Foal Birthweights: Does Size Matter?

Have you ever wanted to breed a sales-topper? Or, better still, raise a stakes winner? Without promising to offer any secret formula to the aforementioned objectives, a recent study published by Dr Joe Pagan of Kentucky Equine Research in association with Saracen Horse Feeds takes a deep dive into the subject of how the size of thoroughbred foals ultimately effects their future sales price and racing performance.

Using tens of thousands of data records from prominent breeding nations across the world, Dr Pagan not only looks at the relationship between foal size and their achievements in the sales ring or on the racecourse but also, and perhaps most crucially, skeletal soundness.

With the benefit of international analysis on a scale never before seen in this field, the findings are significant; providing breeders and stud farmers with the knowledge to potentially adjust practices in pursuit of most favourable future outcomes.

What affects a foal’s birthweight?

Before digging into the correlation between foal size and performance, the research initially highlights the factors affecting a foal’s birthweight in the first place. 

Assessing a pool of over 3,000 birthweights, the basics show that in every region fillies were 1.5kg smaller than colts and America bred the heaviest foals, followed by the UK, with the lightest being born in Australia.

In addition to gender and region, one of the most significant elements to affect birthweight is the number of foals that a mare has had before – also referred to as parity. It may come as little surprise that maiden mares produce the smallest foals but most interestingly these first foals were on average 15% lighter than those out of mares that had already produced more than two foals previously (multiparous mares); adding scientific significance to the anecdotal ‘first foal’ theory.

The month of birth had a considerable impact on the weight of foals as well, with those born earlier in the season much lighter than those born later on. Of course, the likelihood that maiden mares are often bred towards the beginning of the season might explain some of the reason why January and February foals were lighter. However, equal gravity may be given to the availability of better pasture for mares foaling in springtime (April and May), with Dr Pagan suggesting that nutrition of mares at the time of foaling is also an important determinant of birthweight. 

Does yearling size affect sales price and racing performance?

As the study continues, percentiles and quartiles are referred to in order to make comparisons among varying sets of data. In simple terms, percentiles rank the size of an individual based on its age and gender in relation to a wider population on a scale from 1 to 100, while quartiles divide this further into quarters. The first quartile is the bottom 25%, the fourth quartile refers to the top 25% and so on.

Dr Pagan’s research goes on to look at the relationship between the size of yearlings – measured by both body weight and wither height – and sales price, as well as racing performance. 

At the Kentucky yearling sales, a premium was paid for the biggest yearlings available – 55% of yearlings purchased for over $250,000 were in the fourth quartile for weight and 75% of those purchased were in the third and fourth quartiles for height. 

In the UK, the situation was a little different. Purchasers at the sales on this side of the Atlantic preferred a lighter yearling, with 70% of those sold for over £200,000 being in the second and third quartiles for weight; the same was evident for wither height; UK buyers favouring a slightly shorter type (59% being in the second and third quartile). 

Regular transatlantic racegoers and sales attendees might find these differing trends unsurprising given the powerful model of equine athletes that run over dirt in the US versus the slighter frames of those that compete on the turf in the UK. 

And here’s the thing: 43% of Kentucky stakes winners were also in the fourth quartile for weight as yearlings and 62% were in the third and fourth quartiles for height. Proving that, fundamentally, the heaviest and tallest yearlings which found the highest favour at the sales in Kentucky, also achieved greatest success on the racecourse.

Reassuringly, racecourse results in the UK also stack up with yearling sales trends, whereby horses in the second quartile for yearling weight also took home the highest number of stakes races on home turf. 

How does foal size affect OCD and racing performance?

Having established factors affecting birthweights and, subsequently, yearling size in relation to sales results and racing prowess, the study moves on to analyse trends in skeletal soundness. For the purposes of this study, this may be defined by the incidence of surgery for osteochondrosis dissecans (OCD). 

When looking at foal size and its correlation with skeletal soundness, results clearly show that during early life (1-90 days old) foals that had OCD surgery were far heavier than those that did not have surgery or, indeed, those that became stakes winners.

It is important to note, however, that as foals grew to become weanlings and yearlings (241-360 days old) weight no longer had a significant effect on OCD surgery. When it came to height though, the tallest foals at all age ranges (0-360 days old) had the highest incidence of OCD surgeries than any other groups.

When solely looking at birthweights, research highlighted that the very heaviest foals (born in excess of 64kg) were twice as likely to have OCD surgery than the rest of the population (foals born under 64kg) and only one-third as likely to win a stakes race.

Put simply, skeletal unsoundness was most prevalent in the heaviest-born foals but had less significance in foals that were of normal size and grew to become heavy weanlings or yearlings. And while weight caused most issues in the early life of foals, height was relevant throughout the first year, with the very tallest foals being at most risk and having the highest incidence of OCD surgery.

These findings neatly tie-in with the fact that foals that went on to reach the top of their game and win stakes races were a normal size as foals and became heavier, but not taller, as weanlings and yearlings. Still following? Basically, you’re better off having an average-sized foal at birth that becomes a big yearling than a big foal from the start.

Does month of birth and parity have an impact on skeletal soundness?

The early part of Dr Pagan’s study clearly showed that a foal’s birthweight was significantly affected by the number of foals a mare had had previously, and also their month of birth. It goes on to highlight the strong relationship between these factors and the likelihood of OCD surgery. 

In Kentucky, foals born in April had by far the highest incidence of OCD surgery, while in the UK it was May-born foals that had most skeletal unsoundness. It is then perhaps no coincidence that the heaviest foals were born in April in Kentucky and May in the UK – emphasising further still the correlation between birthweight and OCD surgery.

Evidence showed that primiparous (maiden) mares produced foals that were less likely to have OCD surgery than mares that had had foals previously which would also make sense, given what we learnt earlier about maiden mares producing lighter foals compared to their more experienced counterparts. 

While parity in mares, can go some way to explaining why birthweights are higher and OCD surgery is more likely in foals born in the spring – because maiden mares are generally bred earliest in the season – the study places as much importance on the level of forage available around the time of foaling which can impact the nutrient intake of mares, foetal growth and milk production.

It suggests that being able to adjust a controllable element such as nutrition through understanding how a mare’s due date and parity could affect its foal’s birthweight, there is potential to improve sales-ring success and athletic ability, not to mention skeletal soundness.


So, what’s the take-home message? In truth, there are many. And to reduce this level of research into a single sentence is possibly a disservice. But in basic terms, size really does matter, and it matters most during the early life of foals. So where size – and specifically birthweights – can be manipulated through feeding practices then so much the better.

Presentations by Dr Joe Pagan and Nick Wingfield Digby MRCVS at Saracen’s Thoroughbred Growth Forum last June can be found via this link.

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