Stallions usually reach sexual maturity in the Spring of their second year i.e. around 18 months
Daily sperm output is correlates directly with testicular size.
Semen quality varies form stallion to stallion and even from season to season in the same stallion. Semen quality also deteriorates with the horses age but this, again, is very variable with many stallions remaining fertile well into their twenties.
There are four aspects to sperm quality-
Total sperm numbers; progressive motility; sperm longevity and sperm morphology.
On average the volume of a stallions ejaculate is around 80ml with a sperm concentration around 250million/ml. About 2billion progressively motile sperm are considered necessary to achieve a successful fertilisation, which can be achieved from about 10ml of semen. (this is the reason why several mares can be covered from one semen sample using AI)
When a semen sample is examined under a microscope several sperm will be dead, some will be deformed and some will be moving in circles. An assessment has to be made of the percentage that are active and moving forward - termed progressive motility. A good quality sample has a 70-80% progressive motility. Anything less than 50% is very poor quality.
Semen is ejaculated in three phases:
Sperm are not distributed evenly throughout the semen sample; the sperm-rich faction is the last to be ejaculated. It is not unusual for a stallion only to ejaculate the pre-sperm faction, made up of secretions from the prostate gland and seminal vesicles, and for it to have virtually no sperm in it. These stallions often dismount early and this can be due to hindquarter/spinal pain, an unco-operative mare or a poorly adjusted A-V. Obviously, if the stallion is covering naturally this goes undetected and the mare will return to oestrous in three weeks time. If the semen is collected using an A-V then this problem would be detected and further attempts can be made to obtain a decent sample.
Cause of poor semen quality
1] Infection. Obviously, genital tract infections will have a direct impact on semen quality but any systemic infection that causes a rise in body temperature above 40C will result in a significant drop in sperm output, which may take several weeks to resolve.
2] Drugs/medication. Most routine medicines are safe but cytotoxic drugs and some hormones will interfere with spermatogenesis.
4] Inherited/genetic defects
Training stallions to use a dummy mare.
The greatest advantage of AI is safety! The stallion is not going to be hurt by an uncooperative mare; the mare is not going to be hurt by an inconsiderate stallion and there is less likelihood of the handlers/attendants getting hurt by either of them!
We train a stallion to use a dummy mare by teasing him with a mare that is in season. We are lucky because we have an ovariectomised mare that we can bring into season artificially at any time. We also try to establish a ritual that the stallion can associate with the fact he is going to be covering. Once aroused some stallions will jump the dummy without much encouragement but if they are confused (and who can blame them!) we bring the mare alongside the dummy so that the stallion will mount the dummy in an attempt to cover the mare. We repeat this process over two or three days until the stallion associates the ritual with mounting the dummy and once the routine is established he will usually mount the dummy whenever required, sometimes without the presence of a tease mare. The samples collected are assessed for quality and longevity in a variety of extenders.
A Stallion being teased by a mare in season and then mounting the dummy for a semen collection. This stallion has left the dummy early, before the sperm faction has been ejaculated, which is why the AV has been held in place after he dismounted.