The Stallion: The Forgotten Part Of The Equation
An interview with Angus (back in 1995) Caitlin and "Beau" at GVEH Feb 2013
Q: What contribution does the stallion have to fertility ?
A: The stallion is forgotten in the fertility equation despite being responsible for half of the probability outcome in each breeding. If a group of matings results in an outcome of 80 % of mares becoming pregnant per cycle then the mare and stallion fertility contributions are most likely 0.9 and 0.9. When the fertility of stallion drops then the outcome is affected. If his fertility is halved then only 40% of mares will go in foal per cycle. However because people have the mistaken belief that there is nothing we do to influence stallion fertility, they ignore his contribution. It is disappointing to recognise that within Australia, only a few people are adequately trained to evaluate stallion fertility parameters. Perhaps that is another reason why the stallion is too often forgotten.
Q: What makes one stallion more fertile than another one?
A: Management has a huge impact on fertility. Well managed farms have good pregnancy and foaling rates. However, when pregnancy rates per cycle on the same farm are compared between stallions quite often there are major differences in the figures. In those cases, providing the percentages of mares in the barren, maiden and foaling groups are similar, the stallion is demonstrating his "inherent" fertility. That is something he is born with. I am sure many of us have seen the stallion that just seems to have to sniff his mares and they go pregnant. Compare him with the stallion that takes two, three or even four cycles to get mares pregnant. They can end up with the same number or percentage of mares pregnant at the end of the breeding season, but it is allot more work with the less fertile stallion and his progeny may have an average foaling date that is later than acceptable for commercial foal sales. Nobody knows what the real differences in fertility are caused by. However, quite frequently we see a difference in the characteristics of progressive motility and good tense sperm producing testicles between the stallions. Despite this, there are sometimes are no obvious differences and the only explanation is that one stallion is able to produce more fertile sperm than another.
Q: How can you tell a stallions real fertility?
A: The farm has to be well managed. Then the only accurate test is the pregnancy rate per cycle.
Q: What if both stallions have 90% of mares in foal at the end of the breeding season? That sounds quite good?
A: If one stallion gets mares in foal at 75 % per cycle then after two cycles for each mare (eliminating mares pregnant), then after two cycles 93% of mares are pregnant. If you have another stallion that has a pregnancy rate per cycle of only 40%, then he can achieve a 93% pregnancy rate after 5 cycles. The difference is huge in relation to agistment and veterinary costs, not to mention lost opportunity costs because the foals are born so much later.
Q: So to summarise what you have just said, it is the pregnancy rate per cycle that truly reflects a stallions fertility and the number of mares pregnant at the end of the breeding season maybe an inaccurate estimate ?
A: Yes, that is correct. When you see the fertility reported as the number of mares pregnant at the end of the season, we would need to know how many cycles were necessary to obtain that result. Each year we see stallions with low numbers of mares booked get the majority of them in foal, but what people are not told is that the stallion may need to keep breeding late into the season to achieve this. One stallion I had lots to do with had history of only getting twenty mares in foal for his best season. When we started working with him we realised that no matter how many mares we bred (within reason) he was always going to get about 20% in foal per cycle. We ended up with 48 mares in foal out of 60 bred (80%). But can you work out how long we had to breed to achieve this result ? Seven cycles for some mares, almost 5 months.
Q: Why is it some stallions seem to get more twins than others ?
A: Mares that conceive twins do so only from multiple follicles. The twinning rate varies by breed because some mares have more follicles ovulate per cycle i.e. Thoroughbreds have as many as 15% of pregnancies start out as twins and ponies will have almost zero. Within a breed the number of mares ovulating two or more follicles per cycle is relatively consistent, so the more fertile the stallion the more twins we will see per cycle. Early in the breeding season with a new stallion we really like to see some twins because it tells us we have good fertile stallion.
Q: When is a stallion his most fertile ?
A: In his early years, i.e. 4-7 YO. Sperm numbers may increase until the stallion is around 13 -14 years old, however the ability of the sperm to get mares pregnant declines with time due to effects of cumulative insults, effects of aging and at present unrecognised factors.
Q: Does that mean a stallion of 15 needs to have his book of mares reduced.
A: No, not necessarily. Some stallions tend to have good quality sperm and good numbers well into their late teens. There is evidence within a breed that some sire lines have longer breeding capability. It is reasonably well known that the Northern Dancer line of stallions start to slow down in their late teens whereas the Raise a Native line tend to breed on longer. Of course their are exceptions to this trend.
Q: How do you evaluate the fertility of stallions ?
A: Fertility evaluations involve analysis of breeding records, semen collection, external genital and maybe a physical examination and occasionally an internal examination. There are many places were the production or delivery of sperm may go wrong. The important thing is to recognise when to call for an examination. When I was working at Colorado State University, we (Bill Pickett and Jim Voss) would go to Kentucky at the beginning and end of each breeding season and examine semen from as many stallions as they were requested to. They were able to recognise problems developing and make adjustments to the numbers of mares or on occasion treat the problems. On farms that do not perform a pre and or post season analysis of semen, the first signs of problems are usually 5 or 6 mares coming back in season or being checked negative with ultrasound at 14 days. Semen analysis will often show where the problem is occurring. On some occasions it may be the delivery of semen and the stallion is not ejaculating well. It is difficult to tell if a stallion has completely ejaculated in some instances. I remember one stallion that "cheated" all the time, it was a real hit and miss thing if mares went into foal or not. When he bred he would have fluid like ventral penis pulsations and would flag (pump his tail up and down), he would relax on the mare for a few seconds afterwards and would have gel discharged from his penis as he dismounted. Despite all these normal signs of ejaculation he still either partially or incompletely ejaculated. It took collection of semen with an artificial vagina to work this out. When we started to ultrasound his mares after breeding we found we could determine when he had ejaculated properly by looking for fluid in the uterus. From then on we monitored the mares after breeding, the mares started to go into foal again and everyone was happy.
Q: Can we go back to the tests again and describe some of the things you perform with semen?
A: The volume of the ejaculate multiplied by the concentration gives us the total sperm. Volume is usually between 40 and 200 ml, concentration is usually in millions and the total sperm is reported as billions. We know it takes ~500X106 progressively motile sperm to achieve maximum pregnancy rates. So after we have calculated the amount of sperm and the progressively motility (those sperm swimming forward properly) we can see if there is enough sperm in the ejaculate to breed a mare or in an AI program just how many mares can be bred. Commonly we see about 5 X 109 in the second of two ejaculates collected an hour apart. If 50% were motile the ejaculate could be used to breed as many as 5 mares with artificial insemination. Many Thoroughbred breeders turn off when AI is mentioned, however it may be a suitable place for timely reminder that it was AI that has been used to develop more recent research models and results of this work has been of a huge benefit to the Thoroughbred industry, particularly in the area of endometritis and sperm transport etc. While on the subject of AI and Thoroughbred’s, I have to say that it is frustrating and annoying to be restricted by breed association rules that are archaic and fundamentally short sighted. The Standardbred industry certainly has led the field by adopting AI and legislating for it’s use. Perhaps the recent push from within the Stud Book of Dr John Digby will be treated with the respect that it deserves. I know that in 100 years people will look back and laugh at our indecision to adopt and legislate for this.
Q: Back to the tests ?
A; Other tests used are; morphology, which is the anatomy of the sperm: total scrotal width which is a measure of the potential of the testes to produce sperm; pH of the sperm which may indicate incomplete sperm release or infection and bacterial cultures which may identify a stallion that is carrying a bacteria that either may infect mares bred by him or may a recognised venereal pathogen such as the organism (Taylorella equigenitalis) that is the agent causing "contagious equine metritis" (CEM). It is thought that recent relaxation in bacteriological examination of stallions and semen was responsible for the outbreak of CEM detected in Newmarket this breeding season.
Q: AI would be useful in controlling venereal infections wouldn’t it ?
Q: Are there other tests you want to inform us of ?
A: Yes there are lots of other things that can be examined and are mostly used either experimentally or after the other tests have not been able to confirm the problem. Some of the new tests are: penetration of dead female eggs with sperm (can compare the different ability of different stallions); sperm chromatin assay and computer aided motility and swimming pattern analysis studies. Some times we examine sperm with an electron microscope (called either transmission or scanning EM) and on occasions we submit samples for chromosomal analysis.
Q: So, after all these tests can you usually say why a stallion has gone infertile?
A: Almost always.
Q: How about when used as a pre purchase examination to predict fertility of an untried horse?
A: Pre purchase semen evaluations to predict fertility are a little more risky. We can usually say that a horse has good semen parameters that should result in normal fertility, however sometimes a stallion has good looking sperm with normal numbers etc, and can breed mares that either do not go in foal or go in foal only slowly. The only really accurate test of fertility is to breed mares and measure the pregnancy rate per cycle.
Q: Last question. Where do you see an immediate improvement possible in relation to stallion handling ?
A. In management. People believe that the stallion is just a sperm producing machine. They have little thought for the normal horse behaviour. For example some stallions are intimidated by another stallion, they do not have to be in next door stalls or paddocks for this intimidation to occur. In addition stallions like to see mares and not be isolated. A study of free ranging horses showed that the dominant stallion has little or no work in stopping the other males from breeding. However as soon as the dominant stallion has left the vicinity the next stallion in line takes his place, no fuss, no fighting, he just starts breeding. We have seen stallions start to cheat on service and develop bad breeding behaviour simply because they were moved or another stallion was placed closer to them. These signs may be subtle, so it is a caring, thinking approach that we need to engender into our stud grooms. Management comes back to the veterinarian as well. To often we see mares being re-bred on the same cycle. We need to aim for one service only per cycle and we need to consider that some stallions do not have enough libido to breed 3 times daily for the whole season. An interesting study was published in 1982 by Frank Bristol, in Canada. He showed his Belgian stallion could breed as often as 18 times a day (ejaculations). This was considered to be huge news by all of us and every one started saying that it was handling of stallions that contributed to their lack of interest when asked to breed as often as 5 or 6 times a day. Later a study at the GVEH clearly demonstrated that Frank’s stallion was a little unusual. He had been locked up and then turned out with 20 mares in heat. In an identical study at the GVEH, we compared an experienced versus an inexperienced pasture breeding stallion. The experienced breeding stallion spent the first few days rounding the mares up and only bred once or two times a day. The inexperienced stallion, who was younger, bred everything he could for a few days and then settled into a routine of 2 mares a day. When we collected semen from them at the end of the experiment they were still only breeding twice per day but were easily coaxed into three collections of semen only 30 minutes apart. Our conclusion was individual stallions have varying libido and we need to cater for their sex drive. Especially the shuttle stallions who get confused as to which season they really are in.