Breeding Soundness Examinations
By B. W. Pickett, Ph.D.
Professor Emeritus, Colorado State University
Revised December 2006

Although some of you may feel that the breeding season has just ended, it is time to begin thinking about the next breeding season.

Good management is essential for maximum reproductive efficiency in any livestock enterprise.  In beef cattle and sheep ranching, where fertility of the animals is closely related to profitability, a thorough breeding soundness examination is conducted on all breeding males well-before the breeding season.  However, only a very small percentage of the females are bred utilizing artificial insemination. In contrast, very few stallions are given a thorough breeding soundness examination inspite of the fact that a significant percentage of the mares are bred utilizing artificial insemination.  Not to do so is poor management.

Stallions: Just because a stallion will mount quickly and copulate vigorously does not mean he is fertile. It is essential to examine the breeding records. Unfortunately, many farms keep excellent records, but fail to analyze them properly.  To merely determine the percentage of mares that became pregnant during the breeding season is a poor measure of fertility.  In spite of the fact that 90% or more became pregnant is not nearly as informative as the efficiency with which those mares became pregnant.

For example, presented in Table 1 are the records of two hypothetical stallions (A and B), each bred to 100 mares. This emphasizes how erroneous some assumptions can be made from overall pregnancy rates only. No one would dispute that overall pregnancy rates of 99 and 97% for Stallions A and B, respectively, are excellent.  However, upon closer observation, a first-cycle pregnancy rate of 65% for Stallion A vs. 35% for Stallion B represents a very large economic difference.  After only two cycles, Stallion A had settled 88% of his mares compared to only 58% for Stallion B. After four cycles, Stallion A had settled 99% of his mares, whereas Stallion B had settled only 82%.  Therefore, the cost of each pregnancy obtained by Stallion B would be much greater than for those impregnated by Stallion A. This cost is borne primarily by the mare owners, and is associated with the routine management cost such as, shipping semen, teasing, palpating and/or ultrasounding.  Further, the average date of foaling would be later, resulting in less marketable foals, and creating a group of mares that will always have a late foal unless held open for a year. In this example, overall pregnancy rates were virtually identical, but there was a tremendous difference in fertility.

Table 1. Pregnancy Rates of Two Hypothetical Stallions (A and B). Stallion A had a 65% pregnancy rate per cycle, while Stallion B's pregnancy rate per cycle was 35%. These calculations represent pregnancies determined by ultrasonography at 15 days post-ovulation.

It required 1.53 cycles for Stallion A to obtain a pregnancy and 1.83 breedings compared to 2.86 and 3.16, respectively for Stallion B.  The difference in cycles and breedings per pregnancy represents 50% to 60% more work in the breeding shed for Stallion B and his breeding crew.  In addition to the real costs for the owners with mares to Stallion B, there was a loss of morale within the breeding shed crew, and a poor reputation for the farm and stallion for coming years. Perhaps this could have been avoided, if a breeding soundness examination had been conducted prior to the breeding season.

All stallions that are to be used for breeding and/or to be sold should have a breeding soundness examination.  Further, it is extremely important that stallions standing at your farm for the first time, especially juvenile stallions, be examined. Your veterinarian should be called to conduct these examinations in November and no later than December. In the event a stallion is diagnosed as potentially sub-fertile, changes in management and/or treatment may significantly improve his efficiency of reproduction.

Diagnosis well before the breeding season is essential, because it takes a stallion about two months to produce a sperm cell and another 11 days for that sperm cell to be in a position to be ejaculated.  Any treatments and/or vaccinations that may cause a spike in temperature should also be completed at least two months before the breeding season.

Another reason for conducting a breeding soundness examination, to become aware if there is a potential problem, is if semen is going to be shipped from your stallion to other mare owners. If a stallion is marginally fertile, such as Stallion B, Table 1, he is very likely to be less fertile in a shipped-semen program than when breeding mares on the farm.

Mares: Those individuals and organizations that do embryo transfers for the public are buying recipients, sorting and preparing them for receipt of embryos. Therefore, it is time for breeders to have a thorough breeding soundness examination done on their barren mares, and take whatever action is necessary to improve the chances of pregnancy. This is especially true of older mares and those recently retired from performance events.

Further, now that some breeds allow an unlimited number of foals to be registered from one mare, in any given year, which many times involves shipped semen it is essential that the donor and recipient mares be fertile. Otherwise, the entire program becomes an extremely costly endeavor.

Shipped Semen: The acceptance of shipped semen by the majority of breed registries has had a dramatic effect on the horse industry. Like many technological advances, or changes, depending upon your point of view, this technology provides additional opportunities and more flexibility in breeding horses, which in turn will ultimately improve the genetic merit of the breeds that are participating.

Although the advantages far outweigh the disadvantages, many of the disadvantages can be eliminated or minimized. The objective is to get your mare pregnant, hopefully on the first breeding, requiring only one shipment of semen. When this does not occur on the second or third attempt a great deal of dissatisfaction ensues, resulting in all sorts of warranted and /or unwarranted accusations.

It is the responsibility of the stallion owner to utilize an appropriate extender, such as E-Z MixinŽ - "CST". This is a milk-based extender, which must be handled properly to provide maximum fertility. Fresh extender is essential, and upon receipt should be placed in a deep-freezer, because all milk and milk by-products will deteriorate, particularly when exposed to excessive heat or even room temperature. Fresh batches are produced throughout the year, and evaluated at Colorado State University and approved before being offered for sale.

Unfortunately, semen from some stallions does not tolerate cooling and shipping. Obviously, it is the responsibility of the stallion manager to use fresh extender and make the appropriate tests prior to the breeding season to assure that his stallion's semen will withstand the rigors of cooling and shipping. It is the responsibility of the mare owner to employ the services of a veterinarian well-trained in equine reproduction to assure that the mare is in the appropriate stage of her cycle for maximum chances of conception. In addition, appropriate handling of the semen upon receipt is an integral part of management. Failure to obtain well-trained personnel with appropriate technical skills is a common cause of failure.

A common source of disagreement between the stallion manager and mare owner is motility of the spermatazoa. Most frequently this is due to the lack of a good phase-contrast microscope by the mare owner. There are a number of good, relatively inexpensive microscopes available, such as the Video Microscope produced and sold by Animal Reproduction Systems. Frequently it is difficult to remember that the sperm must be warmed, sometimes for as long as 20 minutes, to attain maximum motility. Further, not all motile sperm are fertile, therefore a quality microscope, properly cleaned and adjusted is necessary for trained personnel to make a reliable judgment.

When the stallion manager and the mare owner have confidence in one another and a spirit of cooperation, shipped semen has many fewer disadvantages and a much greater chance of success.

  Stallion A   Stallion B
Pregnancy Rates No. of Mares Pregnant Pregnant Total  (%) No. of Mares Pregnant Pregnant Total  (%)
Overall 99 100 99 97 100 97
Cycle 1 65 100 65 35 100 35
Cycle 2 23 35 66 23 65 35
Cycle 3 8 12 66 15 42 36
Cycle 4 3 4 75 9 27 33
Cycle 5 6 18 33
Cycle 6 4 12 33
Cycle 7 3 8 37
Cycle 8 2 5 40
Cycles/Pregnancy 1.53 2.86
Cycles/Pregnancy 1.83 3.16
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