Centrifuging stallion semen on the farm

Centrifuging concentrates sperm and removes most of the seminal plasma. For some stallions it is essential. For many it is extra time and handling they do not need. Exodus’s advice was to decide case by case.

Does your stallion need it?

Centrifuging is worth considering when:

• the ejaculate is large but the concentration is low. Exodus used under about 100 to 125 million per mL as the point to consider it, because a dilute stallion cannot be extended enough without the dose becoming too large.
• the semen looks shocked from collection onward, with poor motility within minutes. That can mean the stallion’s own seminal plasma is harming his sperm, and it needs to come off quickly.

If neither applies, careful counting and broad extension may be all your stallion needs.

Set g-force, not RPM

What separates sperm cleanly is g-force (relative centrifugal force, RCF), not rotor speed. The same RPM produces different forces on different centrifuges, so work it out for yours:

• RCF = 0.00001118 × r × RPM²
• RPM = √(RCF ÷ (0.00001118 × r))

Here r is the radius in cm, measured from the center of the rotor to the tip of the tube. Example: with a 10 cm radius, 3,000 RPM gives about 1,000 × g.

Use a centrifuge with a swing-out rotor, so the pellet forms at the bottom of the tube rather than smeared up one side.

The basic spin

From the Next Generation extender directions:

1. Extend the semen 1:1 (equal parts) with extender at the same room temperature as the semen.
2. Divide it evenly between centrifuge tubes and balance the rotor.
3. Spin at about 500 × g for 12 to 15 minutes. Higher-volume, lower-concentration stallions may need more, up to about 1,000 × g.
4. Draw off the supernatant down to about ¼ inch above the pellet.
5. Resuspend the pellet in fresh extender to 25 to 35 million sperm per extended mL.

Spinning onto a cushion

A centrifugation cushion protects sperm at higher g-force and gives better recovery. Exodus’s method:

1. Extend 1:1 in 15 mL or 50 mL conical tubes, cap and invert slowly 3 or 4 times.
2. Lay the cushion under the semen. With a syringe and a long catheter, slowly deposit the cushion into the tip of each tube: about 1 mL for a 15 mL tube, or 2 to 3.5 mL for a 50 mL tube. Follow the cushion’s own directions.
3. Spin at 1,000 × g for 12 to 15 minutes.
4. Take out the cushion first. Slide the catheter down the wall of the tube into the cushion and draw it out slowly, so the pellet drops into the cone.
5. Remove the supernatant from the top down to just above the pellet, without dipping the catheter more than ¼ inch below the fluid line.
6. Resuspend in fresh extender to 25 to 35 million sperm per extended mL.

Next Generation extenders can be used both for the spin and for the final resuspension, which keeps things simple.

Freezing

Centrifuging is also the first step in freezing semen. Freezing protocols vary a great deal between stallions and between freezing extenders, so follow the directions for the freezing extender you use and work with your veterinarian or a freezing lab to test what suits your stallion.

Adapted from Exodus Next Generation Cryo-Preservation System guidelines and extender direction sheet. This guide covers handling and use only; breeding timing, insemination and anything involving medication are for your veterinarian.