VETERINARY FEATURE - SWITCH ON THE LIGHTS

Increasing reproduction efficiency of dry broodmares

Every year disappointment and frustration is felt by clients, staff and, yes, even veterinarians when dry mares fail to cycle normally (or maybe even not at all).
There are manifold reasons to try and ensure as early a cover as possible – on or soon after 1 September – and the heartache mounts as those precious weeks starting ticking away with no action.
It is easy to blame feeding, management and a range of imponderables in an attempt to find a reason for this ‘abnormal’ breeding behaviour.
However, and this is a big however, the majority of these mares are behaving completely normally!
Indeed, it is our expectations of how they should be cycling which requires review.

BACK TO BASICS

The horse, in reproductive terms, can be fully described as a seasonally polyestrus breeder: fancy terminology to indicate that she can breed in a cyclical fashion (unless she becomes pregnant) every 21 days or so for a certain and distinct period of the year.

During this time she can be described as reproductively competent.

The ‘seasonal’ part of the seasonally polyestrus description is what is important.

What is the season and what controls it?

Everyone knows the answer to the first part – spring and summer is when horses breed … but why? Warmer temperature? Better nutrition? Better body condition?

All of these factors play a role, albeit minor roles, compared to the main player ... photoperiod (or day length).

Increasing natural day length, or photoperiod, will bring mares from reproductive incompetence (anoestrus) through the vital transition period into the breeding season proper, characterised by regular cycling and ovulations.
(Remember, however, that intercurrent disease may be a potential limiting factor in some individuals – for example the elderly mare with Cushing’s disease may not cycle normally.  Individual medical problem must be weighed up by client and veterinarian and their possible significance - if any - explored).

The mechanism by which photoperiod brings about the resumption of cyclicity is via a highly complex intricate symphony of hormonal and neural reactions and pathways. It is the recognition and understanding of some of these processes which can allow us to attempt to modify (advance) the breeding season.

In August and September, in this part of Australia, natural photoperiod is insufficient to allow the breeding season to be up and running with our artificially imposed 1 September start date.  Therefore, for better or worse, we have to artificially ‘trick’ the mare into believing she should be cycling in August in anticipation of early September breeding.

Work conducted in the mid 1940s was the first to demonstrate the effect of photoexposure (putting under lights) to artificially advance the breeding season.

There are a number of ways of doing this, traditionally the 16 hours of light, 8 hours of darkness regime being the most commonly employed, and the one that we find the easiest to manage at Eliza Park.

One important point must be borne in mind, however. The complexity of the process of transition to breeding must, by definition, be a long process, often taking from 6-8 weeks. It is imperative, therefore, to put the mare UNDER LIGHTS EARLY.

In practice, this means exposure should begin no later than the winter solstice, and perhaps even much earlier.

At Eliza Park we normally commence lights at the beginning of July.

Different protocols may be more appropriate or manageable on smaller farms away from the main breeding farm, and I would encourage any mare owner - contemplating using lights on their own properties prior to mares coming to Eliza Park - to speak with me if you require further advice or discussion regarding the practicalities of the process.    

So what does putting mares under lights and advancing the breeding season really do for the mare and breeders at the end of the day?

By doing so, mares ideally enter the ovulatory season one or more cycles before intended breeding.
This has two advantages.

• It allows us to evaluate, and if necessary, treat the ‘normal’ cycling uterus prior to breeding. More treatment options in potentially problem mares are open to us if the hormonal environment of normal cyclicity is established.
• It eliminates the requirement to predict the first ovulation of the breeding season. This can be notoriously difficult in the transitional mare for many complex reasons and even the use of ovulating drugs can be inconsistent. In fact, herein lies the main advantage: covering mares with a much better chance of normal ovulation (as they have already cycled)

Regression of large, apparently normal preovulatory follicles or abnormal (infertile) ovulations are common when mares are bred at the end of the transitional phase, and the rate of cross cover is thus higher. This is neither good for the mare (especially the older problem mare where we may wish to absolutely minimise the number of covers), nor for efficient use of the stallion.

Multiple covers for an eventually failed ovulation is a ludicrous and inefficient use of a stallion, especially one with a large book of mares.

Bear in mind too that stallions are also subject to the effects of photoperiod! Early in the breeding season the maximum sperm count for any individual stallion has not been reached, therefore we want them to cover mares with the optimum chance of normal ovulation, optimum chance of responding to ovulating drugs and, therefore, ultimately providing them with the best opportunity to conceive!

If we cover on a late transitional follicle, which ultimately goes on to ovulate normally, is this less likely to result in conception?

No evidence points to this being so in the horse (as it is in some other species) but the advantages lie in improving the breeding management during a difficult time of the year on the stud farm

• Decreasing cross cover rate
• ‘better’ follicles, more likely, to ovulate normally and therefore
• better chance of conception

Although a whole story in itself, drugs can, and sometimes must be used in the late transition to provide an additional helping hand. At Eliza Park last season we looked at an old treatment (progesterone therapy) in a new way with some promising results which we are currently in the process of analysing.

So much for the advantages, but what about the disadvantages? Prima facie, the perceived COST. It can be expensive to keep mares under lights for the required period. However, remember the old adage – you have to spend money to make money! In this case, not really making money but rather saving money in the long run through:

• Decreased length of time at stud
• Decreased number of veterinary examinations
• Ultimately smaller agistment bills
• Earlier foals the next year, with potentially better returns at the yearling sales due to body size, exposure to the better sales etc.

Remember, too, all mares are individuals and we endeavour to take them from under lights as soon as cyclicity has begun, thus minimising your costs.

Weighing up all of the above, a simple formula appears: NO LIGHTS = NO BRAINER!

- Dr Alan Clark B.V.M. & S., M.R.C.V.S