Six weeks into the covering season, broodmare owners across the country are leaning over the crush rail, eyes fixed on the ultrasound screen, willing that faint shadow to appear. They’re hoping for a 15-day embryo - the tiny beginning of a viable foal - and the first sign that months of planning are paying off.
A colt earns his place at stud on the track, but it’s in the shed that he must prove himself as a stallion. As Dr Allan Gunn, Associate Professor in Large Animal Theriogenology and Production Animal Medicine at Charles Sturt University, puts it; “The ultimate test of a stallion’s fertility is getting mares in foal.”
To struggle with this can be put a stallion’s career at risk, but the likes of Extreme Choice - who boasts a strike rate of 11.9% stakes winners to runners, and an unbelievable 4.44% Group 1 winners to runners - are proof that sub-fertility doesn’t mean failure. But it does require management.
So what happens when a stallion’s numbers start to slip?
Extreme Choice | Standing at Newgate Farm
The balancing act of stallion fertility
“Stallion fertility is multifactorial,” says Arrowfield Stud’s Head Veterinarian Dr Wendy Perriam. “It’s influenced by genetics, environment, behaviour, management — and just like mares, it can fluctuate through the season.”
Dr Wendy Perriam | Image courtesy of Arrowfield Stud
There’s no single cause for sub-fertility. Age, injury, stress, disease, even heat can disrupt reproductive function. Over fifteen, many stallions experience natural testicular decline, but plenty stay potent well beyond that. The key is knowing when, and how, to intervene.
“The job is far more physically demanding than people think, especially for a stallion with a reasonable book size,” Dr Perriam says. “They specifically need to be sound in the hind limbs and in their back.”
Routine breeding soundness exams are the starting point: palpation, ultrasound, and culture tests to detect early problems. “We look at the testes’ structure, blood flow, and any asymmetry,” she explains. “That can be the first sign something’s changing.”
When metrics fall below
“Something most studs traditionally do every year is collect each stallion’s semen and examine it for sperm numbers, sperm motility, and sperm morphology,” says Dr Perriam.
Motility - the sperm’s movement, and morphology - its shape, are major fertility indicators. Abnormal heads, coiled tails or sluggish swimmers can all undermine conception rates.
“In covering sheds, dismount samples are routinely checked to make sure the stallion has ejaculated normally,” adds Dr Gunn. “Then they look at dismount samples to check that the spermatozoa has normal motility and normal morphology.”
This means advanced lab work. The University of Newcastle runs cutting-edge tests on factors now recognised as critical.
“There's a reasonable percent of stallions that will have these parameters all fall within the normal range and they still can't achieve an acceptable fertility rate,” Dr Perriam said. “That's where these more advanced tests really can give you a little bit more information.”
“Sophisticated tests that can, for example, look at DNA stability and compaction, which is a very important part of the fertilising ability of the sperm and its ability to produce a viable embryo.”
“By working out his daily sperm output and his testicular efficiency, combined with his libido, we can work out what his book size can be,” she said.
Allan Gunn | Image courtesy of Charles Sturt University
In combination with examining the sperm themselves, it is important to look at the stallion’s covering records to establish if there was a particular event that could have caused a drop in performance.
“It takes 12.2 days for a cycle of semen (to be developed) plus four and a half weeks’ epididymal time before, if you have a problem with the testes, the sperm that has been affected will be ejaculated,” said Dr Gunn. “So if you are able to fix the problem with the testes, there will still be around 55 days (lag time) until the ejaculated sperm will be normal.”
Fixing any problem
Once a fertility issue is identified, the focus shifts from diagnosis to management.
“It’s about working out why,” Dr Perriam says. “Nutrition, workload, collection frequency... everything plays a part.”
Diet is one of the biggest levers. Omega-3 fatty acids like DHA have been shown to support sperm health, as have antioxidants and L-carnitine, which aids energy metabolism and sperm longevity. “We need to look very, very carefully at their nutrition,” she notes.
Behavioural management also matters. Young stallions with early negative experiences can develop long-term aversions or performance anxiety in the shed. Skilled handlers are critical: “They’re the ones who notice changes in behaviour, libido, or output,” Dr Perriam adds. “The relationship between vet and handler is vital - they see what we don’t.”
By calculating daily sperm output, testicular efficiency, and libido, studs can tailor how often a stallion covers - and how large a book he can safely manage.
But even the world’s most fertile stallion can’t overcome poor timing.
“The mare’s management is half the equation,” Dr Perriam says. “We use ultrasonography and ovulation-inducing drugs to make sure she’s covered at the optimal time - ideally within 48 hours of ovulation.”
One cover, perfectly timed, is usually all it takes. Rosemont Stud’s handling of Starspangledbanner is the textbook example: by waiting until mares were ready to ovulate before allowing him to serve, his fertility rate jumped from 37% to over 50%.
Starspangledbanner | Standing at Coolmore, Ireland
“Generally speaking, we manage the mare by frequent ultrasonography to confirm when that mare has a pre-ovulatory follicle,” Dr Perriam said. “We also use ovulatory induction drugs to ensure that the mare ovulates within a timely fashion. What we are aiming for is one well-timed cover for the stallion, and that goes for any stallion, but particularly useful if he is limited by how many covers he can do each day.”
“What we are aiming for is one well-timed cover for the stallion.” - Dr Wendy Perriam
After covering, Dr Perriam explained that it is also normal for the ejaculate to cause an inflammatory response in the uterus, which can be lavaged with a sterile solution to help the mare’s body recover and return to being an optimum environment for an embryo to settle.
“What we have achieved over the course of time is that we use far fewer antibiotics during reproduction, instead we use far more lavage and other non-antibiotic therapies to help the mare with inflammation,” Dr Perriam said.
“I think that’s been a big improvement in the last 10 years. It's extremely important that we only use antibiotics in a targeted fashion and when we know that alternatives are not going to be helpful.”
The opportunity to serve
“When we talk about overall industry fertility, we need to be very careful with what statistics we use that will be most representative of what’s going on,” Dr Perriam said. “One of the better measures of true fertility is what we call pregnancy rate per cycle - how many mares conceive when they're bred by the stallion on any individual cycle.”
There is no set number for this, but Dr Perriam indicated she typically liked to see a 60 to 65% pregnancy rate per cycle from a stallion with good fertility. Another metric often used by studs is seasonal fertility - how many mares are in foal at the end of the covering season - but Dr Perriam considers this measure to not be wholly representative of actual fertility.
“One of the better measures of true fertility is what we call pregnancy rate per cycle - how many mares conceive when they're bred by the stallion on any individual cycle.” - Dr Wendy Perriam
“The number of mares in foal at the end of the season can be severely impacted by the opportunity that mare gets to actually be bred to the stallion,” she said.
“In the Southern Hemisphere, we have quite an abbreviated season (compared to the Northern Hemisphere) [3.5 months vs 5 months], and that can really impact seasonal fertility statistics. Because our season is short, some mares will not get enough opportunity to be presented to the stallion for more than one or two cycles.”
Work by the late Dr Dave Hanlon in Waikato in the 2000s indicated that variations in end-of-season pregnancy rate reflected on the mare in 92.5% of cases, as opposed to the stallion or stud farm management.
“David Hanlon determined that mare factors were one of the biggest influences on fertility, so we need to look at maximising her contribution to the fertility equation,” Dr Perriam said.
The impact of heat
Perhaps the most underestimated fertility disruptor is heat.
“Researchers have found that, in Australia, there tends to be decreases in fertility rates in November and December, correlated with increases in diurnal temperatures,” says Dr Allan Gunn.
He explains that high temperatures can interfere with the cooling of the testes, affecting sperm production and quality.
University of Newcastle’s Dr Aleona Swegen has been leading research into this phenomenon, funded by AgriFutures Australia. Her findings show it’s not just daytime heat that matters - it’s the lack of a cool period overnight.
“Specifically, it was the lack of a cool respite period during the night that was most detrimental (to fertility) – hot days were okay as long as the stallions could cool down at night. But what we also noticed was that horses stabled at night were actually exposed to higher temperatures than they would have been had they been outside.
“We’re also looking into the mechanisms and the timing of the effects of heat stress. The concern is that not only does fertility drop, but we see effects on the DNA that the sperm are carrying, which could be leading to pregnancy losses or compromised performance or health in the offspring.”
The practical fixes are simple but often overlooked: fans, misters, better airflow, and - counterintuitively - letting stallions spend nights outside rather than in heat-trapping stables.
“Some stallions are more resilient to ambient heat than others,” says Dr Swegen. “Recognising that even temperatures we wouldn’t typically consider a ‘heat wave’ can be detrimental in a select population of horses is quite helpful.”
The causes of subfertility are various and complex, but mare owners should be encouraged with the continuing research in the area and the high standard of care delivered on stallion farms across the country that seeks to maximise a stallion’s capacity to conceive.
And you never know where your next champion might come from. But you do have to conceive it first.
