How to evaluate stallion semen motility

Motility is the movement of individual sperm. Some swim in circles (“spinners”), some drift or move backwards, and some move steadily forward. A dose is built on the forward movers, so this is the number that matters most.

Get the sample right first

• Look at it promptly. Motility changes as semen cools.
• Keep slides and coverslips at the same temperature as the sample. A cold slide shocks the sperm and makes motility look worse than it is.
• Use the same magnification and the same routine every time, so this week’s reading can be compared with last week’s.

The 1-2-3 method

Exodus taught a simple routine for estimating progressive motility without a computer system. Look at one field, and for each step close your eyes for about 10 seconds, then open them and make your estimate fresh:

1. Passively motile. Estimate the percentage of sperm that are dead, spinning or sluggish.
2. Slightly motile. Estimate the percentage moving in a circle or only loosely forward.
3. Progressively motile. Estimate the percentage moving strongly in a straight line across the field.

Repeat over several fields and average. Closing your eyes between estimates stops one reading from colouring the next.

Reading the numbers

• Total motility is slightly motile plus progressively motile.
• Progressive motility is the strong, straight movers on their own.
• A reading of 80/70 means 80% total motility and 70% progressive motility.
• Usable sperm per mL is concentration × progressive motility. That is the figure a dose is built from. See counting sperm and calculating a dose.

Things worth noting on the sheet

Exodus’s notes flagged a few observations that are worth writing down and discussing with your veterinarian if they keep appearing:

• Clumping. Sperm sticking together can mean the semen has been shocked, or that the extender’s osmolarity does not suit the stallion. Try the other formula, and check that semen and extender were at the same temperature when mixed.
• Droplets on the tail. Small bumps at the base of the head or midway down the tail usually mean immature sperm.
• A collection with very low motility. Think back over recent months for anything that changed: a move, feed or water, heat, illness or stress. Your veterinarian can help work out the cause.

A full morphology assessment, which classifies individual defects at high magnification, is a job for your veterinarian or a reproduction lab.

Adapted from Exodus "How do I Analyze Motility" and semen terms glossary. This guide covers handling and use only; breeding timing, insemination and anything involving medication are for your veterinarian.