How to Improve Stallion Fertility?

Q. I have a young (4-year-old) Dutch warmblood stallion with some fertility issues. However, he has had good fertility and a good sperm cell count. He has low motility, some poor morphology, and some dead and immature sperm cells. The longevity of his semen is good.

We want to ship the best-quality semen we can achieve. We have tried different extenders and collecting him into extender (which makes counting sperm cells inaccurate) without improving the quality. Is there anything we can do to improve motility? What do you think of using agnus castus (vitex)?


A. In evaluating stallion fertility, several step-by-step questions need to be answered to sort out the key issues. First, what is good fertility? We usually look at first cycle conception rate as an indicator of fertility. This is the percentage of mares that get pregnant the first cycle they are bred. The higher the value, the fewer breedings needed to get mares in foal. This percentage should be 65% or greater.

Is the "good" fertility achieved with shipped semen or with extended semen used to inseminate mares on the farm? If some mares are bred on the farm and some with shipped semen, they must be looked at as different groups to calculate fertility.

Are all mares bred via artificial insemination, or are some bred with live cover? Obviously, the number of sperm the mare receives is greatly different in these two instances.

What is the insemination dose in progressively motile sperm? The important number is the number of progressively motile sperm in the ejaculate. This is the percentage of sperm that move forward in a relatively straight manner.

What are the morphologic issues? Some changes have minimal effect on fertility because those cells can't fertilize an egg anyway. Other changes can decrease fertility because the abnormal cells can bind to the egg and prevent a normal sperm from doing so. The other factor is the percentage of these abnormalities. A relatively low percentage of abnormal cells (about 10%) will have minimal effect if that is the only abnormality present.

Do morphology and the live cell number change over time? How often is he collected? Some horses are sperm accumulators, in which sperm accumulates in the epididymis. Ejaculates from these horses typically have a high concentration of sperm with a high percentage of dead sperm, many detached heads, and low motility. When these horses are collected or bred several times a day for several days, semen quality usually improves: The concentration decreases, the percentage of abnormal cells decreases, the percentage of live cells increases, and motility increases.

The number of immature sperm can be related to the horse's age, testicular insult, or testicular degeneration. An increased number of immature sperm cells are typically seen in young horses and old horses. An insult such as trauma, fever, systemic illness, or a variety of drugs can cause testicular degeneration that results in more immature cells. Testicular degeneration can be seen in family lines as well.

Do you know anything about the semen quality of the sire or grandsire? He is still relatively young for a warmblood, and his semen quality might improve somewhat over time.

How concentrated is his ejaculate? A dilute ejaculate exposes sperm to an increased amount of seminal plasma, which can decrease motility. Sperm numbers can be counted in an extended sample, but they must be counted manually using dilution and a hemocytometer. The concentration cannot be performed using a densimeter because the densimeter measures the amount of light that passes through the sample.

Regarding feeding agnus castus (vitex) or other supplements, there are many anecdotal reports of various supplements improving "semen quality." However, I know of no controlled studies evaluating supplements and their effect on semen quality.

A breeding soundness examination will help you sort out the questions regarding your stallion. There are several issues that need to be examined and resolved. Then a plan can be developed to utilize his semen efficiently.

About the Author

Pete Sheerin, DVM, Dipl. ACT

Pete Sheerin, DVM, Dipl. ACT, is a practitioner at the Rood and Riddle Equine Hospital in Lexington, Ky.

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