Managing The Seasonality Of The Horse

Running head: MANAGING THE SEASONALITY OF THE HORSE 1

Managing The Seasonality of the Horse

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MANAGING THE SEASONALITY OF THE HORSE 2

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Introduction ......................................................................................................................................3

The endogenous circannual rhythm of reproduction .......................................................................3

External factors entraining the annual reproductive rhythm ............................................................4

a. Photoperiod ....................................................................................................................4

b. Temperature ...................................................................................................................5

c. Nutrition and body condition Temperature ....................................................................5

Neuroendocrine control of seasonal reproduction ...........................................................................6

Conclusion .......................................................................................................................................7

References ........................................................................................................................................8

MANAGING THE SEASONALITY OF THE HORSE 3

Managing The Seasonality of the Horse

Introduction

The horse is one of the species that have seasonability in their reproduction cycle. It

experiences its breeding season in spring which is influenced by factors such as increased daylight,

temperature and adequate food. The reproductive activity takes place between April and

September around the Northern Hemisphere. Therefore, many foals are born on January 1 and the

official breeding season occurs between February and June. This results in a similar age for all the

foals born in a particular season. Horse breeders end up experiencing a lot of economic pressure

in breeding the mares. They have to breed them early enough within the year in order to gain a

benefit on the age over the ones given birth to as the year ends. Foals born early enough have a

better performance than those born later (Nagy, Guillaume & Daels, 2000).

However, research has proven that the seasonality of the horse can be adjusted by coming

up with various induction methods that make the breeding season in horses take place early

enough. These methods include comprehending the endogenous circannual rhythm of

reproduction, the external factors entraining the annual reproductive rhythm, and using the

neuroendocrine control of seasonal reproduction method (Silva, 2016).

The endogenous circannual rhythm of reproduction

Seasonality of reproduction in horses is sometimes caused by the endogenous circannual

rhythm of reproduction which is caused by various environmental factors such as temperature,

photoperiod, body condition and nutrition. Circannual reproduction cycle has been found to exist

in horses. Mares which have been kept in constant long day photoperiod starting at the summer

solstice get into anestrus. Likewise, mares which have been kept under short day photoperiod

MANAGING THE SEASONALITY OF THE HORSE 4

starting at the winter solstice get back to the cyclic ovarian activity (Wilson, 2010). Although doing

away with the pineal gland never removes the seasonal reproductivity in horses, their reproductive

cycle is not affected by photoperiodic alterations anymore.

External factors entraining the annual reproductive rhythm

The annual reproductive rhythm in horses is affected by various factors such as

photoperiod, temperature, nutrition and body condition.

a. Photoperiod

Photoperiod of horses is believed to be a very crucial environmental factor which affects

the endogenous circannual reproductive cycle. In winter and the beginning of spring, extra light is

exposed to horses. This exposure of light stimulates the activity of the ovary, especially in

anestrous mares. This, therefore, brings an advancement to the onset of their breeding season.

However, the ovarian activity is not always as a result of long days’ exposure (Wilson, 2010). The

stimulatory photoperiod depends on various factors such as the condition of refractoriness to

changes in photoperiod, the history of photoperiod and the availability of photosensitive phase in

the dark.

Refractoriness is a condition whereby mares maintained in constant lighting conditions still

get back to their circannual cycle regardless of their lighting conditions meant to stimulate or

inhibit the process. Refractoriness makes the mares unable to respond to any current photoperiod.

Therefore, mares maintained under 16L:8D at the beginning of winter or summer still go back to

summer anestrous. The way photorefractoriness develops can be extended and reduced by altering

photoperiods at various definite levels. For mares, the photorefractoriness is still preventable

through reducing any more light exposed to them every day (Silva, 2016).

MANAGING THE SEASONALITY OF THE HORSE 5

The ovarian activity is usually induced in anestrous mares which are seasonal through by

using the 1- to 2- h light flash not more than 10 hours after darkness begins. This implies that just

like the other species which are seasonal, mares also possess a phase which is photosensitive even

when there is darkness. Some photoperiod conditions occur naturally. During such conditions, the

alteration made in the light and dark phase every day is very gradual. Darkness intensity also

experiences a considerable change hence gradually altering each phase’s length (Kirkpatrick &

Turner, 2010).

b. Temperature

The seasonality in the reproductive activity in horses can also be adjusted through

temperature. The activities in reproduction are significantly affected by the range in environmental

temperatures, that is, the maximum and minimum temperatures. Cold weather slows down the

transition of the spring in a couple of years. Therefore, apart from the photoperiodic activities,

nutrition and the management cycle, temperature contributes a lot towards the adjustment of the

seasonality of mares (Aurich, 2011). It provides a very good timing in the endogenous circannual

rhythm of reproduction.

c. Nutrition and body condition

Mares which take supplementary diet such as concentrates tend to experience earlier

ovulation just after winter anestrous that those control mares which do not take any supplements.

It has also been observed that mares which add on weight at the beginning of spring tend to have

a short period of anovulation (Kirkpatrick & Turner, 2010). Taking in high levels of energy makes

the first interval of ovulation shorter in transitional mares whose body fat level is low. However,

moderate or fat bodied mares do not benefit from it. Poor body condition influences how long each

MANAGING THE SEASONALITY OF THE HORSE 6

phase of transition takes and the first spontaneous ovulation. It also reduces the administration

efficiency of equine pituitary extracted to bring about induction for ovulation to occur.

Apart from the intake of energy, the onset of the breeding system is also affected by the

amount of proteins in the diet. Mares which take in a diet with proteins of high quality tend to

secrete high amounts of Follicle Stimulating Hormone (FSH). This hormone in return makes them

ovulate about three to six weeks before those mares that take in the diet with protein of low quality

(Aurich, 2011). Grazing mares on pastures just before they receive their first ovulation also reduces

the time they take before the ovulation occurs. Anestrous mares fed on green pastures are likely to

receive their faster ovulation earlier than mares pastured on hay. In general, nutrition and body

condition play a very great role in the onset of the breeding season.

Neuroendocrine control of seasonal reproduction

Melatonin plays a very important role in controlling seasonality in reproduction in horses.

In mares, the secretion of melatonin is higher at the start of the dark phase. Its secretion is

stimulated by norepinephrine. Melatonin has higher concentration during winter and autumn as

compared to summer and spring. Daily and long term administration of melatonin affect the

seasonal reproduction I horses. Implanting melatonin at the beginning of summer advances the

season of ovulation in adult mares (Kirkpatrick & Turner, 2010).

Neurotransmitters also play a very important role in adjusting the reproductive activities of

horses. This is achieved by stimulating the secretion of GnRH which is facilitated by several

neuronal systems found in the hypothalamus. In mares, this has been proved by studying the effects

of agonists and antagonists on the secretion of gonadotrophin. They are applied for long periods

in order to determine the ovarian activity and the time taken for the first ovulation to occur. More

MANAGING THE SEASONALITY OF THE HORSE 7

research conducted suggests that seasonality in horses can be managed by also understanding the

role of opioids and thyroid hormones in the reproductive system of the horses (Aurich, 2011).

Conclusion

Understanding the seasonal reproduction in mares is very important. After that, one can be

able to apply some of the above-mentioned methods in order to adjust their reproductive activities

as desired. Strategies about better treatment can also be administered in the induction of the ovarian

activity early enough.

Links to the articles:

https://pdfs.semanticscholar.org/5e6f/5bcfea39dc46e86b642a83f6ef473ccda37b.pdf

https://pdfs.semanticscholar.org/f2c4/4b44f0c8dfd24c4ea4f4475bc9a016e618e0.pdf

https://academic.oup.com/jas/article/80/11/2949/4789440

MANAGING THE SEASONALITY OF THE HORSE 8

References

Aurich, C. (2011). Reproductive cycles of horses. Animal Reproduction Science, 124(3-4), 220-

228. doi:10.1016/j.anireprosci.2011.02.005

Kirkpatrick, J. F., & Turner, A. (2010). Absence of Effects from Immunocontraception on

Seasonal Birth Patterns and Foal Survival Among Barrier Island Wild Horses. Journal of

Applied Animal Welfare Science, 6(4), 301-308. doi:10.1207/s15327604jaws0604_4

Nagy, P., Guillaume, D., & Daels, P. (2000). Seasonality in mares. Animal Reproduction

Science, 60-61, 245-262. doi:10.1016/s0378-4320(00)00133-0

Silva, M. A. (2016). Advances in Diagnostic and Therapeutic Techniques in Equine

Reproduction, An Issue of Veterinary Clinics of North America: Equine Practice, St.

Louis, MO: Elsevier Health Sciences.

Wilson, D. (2010). Clinical Veterinary Advisor - E-Book: The Horse. St. Louis, MO: Elsevier

Health Sciences.

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