b4breeding

As I touched on in my last blog, we’ve got a good range of sizes of sires now, including several that are only just on 15 or 15.1 hands and some that are over 16 hands.

I know that yearlings who are “good size” will catch the buyer’s eye, but my own ignorance about what determines foal size has led me to do a wee bit of research on the topic. Do small sires leave small foals? Do big mares always leave big foals?

Mare Sophie’s Choice with her newly born Tintin In America colt foal October last year

I’ve always followed the rule that you don’t put a very big sire to a very small mare because of the risks of rupturing the mare or making the birth more difficult, and that you are careful around your choice of sire for a maiden mare. Likewise I’m aware that people recommend a smaller sized sire for a particularly big mare, or at least a sire that seems to leave smaller types.

However in practice, finding a sire that leaves a consistent size of foal (big, small or medium) is really difficult. Most of the great ones can stamp in terms of type, but not necessarily in terms of size. Think Bettor’s Delight. Or Courage Under Fire. They often have a certain look about them, but not necessarily size.

It’s more common to find a mare who stamps her foals according to size, particularly if she is big.

And that’s pretty much what the research seems to show. The foals size is more in the mare’s hands – or rather, in her uterus.

But even then, consistency of size is not something sires or mares seem to care about much. Many sires are known for “throwing them all sizes” and mares can be likewise.

My own experience with a medium sized mare over a number of foals is an example. Zenterfold’s her first foal (Tintin In America by McArdle, 16h) was a strong, nuggety foal, not tall but with a decent length of barrel and grew to just 15.1h. Next foal was Zenstar (even smaller in size, by Falcon Seelster, 15.3h). Zenterfold also has left a couple of medium sized types (both by Grinfromeartoear, 15.3) and she has also left a tall colt (by Real Desire, 15.2h) who could end up well over 15.3h, plus a very big type to Geoff and Aria Small who was a colt by Elsu – and of course Elsu is only 15.2h himself.

So there are no golden rules.

Where size does seem consistent, there may be strong enduring family traits (genetic) that are influencing things beyond the immediate match of that mare and that sire.

Here’s a readable article that looks at a pretty scientific study about the comparative size of foals from older maiden mares and foals from mares who had bred several foals.

Here’s a quote from another article that looks at whether crossing a big with a little means you get a medium….

Let us, for a brief moment, take a step back into history and look at the work of Gregor Mendel. He decided to produce hybrid plants by crossing purple flowered plants with white flowered plants. He also did the reverse. He fertilized plants that produced purple flowers with pollen from a white-flower plant.

At that time, one of the prevailing theories was that if plants with opposing colors were crossed, the result would be a plant that was intermediate in color. In the case of the white-flowered plant being crossed with the purple-flowered, the result might have been expected to be a light lavender.

Mendel proved that theory to be groundless. He did not get any plants with intermediate coloration with a crossbreeding program.

All of this brings us back to our earlier scenario involving the breeding of a short mare to a tall stallion. Genetics will dictate that we won’t get the in-between horse we have been seeking. The offspring would have inherited one or the other of the parent’s genes for growth. If it received the mare’s gene for growth, the offspring will likely wind up being short. If it inherited the stallion’s growth gene, it will likely be tall. This does not mean that the offspring will be the exact size of one of the parents. It means that one or the other of the genes-tall or short-will be expressed, and it is unlikely that the resultant foal would wind up midway between its parents in size.

Read the full article here

Another study of sport horses (Brazil, April 2015) focus just on the foal’s weight soon after birth, and is reported by Kentucky Equine Reasearch

Analysis of data showed that bigger, older, and multiparous mares had larger foals than smaller, younger mares foaling for the first time. The foals from larger mares were taller and heavier and had greater thoracic perimeters at birth and also at 7 and 30 days of age. Older and multiparous mares also produced heavier placentas, which is possibly related to the better ability of these mares to allow the uterus to expand during pregnancy. Fetal nutrition is provided by the placenta, and a small or poorly developed placenta may be less able to deliver nutrients and support fetal growth.

There were no significant relationships between sire characteristics and neonatal foal characteristics for the horses in this study.

Or this article in the journal Reproduction – “The influence of maternal size on pre- and postnatal growth in the horse: III Postnatal growth” by W R Allen, Sandra Wilsher, Clare Tiplady and R M Butterfield, which compares the outcomes from ‘restricted’ in utero existence with ‘luxurious’ in utero existence, and concluded their were enduring differences in size but these were not statistically significant.

Check this range of views and evidence about size of mares, sires and foals from this (archived) forum.

And at the other extreme here is the 2008 masters thesis on NZ thoroughbred mares by Luciana de Freitas Aiex.

What seems to be common ground is that the size of the mare’s uterus does have a big influence in most cases the size of a foal at birth, and that is nature controlling things so a mare is not damaged during birthing (although this is not necessarily a fail-safe as we know).

And most studies seem to show that foals born smaller often don’t take too long to catch up. The initial size on the ground is not what will determine how big they grow in the end.

One reply in a forum struck me as very clear and sensible:

“A longer bodied wide ribbed mare will give more room for a foal to develop than a short bodied narrow ribbed mare will. There are always variables such as proper nutrition and mare care. Once the foal is born, again, proper nutrition, worming and other necessary vet type care are will affect the size of the foal. However after that, the final size of a foal seems to be a mix of a range of factors.” (including genetics).

Sophie’s Choice x Tintin In America colt now a strapping type of yearling

So is there a clear answer? It is certainly not as simple as:

• SMALL MARE X BIG SIRE = MEDIUM SIZED HORSE

• SMALL SIRE X BIG MARE = MEDIUM SIZED HORSE

As most breeders will tell you, it is better for a broodmare to have a longer and wider body than to be narrower and short-coupled, regardless of her height.

In other words look for SHAPE rather than size.

The wellbeing of a mare and foal during the whole process of pregnancy (and in particular the proper nutrition of the mare during her pregnancy) and the raising of the foal are just as important in building a horse to the best size it can be.