton in Victoria. Apart from being actively involved as a veterinarian in reproduction clinical problems of mares and stallions, Dr. McKinnon performs extensive research projects and is currently being supported by RIRDC. Dr. McKinnon spoke with Prof. Reuben Rose, the Equine Program Coordinator, about the some of the major issues in horse breeding.

Q: Why is it that the relationship between conception rates in the mare and the number of live foals born is so low?
A: There are such varied levels of management, it's really hard to make direct comparison between breeding farms. On the breeding farms serviced by veterinarians familiar with sophisticated reproduction techniques, the pregnancy rates per cycle are very good, and the number of mares that foal is quite high. On some farms mares are bred on foal heat, before they are physically ready to nurture a foal, and many mares with chronic uterine inflammation, are bred and either they don't go in foal or if they do, the uterine environment is so poor for the developing foetus that early in pregnancy or later the mare will abort.

Q: Is there anything in your work or in the work of others that suggests that we are never going to get to 95% foaling rate in mares. Was the mare just never designed to have a foal every year?
A: We are not breeding for fertility, we are breeding for the elite athlete. Mares that have difficulty getting in foal, because of heritable conditions such as conformation and maybe even ability to resist uterine infection, will pass on these traits. So while we are breeding the fastest animals we are never going to breed the most fertile animals. I think that on well managed breeding farms where problems are attended to, mares will have foals most years. Endometritis is by far the most common cause of the old barren brood mare. We have had spectacular results by treating them every month with antibiotics. While I have not got any statistical data, with many of these chronically infertile mares, I put them on antibiotics if they've aborted for 3 years or more or had early embryonic death. It is expensive, but the results are well worth it, and it's a rarity to have those mares slip.

Q: Could you outline the major reasons why mares fail to get in foal or "slip" the foal early?
A: I believe that the main cause is a chronic inflammatory reaction in the uterus usually due to infection. This can occur due to reproductive conformation problems which allow air into the reproductive tract. It may also occur if the mare has been poorly or inappropriately treated after foaling or may occur due to repeated serving by the stallion during the one breeding cycle. Many studies show that natural service results in high levels of contamination: one million bacteria per millilitre of semen is quite common when semen is cultured after natural service. However, there are many other causes for mares failing to maintain a pregnancy and to investigate these causes we can take a biopsy or do a complete reproductive exam.

Q: Could you outline the techniques that have become available recently that have helped to be able to make a better assessment of the likelihood of the mare getting in foal?
A: One of the things that has been around for a long time that people have not appreciated until more recently is the effects of good management. The single biggest thing that we have done differently is to change management practices and I believe that this has been very beneficial in improving fertility. We place a lot more emphasis now on the history of the mare, this includes factors such as her foaling history, the fertility of the stallion and the management of the farm where the mare was served. So, history first, then we assess the mare's physical condition to make sure she's not suffering from a disease that may be affecting her ability to cycle, such as laminitis or a pituitary problem. We assess her external reproductive conformation and whether or not she's been appropriately treated. The Caslick operation is still one of the most useful tools to manage mares with conformation problems but it is often inappropriately done.

Then we do a rectal exam to feel the tone of the uterus and also to find out if the mare is pregnant. We sometimes have mares presented for infertility and we find them pregnant! In cases like that, everyone is quite embarrassed but the owner is usually quite happy. The next part of the exam is the ultrasound, we look at the ovaries to see if they're normal in size and shape. We then do a vaginal exam to feel for cervix adhesions or lacerations or for places where the foal may have torn the mare at birth causing weakness in the walls of the uterus or birth canal. Next we do a biopsy where we take a small sample of cells from the uterus to see if the mare has any problems such as infection. After this it depends on the history of the mare, results of the biopsy and other tests as to what we will do next. We may use an endoscope to have a look high up in the uterus at the uterine-tubule junction. In some cases where the mare has never had a foal we may take blood for a chromosome analysis, to see if there is a genetic problem causing sterility in the mare. We may look at the hormone balance if the mare isn't cycling or has unusual cycling. Most of the work we do however, focuses on the chronic inflammatory changes in the uterus, which we'll pick up with the biopsy. We try to assess how bad the inflammation is, how long it's been going on and whether it is worth treating. If we think that the mare has a poor probability of going in foal then I think we've done the owner a very good service by saying that you have a 5 or 10% chance of this mare maintaining a pregnancy. An informed decision can then be made about what they do with her. It saves frustration for the owner, saves the owners finances and saves the mare being subjected to a stud environment that's not necessary.

Q: Many people think that when mares abort or "slip" the foal, that there's a hormonal problem. What's the value of hormone treatments?
A: Cliff Irvine who is a researcher in Christchurch New Zealand, has done a study in a large number of mares, examining causes of early embryonic death. Only one mare in this study had a hormone imbalance at the time of embryonic death. All other cases in his study showed hormonal changes after the pregnancy had been lost, but not before. Progesterone is the hormone that maintains pregnancy and most people think that low progesterone is the cause of early embryonic death. However, we've produced a lot of clinical and experimental data to say that the progesterone level comes down after the foetus has died and not before. We've had mares that have been treated with progesterone and we've had mares that have not been treated and seen similar embryonic loss rates in both groups, even in mares with a history of embryonic loss. What we haven't looked at yet, is what other things are able to prevent embryonic loss and one thing we've been looking at is antibiotics. If results from the mare's biopsy show that she has an infection, we'll treat her with neomycin penicillin for three days every two weeks throughout pregnancy. These mares usually do very well and in most cases will produce a live foal.

Q: These are mares that presumably have evidence of a chronic uterine infection?
A: Yes, and that's the reason for putting them on the antibiotics. I believe that without proper treatment, bacterial endometritis does not go away in some mares and I think that it can cause placental separation in pregnant mares, foetal retardation and abortion or early in pregnancy you get lack of nourishment and the foetus shrivels and dies. The single biggest thing we've done is identify these mares with biopsies and put them on antibiotics and we've done quite excitingly well with this treatment.

Q: There seems to be problems associated with lower pregnancy rates when mares are bred at the foal heat. Is there anything that can be done to improve this?
A: We did a study using ultrasound technology and this study gave us a management tool to handle mares on foal heat and it also gave us the ability to accurately inform our clients about when to breed their mares. We can show them that foal heat pregnancy rates are lower than mares bred at a later time after foaling and that embryonic death rate is higher in the unmanaged mare. After foaling, the uterus is inflamed, contains fluid and it needs to contract and repair itself before it is ready to accept another embryo. If the mare is bred on foal heat, the embryo will arrive in the uterus at about day 16-18 after foaling. It is a huge effort to have the uterus ready in this time frame. By using ultrasound we can identify the mare with a uterus that contains fluid and hasn't contracted, we can stop them from being bred on foal heat so that they can be bred later for a greater chance of a successful pregnancy. We only breed 5-10% of foaling mares on foal heat.

Q: In general, is the principle: don't breed the mare on the foal heat if you want a successful outcome?
A: Unless you're prepared to manage the mare carefully and treat her and ultrasound her. Our foal heat breeds this year have been very good. We would have an 85% pregnancy rate per foal heat cycle bred and we've not lost any pregnancies that we know of to date. However, those mares are very carefully selected and managed.

Q: What percentage of mares in the general population are bred on foal heat?
A: Only about 10% of the population of foaling mares are bred on foal heat.

Q: The stallion does seem to be the forgotten area in reproduction. Are there many reproductive problems with stallions?
A: Stallions have lots of problems that can identified and treated. Many people don't realise the technology to fix stallion problems is here. We get more stallion referrals in proportion to the number of mares. There is one stallion for every thirty or forty mares, so you'd only expect to see 3 or 4 stallions if you were referred a hundred or more mares. However, we probably see 25 to 30 stallions per year and between a hundred and three hundred mares for reproductive problems. It's interesting that there's a big imbalance.

Q: What is your assessment about the percentage of stallions with poor semen quality?
A: The older the stallion, the poorer the semen gets. In the stallion population less than 13 years, the percentage of stallions that have got fertility problems associated with poor semen is less than 5%. Once the age of the stallion climbs then the percentage of problems increases dramatically and by the time you have a stallion population aged over 18 years, you've got at least 50% of that population with fertility problems. The only accurate reflection on fertility on well managed breeding farms is pregnancy rate per cycle rather than pregnancy rate at the end of the year. So, a stallion that's got a 60% fertility rate per cycle may end up with the same number of mares pregnant at the end of the breeding season as a stallion that has an 80 % pregnancy rate per cycle, but clearly there's a big difference in the fertility of the two stallions and the number of services required to produce a pregnancy.

Q: If you had to make a list of what you regard as the major problems in reproduction and fertility, what would your list be?
A: I would say the level of management and the education of the managers on breeding farms is probably the most important problem that we face. On well managed breeding farms that have got good farm personnel and good veterinary liaison, most mares will go in foal. On the other side of the coin there are farms where the veterinary input is minimal, sometimes there is no early pregnancy scan, sometimes only a final pregnancy test and the owners themselves decide when to breed the mares and how to treat mares. In these cases the overall fertility on the farm is low.

Q: So in the breeding industry, information transfer must be important. The information is out there but is getting it to the industry the critical thing?
A: Exactly and a great example of this is the new drug OvuplanÒ I've been trying for years to enthuse people to use these types of drugs to ovulate mares to reduce the number of times that a mare is bred per cycle. On most farms mares are being rebred a number of times in the same cycle and that is a waste of manpower and stallion semen. It decreases the stallion's interest in breeding and increases infection. In addition, the mares time spent on the stud is increased, handling of the mare and foal is increased, and there are more problems associated with foal diseases and injuries. The information and techniques are there, we've written about it, we've talked about it but it takes a new drug to produce a change in what people think and do.

Q: What about the foal?
A: The foal is often forgotten and many breeding farms are too interested in getting mares in foal rather than looking after the foal that's on the ground. I think this foal is more important than the one we're trying to create and so issues of preventing and treating infection are really critical.

"Reprinted from the RIRDC Equine Research News with the permission of the Rural Industries Research and Development Corporation".