Management of the anoestrus and transitional phases in the mare.

Intro

Welcome to the Bova UK podcast, where we discuss disease diagnosis and management, tailored for registered vets and veterinary nurses. If you’re listening as a pet owner, please remember: if you have any concerns about your animal, always consult your local veterinary surgeon.

Host:

Welcome to today’s podcast. We’re joined by Dr. James Crabtree, Director at Equine Reproductive Services (UK) Ltd in North Yorkshire. James is an advanced practitioner in equine stud medicine. Welcome, James. How are you today?

Dr. James Crabtree:

Very good, thank you. The sun’s shining today, which is nice after a few stormy nights.

Host:

Lovely to hear. You’ve spent time in New Zealand and Australia. Could you tell us a bit about that?

James:

Yes, I moved into equine practice after starting in mixed practice and was always attracted by the breeding seasons in opposite hemispheres. I went to Australia initially and then alternated between Australia and New Zealand for several years, which gave me a huge amount of experience—essentially doing two breeding seasons per year. It was a great opportunity both professionally and personally.

Host:

So stud work has always been your focus?

James:

Yes, though I originally worked across species. Over time, equine work became my focus—particularly reproductive techniques. The development of assisted reproduction in horses really caught up with that of cattle, and that pulled me further into the field.

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Understanding the Mare’s Reproductive Cycle: From Anoestrus to Transition

Host:

Let’s talk about the cyclic activity in mares. What are the four phases?

James:

On an annual cycle, mares go through:

• Anoestrus – A winter phase when the reproductive system shuts down.
• Spring Transition – A variable-length phase as they begin to cycle again.
• Oestrus Cyclicity – Regular cycles from roughly April to August/September.
• Autumn Transition – The return to anoestrus as daylight shortens.

These are influenced by daylight length because horses are long-day breeders. In contrast, sheep, for example, are short-day breeders.

Host:

Is that cycling pattern consistent for each mare?

James:

Generally, yes, although environmental and management factors—like light exposure and nutrition—can shift the timing year to year. Some mares, particularly those in warm barns with artificial light, may cycle year-round, though that’s not typical.

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Why the Management of the Transitional Phase in the Mare Is Crucial

Host:

Why is managing the transitional phase important?

James:

It’s crucial in the thoroughbred industry. Since all foals have an official birth date of January 1st, being born early gives them a competitive advantage both for racing and sales. The goal is often to get mares cycling early enough to breed them in February, which means manipulating the cycle as early as December.

Host:

And that’s achieved through lighting?

James:

Yes. Light is the main environmental cue. Providing 16 hours of light and 8 hours of darkness from early December can bring mares into season by February. This mimics spring conditions and suppresses melatonin, stimulating GnRH release and triggering the reproductive cascade.

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Techniques for the Management of the Anoestrus and Transitional Phases in the Mare

Host:

Let’s talk about methods to manage this transition. What are the main techniques?

James:

We divide them into two categories:

Environmental (e.g. lighting):

• Artificial lighting programs (stable lights or blue-light masks) work well if implemented by December.
• Equilume masks (blue-light masks) are battery-powered and provide light to one eye—enough to suppress melatonin.

Pharmacological:

• GnRH analogues (e.g., Buserelin, Deslorelin): Can work in mid- to late transition but are off-label in horses.
• Progestogens (e.g., altrenogest / Regumate): Useful when mares already have 25mm estrogenic follicles. Not effective in early transition.
• Dopamine antagonists (e.g., domperidone): Thought to increase FSH receptor density. Used later in transition.
• Ovulation induction agents (e.g., hCG, deslorelin acetate): Effective only in mares close to ovulation.
• Intravaginal devices (e.g., PRID/CIDR): Studied for use in mares with promising results, especially for 10-day progesterone delivery.

Host:

What about regulating mares pharmacologically too early?

James:

That’s a key point. Starting treatment before the mare is physiologically ready—before she’s in mid-to-late transition—is unlikely to be effective. Examine the mare and time therapy appropriately.

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Future Treatments for Managing the Mare’s Reproductive Transition

Host:

Are there any future treatments on the horizon?

James:

Yes. Recombinant FSH is in development in North America. It has potential to directly stimulate the ovaries regardless of transition phase. This could revolutionise how we manage both seasonal transition and embryo transfer programmes.

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Key Takeaways for Managing the Anoestrus and Transitional Phases in the Mare

Host:

Before we wrap up, could you share some key take-home messages?

James:

Certainly:

• Light is the main driver—start artificial lighting early (by December).
• Assess the mare carefully before beginning any drug treatment.
• Use the prescribing cascade responsibly, especially when choosing off-label products.
• Tailor treatment to the individual, based on examination and history.
• Refer to the latest literature—I’ve published an article in In Practice (October 2021) that outlines dose rates, protocols, and references.

Host:

Fantastic. That paper sounds like a great resource. Thank you, James, for a really detailed and insightful discussion. Best of luck for the breeding season!

James:

Thanks very much—pleasure to be here.

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Outro

These podcasts are aimed at registered vets and veterinary nurses. If you’re listening as a pet owner, always consult your local veterinary surgeon if you have any concerns about your animal. For more information on Bova UK and the formulations discussed, please email office@bova.co.uk to get in touch with your territory manager.