Editing Horse breeding (section)

== Breeding and gestation ==
[[File:Stallion and mare, Pilley Allotments, New Forest - geograph.org.uk - 401096.jpg|thumb|A stallion [[mating]] with a mare]]
While horses in the wild mate and foal in mid to late spring, in the case of horses domestically bred for competitive purposes, especially [[horse racing]], it is desirable that they be born as close to January 1 in the northern hemisphere or August 1 in the southern hemisphere as possible,<ref>The [[Australian Racing Board]] uses August 1 as its standard cutoff date, but also uses the date of conception to determine age. A foal born on or after July 1 of a given calendar year is included in the birth cohort of that calendar year if his or her dam was covered no later than August 31 of the previous calendar year. See {{cite web|url=http://www.australianracingboard.com.au/uploadimg/Rules260909.pdf |title=Rule AR.46 |work=Australian Rules of Racing |date=2009-09-29 |access-date=2010-08-03}}</ref> so as to be at an advantage in size and maturity when competing against other horses in the same age group. When an early foal is desired, barn managers will put the mare "under lights" by keeping the barn lights on in the winter to simulate a longer day, thus bringing the mare into estrus sooner than she would in nature.  Mares signal estrus and ovulation by urination in the presence of a stallion, raising the tail and revealing the [[vulva]]. A [[Stallion (horse)|stallion]], approaching with a high head, will usually nicker, nip and nudge the mare, as well as [[Flehmen response|sniff her urine]] to determine her readiness for mating.<ref name="Equine Reproduction">{{Cite book |last1=McKinnon |first1=Angus O. |url=https://books.google.com/books?id=QPJQT6-g-YMC |title=Equine Reproduction |last2=Squires |first2=Edward L. |last3=Vaala |first3=Wendy E. |last4=Varner |first4=Dickson D. |date=2011-07-05 |publisher=John Wiley & Sons |isbn=978-0-470-96187-2 |language=en}}</ref><ref>{{Cite book |last1=Riley |first1=Christopher B. |url=https://books.google.com/books?id=bwZxEAAAQBAJ&pg=PA127 |title=Fraser's The Behaviour and Welfare of the Horse |last2=Cregier |first2=Sharon E. |last3=Fraser |first3=Andrew F. |date=2022-05-10 |publisher=CABI |isbn=978-1-78924-210-2 |language=en}}</ref> During [[Copulation (zoology)|copulation]], the stallion usually [[Ejaculation|ejaculates]] after 6 to 8 [[Pelvic thrust|pelvic thrusts]].<ref name="Equine Reproduction"/>

Once fertilized, the [[oocyte]] (egg) remains in the [[oviduct]] for approximately 5.5 more days, and then descends into the [[uterus]]. The initial single cell combination is already dividing and by the time of entry into the uterus, the egg might have already reached the [[blastocyst]] stage.

The [[gestation]] period lasts for about eleven months, or about 340 days (normal average range 320–370 days). During the early days of pregnancy, the [[conceptus]] is mobile, moving about in the uterus until about day 16 when "fixation" occurs. Shortly after fixation, the [[embryo]] proper (so called up to about 35 days) will become visible on trans-rectal [[ultrasound]] (about day 21) and a heartbeat should be visible by about day 23. After the formation of the [[endometrial cup]]s and early placentation is initiated (35–40 days of gestation) the terminology changes, and the embryo is referred to as a [[fetus]]. True implantation – invasion into the endometrium of any sort – does not occur until about day 35 of pregnancy with the formation of the endometrial cups, and true [[placentation]] (formation of the placenta) is not initiated until about day 40-45 and not completed until about 140 days of pregnancy. The fetus's sex can be determined by day 70 of the gestation using ultrasound. Halfway through gestation the fetus is the size of between a [[rabbit]] and a [[beagle]]. The most dramatic fetal development occurs in the last 3 months of pregnancy when 60% of fetal growth occurs.

Colts are carried on average about 4 days longer than fillies.<ref>{{cite journal |last=Hura |first=V |date=October 1997 |title=The effect of some factors on gestation length in nonius breed mares in Slovakia (Egyes tényezõk hatása a nóniusz fajta vemhességének idõtartamára)|journal=Proceedings of Roundtable Conference on Animal Biotechnology |volume=XIII |url=http://www.oai.hu/kerekasztal/XIII./hura.htm |access-date= 2008-04-22 |display-authors=etal}}</ref>

=== Care of the pregnant mare ===
Domestic mares receive specific care and nutrition to ensure that they and their foals are healthy. Mares are given vaccinations against diseases such as the [[Equine herpesvirus 1|Rhinopneumonitis]] (EHV-1) virus (which can cause miscarriage) as well as vaccines for other conditions that may occur in a given region of the world. Pre-foaling vaccines are recommended 4–6 weeks prior to foaling to maximize the [[immunoglobulin]] content of the [[colostrum]] in the first milk.<ref>{{cite web|url=http://www.aaep.org/vaccination_passivetransfer.htm |url-access=subscription |title=Vaccination and Passive Transfer |website=American Association of Equine Practitioners}}</ref>  Mares are dewormed a few weeks prior to foaling, as the mare is the primary source of parasites for the foal.<ref>[http://www.aaep.org/health_articles_view.php?id=140 "Expectant Mare: Assuring the Health and Well-Being of the Pregnant Mare"] {{webarchive|url=https://web.archive.org/web/20080415220750/http://www.aaep.org/health_articles_view.php?id=140 |date=2008-04-15 }}</ref>

Mares can be used for riding or driving during most of their pregnancy.  Exercise is healthy, though should be moderated when a mare is heavily in foal.<ref>{{Cite web|url=http://www.esdaw.eu/horse---breeding.html|title=Horse - breeding|website=ESDAW|language=en|access-date=2019-06-11}}</ref> Exercise in excessively high temperatures has been suggested as being detrimental to pregnancy maintenance during the embryonic period;<ref>Mortensen C, Choi YH, Hinrichs K, Ing N, Kraemer D, Vogelsang S, Vogelsang M. 2006. Effects of exercise on embryo recovery rates and embryo quality in the horse. Animal Repro. Sci. 94:395-397</ref> however ambient temperatures encountered during the research were in the region of 100 degrees F and the same results may not be encountered in regions with lower ambient temperatures.{{original research inline|date=August 2014}}

During the first several months of pregnancy, the [[equine nutrition|nutritional]] requirements do not increase significantly since the rate of growth of the fetus is very slow.  However, during this time, the mare may be provided supplemental vitamins and minerals, particularly if forage quality is questionable.  During the last 3–4 months of gestation, rapid growth of the fetus increases the mare's nutritional requirements. Energy requirements during these last few months, and during the first few months of lactation are similar to those of a horse in full training.  Trace minerals such as copper are extremely important, particularly during the tenth month of pregnancy, for proper skeletal formation.<ref>{{cite web|url=http://horse.purinamills.com/products/pregnantlactatingmare.asp|title=Nutritional Management of Pregnant and Lactating Mares|website=purinamills.com}}</ref> Many feeds designed for pregnant and [[lactation|lactating]] mares provide the careful balance required of increased protein, increased calories through extra fat as well as vitamins and minerals.  Overfeeding the pregnant mare, particularly during early gestation, should be avoided, as excess weight may contribute to difficulties foaling or fetal/foal related problems.