Breeding Soundness Exam - cont.

As part of the breeding soundness exam it is important to collect semen from the stallion. Some stallions however have never been collected with an artificial vagina and this would require training time which might not be possible. Since a stallion always has a chance of injury during semen collection or mating, the stallion owner might not want to have the semen evaluated. In this lab we will not evaluate stallion semen as we will be doing this in other portions of the class.

It is valuable to determine if the stallion has sufficient sperm production to breed the number of mares per week planned. What we want to determine is the daily sperm output or DSO. One method already described in the lecture material was to determine the volume of the testes. Since the volume of the testes is directly related to the amount of sperm produced, this is often a sufficient procedure for most stallion owners. The formulas for this method (volume of an ellipsoid) are shown below, with a calculation for each testis or measure one and multiply by 2:

Volume =4/3*3.1416* Height/2 * Length/2 * Width/2

or

Volume = 0.5236 * Height * Length * Width

Measurements are in cm. The width for 1 testis is scrotal width/2. The predicted Daily Sperm Output (DS0) in billions of sperm is:

predicted DSO = [0.024 * (volume Left testis + volume Right testis)] - 1.26

It is not easy to get these measurements and experience is needed. The second method is to collect semen every day for up to 14 days. A careful record of the number of sperm in the ejaculate is then plotted. An example you have seen before is shown below:

Sperm Output the First 14 Days of Daily Collection

In this example, the number of sperm per ejaculate decreases as epididymal reserves, sperm stored in the tail of the epididymis, are depleted. Take a look at the tail of the epididymis from two mature stallions. By day 6, it is clear a plateau is reached. To calculate the DSO you could either average the daily values from day 6 - 14 or only use the values for the last 3 days (day 12 -14). For this stallion the DSO is 3.9 billion sperm. To decide on how many mares this stallion could service through artificial insemination, divide 3.9 billion by the sperm per breeding dose. In the case of fresh semen, this is 500 million motile sperm. If we assume an average sperm motility of 70%, this was routine for this stallion, he could then be used to breed:

• first calculate number of sperm required
  • 500 million / 0.70 = 714 million sperm = 0.714 billion sperm
• next divide DSO by sperm per breeding dose
  • 3.9 billion /0.714 billion = 5.5 mares or 5 mares per day since half a mare does not exist

We have calculated the mares bred per day for artificial insemination because it is more difficult to determine for natural breeding (cover). This stallion however could easily breed 1 mare per day with natural cover and deliver sufficient sperm to get an oocyte fertilized. Artificial insemination provides the option to breed more mares than with natural cover.

You will note that at the start of the semen collections the stallion had over 8 billion sperm in the ejaculate. This is because we were not collecting him daily. Epididymal sperm reserves will build up but once full, excess sperm will simply be released into the urine. Therefore you expect to get more than the DSO in the stallion's ejaculate if you are not ejaculating him daily. In the UW-Madison facility we normally collect a stallion 2 to 3 times a week during the breeding season. This particular stallion for whom the data is shown above routinely produced 8 - 10 billion sperm per ejaculate on this type of collection schedule.

If you collect substantially less than the predicted DSO, you need to evaluate the stallion for 1) potential disease conditions of the testes, epidiymidies accessory glands, 2) libido problems, and 3) collection procedures.