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Reproductive Physiology and Breeding Management of the Mare

By: Dr. Sylvia J. Bedford-Guaus

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Breeding The Mare by Natural Cover

Systems where the intervention of the veterinarian is minimal are usually those where the stallion and mares are located in the same facility or stud farm, and bred by natural cover when in heat. At present, this only happens consistently in registered thoroughbred and mini-horse farms.

Breeding by natural cover can be performed by turning out the horses in a pasture or enclosure, resembling a feral situation, or by bringing mare and stallion in-hand into a breeding shed. Pasture breeding is still performed in some breeding farms and in some research situations, and should not be underestimated as a valuable form of breeding management. Pasture breeding is less labor intensive than other breeding management techniques, since mares and stallions do not have to be handled individually for heat detection or cover. Stallions are usually much more efficient breeders when freely allowed to breed a group of mares in a pasture situation. However, this breeding system is not applicable to all situations and some mare or stallion owners do not feel comfortable turning them out to pasture for breeding.

For in-hand natural cover programs, mares must be teased consistently to ascertain when they come into heat. Once in heat, mares can be covered every other day until they come out of heat. If a stallion has a large book of mares this breeding frequency may be impractical, and a veterinarian can check mares in heat by palpation or ultrasound to ascertain the optimal time for breeding, as close to ovulation as possible. The use of hormones to induce ovulation can also narrow the timing for optimal breeding.

The main advantage of natural cover breeding programs is that pregnancy rates are usually the greatest, considering that both stallions and mares in the program are fertile and that breeding management is appropriate.

Disadvantages of natural cover versus artificial insemination programs include the following:

  • One stallion can cover fewer mares per season
  • There is more risk of injury to mare, stallion or handlers
  • Mares need to be transported and boarded for breeding
  • There is higher risk of disease transmission

    Artificial Insemination

    Artificial insemination programs require more intensive management of the mare, especially when transported or frozen semen is used. Similar to natural cover programs, mares can also be teased for heat detection if a stallion is available. The optimal time for breeding is usually ascertained by your veterinarian through palpation or ultrasound exam of the reproductive tract. Hormones are often administered to narrow the window for insemination in relation to ovulation.

    Pregnancy results for artificial insemination programs with fresh or transported (cooled) semen can equal those obtained in natural cover programs, as long as breeding management is optimal. However, pregnancy results with frozen-thawed semen are inconsistent and there is a high variability amongst stallions.

    Advantages of artificial insemination programs include the following:

  • One stallion can cover more mares per season because on ejaculate can be split amongst several mares
  • It decreases the risk of disease transmission
  • There is less risk of injury for mares, stallions or handlers
  • The mare does not need to be transported or boarded in another facility

    The main disadvantages of artificial insemination programs are that they need specialized equipment and training and are also more labor intensive.

    Administration of Hormones

    There are basically three groups of hormones used to control the mare's estrus cycle:

  • Those given to induce ovulation during estrus
  • Those given to synchronize a mare or group of mares for timed breeding
  • Those given to 'short cycle' or bring the mare back into heat sooner than she would on her own cycle

    The main principle to consider when giving these hormones to mares is that they will only work when given at the correct time during the estrus cycle.

    To Synchronize Breeding

    A mare or group of mares may be synchronized to come into heat at a time when the stallion is available or for embryo transfer purposes. For estrus synchronization the most commonly used hormones are a combination of progesterone, given alone or with estrogen, and prostaglandin F2a. These regimes require daily injections of progesterone/estrogen for about nine days and are reserved for special situations where tight synchronization between mares is of utmost importance, such as with embryo transfer. A prostaglandin injection is usually given in the last day of the progesterone/estrogen series. These regimes have also been used with variable success to advance the start of the breeding season. Mares usually come into heat three to five days after the regime ends and ovulate at variable times after that.

    Progesterone-like hormones can also be given by mouth, avoiding the impracticality of daily injections. However, these preparations are less effective for estrus synchronization, although encouraging results have also been attained in advancing the start of the breeding season.

    Another synchronization regime consists of two injections of prostaglandin given to a group of mares approximately 10 days apart; this will synchronize estrus in approximately 70 percent of the mares in the group. Mares in this regime will also come into heat around three to five days after the second injection of prostaglandin.

    To Short-Cycle The Mare

    Short-cycling the mare consists of bringing the mare back into heat sooner than we would expect in her own cycle. For this purpose, natural prostaglandin F2a and similar synthetic hormones are given to cause demise of the corpus luteum in mares in diestrus. To be effective, prostaglandins must be given in a mare that is at least six days from her last heat (ovulation). In such case, we can expect a mare to come back into heat around three to five days after the treatment.

    To Induce Ovulation

    Two main hormones are given to mares to induce ovulation:

  • Human chorionic gonadotropin (hCG), a hormone purified from the urine of pregnant women that has LH activity in horses and acts on the ovaries to induce ovulation.

  • Deslorelin (Ovuplant), a synthetic hormone that simulates the action of Gn-RH, and stimulates LH and FSH secretion from the mare's pituitary gland. This hormone comes in the form of an implant placed under the skin of the neck and slowly releases deslorelin for a few days.

    To be effective, both hormones must be given in a mare that is in heat and has a follicle at least 33 mm in diameter. Therefore, it is crucial to examine the mare by ultrasound and determine the size of the follicle. Smaller follicles will not respond to the hormones. When given at the right time, both hormones will induce ovulation between 24-48 hours later. This allows breeding the mare very close to ovulation time, without having to perform repetitive inseminations and rely on her own spontaneous ovulation.
            
    In summary, good knowledge of the mare's reproductive system paired with application of old and modern breeding tools and the collaboration of your veterinarian can make your breeding season a total success.

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