Light Effects On Estrous Cycle In Horses Explained

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Last Updated on June 1, 2022

Interested in breeding from your mare? Let’s find out all about breeding horses, including the light effects on estrous cycle in horses!

What Is The Estrous Cycle In Horses?

Horses, like many other mammals, have an estrous cycle. This is a series of hormonal and physical changes that occur within the body over a set period of time, in order to allow the horse to breed. The average length of an estrous cycle in the horse is 22 days.

What Is The Estrous Cycle In Horses

The estrous cycle of horses is divided into two stages:

  • Diestrus

The diestrus part of the estrous cycle of the mare lasts for 15-19 days. During this part of the cycle the mare will not be receptive to the stallion, as she is not fertile.

  • Estrus – Light Effects On Estrous Cycle In Horses

This part of the cycle is when the mare is fertile. Changes occur within the reproductive tract, and the mare will ovulate. Ovulation means that she has produced an egg, ready for fertilization if mating occurs.

Estrus lasts for 3-7 days, and is often referred to as the mare being ‘in heat’ or ‘in season’. If the mare is not pregnant at the end of estrus, she returns to diestrus and the cycle starts again.

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Light Effects On Estrous Cycle In Horses Explained

Horses are seasonal breeders – they do not breed all year round, but instead have a mating season that lasts for several months. This is similar to many large herbivores, who time their pregnancy and birthing period to take advantage of the optimum weather conditions.

The gestation period of a horse is around 11 months, and in the wild, a mare will normally give birth during the spring. This means she must have mated by late spring, to give birth just under a year later. Being born in the spring gives the foal the best chance of survival, as the weather is warmer and food for the mare is more abundant.

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Light Effects On Estrous Cycle In Horses

Seasonally Polyestrous Or Anestrous – Light Effects On Estrous Cycle In Horses

The annual breeding cycle of a horse is referred to as ‘seasonally polyestrous’, meaning that she only cycles during the reproductive season. During the rest of the time, the mare goes through a period called anestrous, where no reproductive cycles happen at all.

Depending on the location, most mares finish anestrous in early spring and start to come into season. These may follow an irregular pattern at first, but will soon settle into a regular cycle. But how does a mare know when the time is right to start becoming reproductively receptive? Well, she has no physical control over this process, but the changes are initiated by light! It is the lengthening of the day in the spring that triggers hormonal changes within the body, kick starting the reproductive period. Then, if she is pregnant by the time fall comes around, the shortening days will trigger the start of anestrous, so she does not cycle during the winter months.

When a mare becomes pregnant, the reproductive cycle also stops, and hormonal changes occur that alter the physiology of the reproductive system. This is termed the gestational period, and the mare will not cycle again until after the foal is born.

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Why Are Lights Used To Alter The Estrous Cycle Of Horses?

In domesticated horses, some breeders use artificial lights to alter the breeding cycle of horses. By gradually extending the length of the day, the hormonal system of the mare can be tricked into thinking it is spring, kick-starting the breeding cycle earlier than normal. This is normally started in the middle of winter and will enable the mare to start cycling by early spring.

The reason for doing this is that it brings forward the earliest time a mare can be mated, and also extends the breeding season. When a mare is mated early in the spring, her foal will be born earlier the next year. In some circumstances, this can be a huge advantage, although early foals do need more care to nurture through the colder months.

Putting a mare under lights also reduces the tricky period where she is cycling irregularly. This can be a problem in maiden mares, making it difficult to assess when they are ready to be mated.

Summary – Light Effects On Estrous Cycle In Horses

So, as we have learned, light effects on estrous cycle in horses triggers the mare to begin her reproductive cycle in the spring. The lengthening days stimulate the hormonal system to start producing reproductive hormones, that cause physical and behavioral changes in the mare. In domesticated horses, artificial lights are sometimes used to stimulate a mare to start cycling earlier in the breeding season.

We’d love to hear your thoughts on the light effects on estrous cycle in horses! Do you have a mare that comes into season at odd times of the year? Or perhaps you’ve got some questions about the best time to breed your mare? Leave a comment below and we’ll get back to you!


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