Frequency of Ejaculation
In consecutive ejaculates, the number of sperm decreases as shown in the graph.
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This observation is also true for a single ejaculate collected on each of consecutive days however in this case the number of sperm ejaculated will eventually stabilize to the daily sperm output at around 7 days. This is shown in the graph below.
The question thus arises as to how often should a stallion be ejaculated? The remaining discussion will assume the stallion has a normal testicular size (scrotal width) for his age as previously discussed. We know that between 100 to 500 million motile sperm are needed for optimal fertility. When a stallion is used for artificial insemination and semen is collected with an artificial vagina, studies have shown that there is no more sperm collected per week for 6 vs. 3 ejaculates. Therefore collection every other day during the week will yield maximal numbers of sperm collected and usually fits with management schedules for shipping semen and breeding mares. The number of sperm in these ejaculates will stabilize over 2 - 3 weeks and then number of mares that can be bred is simple to calculate as:
# mares = # of motile sperm per ejaculate/500 million motile sperm
It becomes more of a problem when a stallion is used for natural breeding, hand mated, as there is no ability to determine how many sperm are being produced per ejaculate. In this case, sperm depletion studies are often conducted. A stallion is bred or semen collected once per day for 10 days. The semen collected on day 10 is then an estimate of daily sperm production. If the number of sperm on day 10 is several billion then the limiting factor is likely his willingness to breed. Such a stallion could likely be used 1 or 2 times a day and sometimes three times. A more precise calculation could be made by collecting 5 ejaculates, 1 hr apart on day 10 of the depletion study. The limit of the number of mares the stallion could breed on a day would then be the number of ejaculates with at least 500 million motile sperm present in them.
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When a stallion is used for pasture breeding, we know that a stallion will mate much more than 2 or 3 times per day. Sperm numbers per breeding will thus go down dramatically when multiple mares are in heat on any day. Data also indicates that a stallion can not distinguish a mare on day 1 or later of estrus and so will mate with any mare in any stage of estrus. Stallions will not select mares closer to ovulation for breeding. In general for a normal stallion, has several billion sperm after the 10 day depletion test, he should be able to handle 50 mares in a pasture during the breeding season. Hand mating as used in most breeding farms can potentially increase this number to 100 or more. This is particularly true if mating is done only within 24 hours before ovulation (see hormone manipulation material).