At West Kington Stud, we are often asked about the optimum number of sperm in a ‘dose’ of semen, to achieve a successful pregnancy. In this article, Chris Shepherd BVSc MRCVS, from B&W Equine Vets, shares his insights into this fascinating subject.
The Journey of Spermatozoa through the Equine Reproductive Tract
The hazardous journey for spermatozoa through the mare’s reproductive tract means that a huge excess must be deposited into the uterus at the time of insemination or covering. A fertile stallion can produce anywhere between 5 billion and 25 billion spermatozoa at ejaculation. Not all will be live, and not all the live ones will be viable but, bearing in mind only one is necessary for fertilisation, there are plenty to spare.
At a natural cover, the whole ejaculate (consisting of the spermatozoa and the seminal plasma produced from the accessory sex glands) is deposited directly into the uterus. If the mare is artificially inseminated, we are able to decide which parts of the ejaculate and exactly how many live spermatozoa are deposited into the uterus. Most ejaculates from a fertile stallion will provide sufficient spermatozoa to inseminate several mares, if they are available and ready at the time.
From Ejaculation to Insemination: Understanding Spermatozoa Numbers
There is considerable debate as to how many live spermatozoa should be inseminated for that mare to have a reasonable chance of conceiving. Traditionally, it has always been accepted that 500 million is a reasonable figure. A stallion that ejaculates 20 billion spermatozoa that has 75% alive and viable would therefore be able to provide enough doses to inseminate 30 mares. However, 500 million may be a generous estimate and the required figure may be closer to 250 million. In reality, the fertility of the mare and the method of insemination will have a large influence on the number of spermatozoa that have to be inseminated. Continue reading