Know how. Know now. EC271 (Revised October 2011) UNDERSTANDING MARE REPRODUCTION Kathy Anderson Extension Horse Specialist University of Nebraska–Lincoln Extension is a Division of the Institute of Agriculture and Natural Resources at the University of Nebraska–Lincoln cooperating with the Counties and the United States Department of Agriculture. University of Nebraska–Lincoln Extension educational programs abide with the nondiscrimination policies of the University of Nebraska–Lincoln and the United States Department of Agriculture. © 1994-2011, The Board of Regents of the University of Nebraska on behalf of the University of Nebraska–Lincoln Extension. All rights reserved. UNDERSTANDING MARE REPRODUCTION Kathy Anderson Extension Horse Specialist University of Nebraska–Lincoln INTRODUCTION FUNCTIONAL ANATOMY Many producers who raise horses find breeding A correctly functioning reproductive tract is es- mares rewarding, yet frustrating. Mares and stal- sential to the potential fertility of a broodmare. The lions are traditionally placed in the breeding herd tract goes through various changes as a mare exhib- due to successful performance records, with little its estrous cycles. A good working knowledge of a consideration for their reproductive capabilities. mare’s anatomy and these changes will aid in early Horses are difficult breeders with an estimated identification of potential abnormalities. These foaling rate of below 60 percent. Various factors changes can easily be monitored through rectal pal- contribute to this, including long-erratic estrous pation or ultrasound by a veterinarian. cycles and an imposed breeding season that does The rectum is located above the reproductive not coincide with the mare’s natural breeding sea- tract allowing for a noninvasive examination of the son. Basic understanding of the mare’s reproduc- cervix, uterus, and ovaries. A diagram of a mare’s tive physiology can improve reproductive efficiency reproductive tract is shown in Figure 1. and foaling rates. A mare manager’s goal should be to get the maximum number of mares pregnant, in the shortest amount of time, and to deliver a live, Uterine horn opened to show Probe marking healthy foal each year. arrangement of muscosal folds uterine orifico of tube Ovarian ligament Fallopian tube TYPICAL CYCLE People who have bred many mares soon realize that each mare is an individual, and there may be many variations in the reproductive cycle length Fimbriated end between mares. There is, however, a basic cyclic Ovary with abdominal Common ostium of tube pattern that most mares will follow. Fimbriated end of uterine Typically, during the breeding season, a mare tube concealing Cervix cavity ovulation fossa will have a 21-day estrous cycle (the interval from External os of one ovulation to the next). This length will vary Bladder the cervix between mares and time of the year. The estrous cycle is divided into two periods, estrus and dies- Vagina trus. Estrus (“heat”) is the period (usually four to Ureter seven days) when the mare is receptive to the stal- lion. During this period, ovarian follicles grow and Outline of urethra develop to between 30 and 50 mm in diameter with Urethral ovulation occurring 24 to 48 hours before the end of orifice estrus. Thus, the mare is still showing “heat” when Orifices of she ovulates. vestibular During diestrus, the period between successive Vaginal vestibule glands estrus (typically 14 to 18 days), the mare is unre- Vulva ceptive to the stallion. Clitoris Figure 1. Reproductive organs of the mare. (From: Evans, The Horse 1990) 2 © The Board of Regents of the University of Nebraska. All rights reserved. Tail A Anusα α Vulva α α α B Figure 2. Desirable (A) and Undesirable (B) vulva conformation in mares. (From: Evans, The Horse, 1990). Vulva Caslick’s operation The vulva is the external portion of the mare’s A simple surgical procedure, known as Caslick’s reproductive tract. The lips of the vulva contain operation, can be performed on mares that have muscles to help keep the opening closed, preventing undesirable vulva anatomy to reduce the chances of feces or other foreign matter from entering. When uterine contamination. The skin on the outer por- the mare is in “heat,” the lips swell and become tion of the lips is removed, then sutured together, moist and relaxed. In some mares the vulva’s mus- leaving a small opening for breeding and urination cle tone is greatly reduced, becoming very loose and (Figure 3). Mares with a sutured vulva can eas- pliable. During diestrus, the muscle tone increases, ily be bred through artificial insemination. Mares keeping the lips of the vulva “closed.” that need their vulva sutured and bred by natural The conformation of the mares’ vulva is fre- service would have to be sutured after determined quently overlooked by producers. Desirable vulva pregnant, or reopened if not pregnant. It is es- conformation (Figure 2A) is characterized by a sential to record this procedure on the mare’s vulva vertical to the pelvic floor, parallel with the permanent records so the vulva is reopened anus, and the lips meet to form a seal. Undesir- approximately four weeks (30 days) before able conformation (Figure 2B) is a vulva lying more her expected foaling date! Failure to do so could horizontal with the pelvic floor, or having a “tipped lead to serious tearing and damage to the reproduc- inward” appearance. The anus will appear sunken tive tract. and in a position predisposing the mare to collect- ing fecal material in her vagina. Vagina Additionally, poor conformation of the vulva predisposes mares to wind sucking (pneumovagina) The vagina is a tubular structure about 6 to 8 and urine pooling. This undesirable vulva confor- inches in length connecting the cervix and vulva. mation can be due to genetics, aging, and/or poor There are no glands within the vagina. Its lining is body condition. Regardless of the cause, poor vulva highly elastic and permits dilation during parturi- anatomy is an important component of subfertility tion. During estrus, the vagina has a more reddish in mares due to the ease in which the mare’s tract color and increased moisture. In diestrus, the va- can become contaminated and infected. gina is typically pale and dry. © The Board of Regents of the University of Nebraska. All rights reserved. 3 A B Figure 3. Caslick’s operation. A, Suturing the lips of the vulva. B, Finished Caslick’s. (From: McKinnon & Voss, Equine Reproduction, 1993). Cervix vagina. The mucus also allows semen to enter the uterus. The cervix of the mare serves as a barrier to the The cervix of a diestrus or pregnant mare will uterus during diestrus and pregnancy, and is also be “whitish” in color and lack the moist appearance. the major site of semen deposition during natural A sticky mucus cervical plug, which aids in sealing service. Visual changes of the cervix can be de- the lumen during pregnancy and diestrus, is pres- tected during estrus by viewing through a vaginal ent. The cervix will also appear very “toned” (rigid) speculum. The vaginal speculum commonly used and be up off the vaginal floor Figure( 4B). Howev- is a disposable, hollow cardboard tube. The mare’s er, with manipulation, the diestrus cervix can still vulva should be washed, rinsed, and dried before be penetrated. Thus, visual evaluation of the cervix passing the sterile speculum into the vagina. through a vagina speculum can be used along with Following lubrication of the rolled end, the other management practices in making breeding speculum is passed through the vulva and into the decisions. vagina where it is placed adjacent to the external opening (exterior os) of the cervix. With a penlight, the cervix can be examined visually. Uterus The cervix of a mare when in estrus (“heat”) is The mares’ uterus is composed of two uterine relaxed and has a glistening red appearance when horns and the body, which are connected almost viewed through a speculum. Often, the relaxation is perpendicularly, forming a T-shaped configuration. such that the cervix appears flattened and lying on The uterus goes through cyclic changes similar to the vaginal floor Figure( 4A). Penetration of the cer- the other areas of the mares’ reproductive tract. As vical lumen is very easy at this time. A large quan- a mare exhibits estrus, estrogen causes a swelling tity of mucus is secreted and acts as a lubricant to and increased folding in the endometrium. Once a facilitate passage of the penis or foal through the mare enters diestrus or becomes pregnant, proges- 4 © The Board of Regents of the University of Nebraska. All rights reserved. A Cervix B 1 1 2 2 Vagina Cervix Figure 4. Vaginal portion of uterine cervix. A, during estrus and B, during diestrus. 1, Viewed through speculum in vagina; 2, side view. (From: McKinnon & Voss, Equine Reproduction, 1993). terone secreted encourages glandular development fertilized embryo to the uterus. The fertilized em- and secretion in the endometrium. It also encourag- bryo remains within the oviduct for approximately es greater muscular tone within the myometrium. 144 hours before entering the uterine horn. If a mare has some type of uterine infection or is not exhibiting normal estrous cycles, her uterine Ovaries tone may be compromised and be more flaccid. A small sample of the uterine lining (endo- The mare has a unique kidney bean-shaped metrium) is obtained during a uterine biopsy for ovary from which ovulation only occurs at the ovu- histological examination. A healthy endome trium is lation fossa (indented area of the ovary). As with critical for high fertility. Infection or damage to the most species, the ovaries have a dual role, being not endometrium often results in failure to conceive, only responsible for production of hormones that abortion, or reduced fetal growth.