Horse Breeding Season [When Is It & How Long Does It Last]

When is horse breeding season? How long do horses mate? How often do horses go into heat?

This is a common question that I’ve been asked a lot. Spending my entire life around horses and doing my own research into the topic, I discovered that the breeding process is very well thought out.

I put together this guide to give arm you all the information you need to get started.

Ready? Let’s get started!

Key Takeaways

• Horse breeding season runs from April to September

• The effects of exercise are important for a foal's development

• horse breeders can artificially manipulate when a mare is in heat

• There is an average of 345 days of gestation in both domestic and wild horses

• A newborn foal born in winter is often smaller than those born in spring

• Horse breeders prefer foals born earlier in the year

• It is extremely rare to see a winter foal amongst feral horses

When Is Horse Breeding Season

Horses, whether domestic or wild, are not year-round breeders. In fact, they only breed for certain months of the year.

In a completely natural environment, horses breed from mid-April until the end of September. However, horse breeding season peaks around mid-June.

This is because mares do not come into heat year-round, which I’ll explain in more detail in a minute. The reason mares do not cycle during the winter is to avoid foals being born when the weather is bad, and there is a scarcity of food.

In a domestic environment, it is possible to change this, so foals are born early in the year. This is done for specific reasons, which I’ll get to below.

But now let’s take a closer look at the reproductive cycle of mares.

CHECK: Do Wild Horses Mate for Life?

When Do Mares Come Into Heat

There are two main periods of a mare’s cycle, estrous and anestrous. Anestrous occurs in the winter and is when there is little to no ovulation occurring. This is the non-breeding season.

In early spring, around March, female horses will transition from anestrous to estrous. This is triggered by an increase in daylight hours. It is nature’s way of telling the horse that better weather and food are approaching.

During this transition period, mares will begin to show signs of heat but will have irregular cycles and may not ovulate. Pregnancy rates are still low during this time.

By mid-April, the mare’s reproductive activity should be fully up and running with regular ovulation. This is when breeding usually starts.

In wild horses or feral horses with no human intervention, this is when breeding season begins.

When a healthy mare is fully in estrous, she will ovulate every 3 weeks, “but they are in heat and receptive to a stallion for only 2 to 8 days.”

She will continue to cycle until early fall, when the reproductive system begins to transition back into anestrous. Female horses reach a breeding peak around mid-June when you are most likely to get a pregnant mare and have higher conception rates.

That means that these mares foal at an ideal time in May when the grass is plentiful and the weather is mild.

Once pregnant, there are 11 months of gestation.

Female horses show certain signs they are in heat. Learn more about it here:

https://www.youtube.com/watch?v=7cAxGqE84Jk

This might have you asking the question - if horse gestation is 11 months and you see that some foals are born in January or February, how can a mare get pregnant is she’s not ovulating?

A great question that is down to science and a modern understanding of how the horse’s reproductive system works. Let’s take a look at how it's done.

Check more horse mating facts.

Can You Alter a Mare's Heat Cycle?

Yes, you can alter a mare’s heat cycle by using artificial light. Since the increase in daylight hours is what triggers a mare to go into estrous, research found that using artificial light can have the same effect.

Doing this requires careful management and using the correct type of light. Mares must come inside before it gets dark and be stabled under lights.

“Research suggests that extending the day length by adding light starting in the late afternoon is better than turning the lights on earlier in the morning and shortening the night length.”

This process should start 2 to 2.5 months before you want the mare to start fully cycling with the aim of exposing the mare to light for 12-16 hours a day.

You also need to use the right type of light. Ontario’s Ministry of Agriculture, Food and Rural Affairs states that:

“A 200 watt incandescent or two 40 watt fluorescent bulbs will generally give adequate illumination in a box stall, if placed within 7-8 feet of the mare.”

There are also special hoods, such as the Equilume eye mask. This mask has a blue light next to one eye. The light is timed, so it is on for the necessary hours.

By triggering full estrous earlier in the year, it means that you can start breeding season sooner, which results in foals being born as early as January or February.

This is pretty cool, and there are good reasons for it, but you don’t want to do this unless you have facilities that can cater to young foals and nursing mares during the winter. If you don’t have facilities, then spring is the best time for foals.

Check out what the Equilume mask, ever more popular with mare owners, looks like.

https://www.youtube.com/watch?v=bR8_EBW0xig

Why Late Spring Is The Best Time For a Foal To Be Born?

To develop healthy joints and growth, the best time of year for a foal to be born is late spring. At this time of year, the weather is usually good enough for mares and foals to spend many hours outside each day.

Foals need to move, which helps them grow well. Spring also means grass growth. Grazing on good grass helps mares produce high-quality milk for their nursing babies.

If a foal is born too early, it can miss out on these important aspects, which can affect its development. Alternatively, a foal born in late summer or fall also misses out on the most nutritious grass and outside time.

If this is so important, and it is, why do horse breeders mess with a mare’s cycle to get early foals? Particularly thoroughbreds?

There is a good reason horse farms with the right management practices in place to deal with a newborn foal do this. Let’s take a look.

Gain insight into the world of horse care and find out why horses have eye covers with our informative article on 'Why Do Horses Have Eye Covers.

Why Are Many Thoroughbreds Born In Winter?

All thoroughbreds and sports horses have the same official birthday. So on January 1, they turn a year older even if they were actually born in June.

Since thoroughbreds and sport horses compete by age when they are young, being born earlier in the year gives them a few extra months of physical development. This is why it has become popular to artificially trigger cycling to breed mares. 

For example, a 3-year-old horse born in February will be a more physically mature horse than one born in late June. This is because horses grow a lot in their first couple of years.

This is particularly applicable in horse racing when horses start to race at the age of 2. A January 2-year-old will be in stronger physical condition than one born in early summer.

Once they are a bit older, say around 7, when they were born, doesn’t have any particular benefits. However, a racehorse born in January is much closer to a real age of 3 than one born in June, which is really 2.5 years old.

Those 5 or 6 months make a huge difference to their strength and maturity.

A study did find that foals born during winter months are smaller than spring foals at birth. But they do, however, catch up in size after about 12 weeks and still end up with the other benefits I just discussed.

When Are Mares Rebred

The period of time a horse is pregnant is 11 months, with an average of 345 days of gestation. However, heavy horses have a slightly shorter average gestation period.

This means that a mare can produce just one foal per year. It is not harmful to the mare to carry a foal every year in most cases. But some mares do need years off.

Many mares are rebred shortly after the birth of their foal. This is called ‘foal heat’. This short period of time is done so that the next foal can be born at the desired time of year.

Foal heat is the mare's first full cycle and ovulation after she gives birth. This happens 7 to 12 days after birth. The disadvantage to rebreeding a mare at this time is lower conception rates.

If the horse doesn’t fall pregnant, mare owners might decide to wait until the following year so that the foal is born at the desired time. This is because the long gestation period can mean a foal is born too late in the year.

FAQs

How many times a year do mares go into heat?

Mares will go into heat 6 or 7 times a year.

Can you breed a 30-year-old mare?

No, you should never breed a 30-year-old mare, even if she is still cycling. Carrying and nursing a foal can be hard on a mare, and the older they are, the higher the risk of serious complications. Even breeding a 20-year-old mare is far too risky, especially if she has never had a foal before.

How many times can a stallion mate in one day?

A stallion can mate as frequently as two or three times a day if he is healthy and mature.

Conclusion

I hope this guide has answered the question ‘when is horse breeding season?’ As you can see, breeding season is not year-round and occurs when mares are ovulating.

Even with artificial manipulation, there are still a few months of the year when a mare won’t get pregnant.

Do you have any questions? If so, leave them in the comments.

Resources

1. Mare Seasonality – Extension Horses . horses.extension.org. Available from: https://horses.extension.org/mare-seasonality/
2. Breeding and Reproduction of Horses - Horse Owners . Veterinary Manual. Available from: https://www.msdvetmanual.com/horse-owners/routine-care-and-breeding-of-horses/breeding-and-reproduction-of-horses
3. Artificial Lighting for Mares . omafra.gov.on.ca. . Available from: http://omafra.gov.on.ca/english/livestock/horses/facts/lighting.htm#:~:text=A%20200%20watt%20incandescent%20or
4. Seasonal effects: “Winter foals” are smaller than foals born in summer . ScienceDaily. Available from: https://www.sciencedaily.com/releases/2017/08/170804091347.htm
5. Horse Breeding on Foal Heat - AQHA . www.aqha.com. Available from: https://www.aqha.com/-/horse-breeding-on-foal-heat