Peter Green MRCVS examines some fascinating new research on the science of size

Vet students are taught that it is the size of the dam rather than the sire that affects foal size.

Classic 20th century experiments revealed that a shire stallion crossed with a Shetland mare produced much smaller offspring than the opposite mating.

But does this follow with all breeds and in less extreme crosses, or where embryo transfer is used? Is the effect just a function of the larger uterus of the larger mare, and does increased size last through life?

French vets bred 21 pony foals, measuring foal size and growth rates from birth to weaning at 6 months. They did the same with 28 pure-bred saddlebred foals.

They then transferred purebred pony embryos into 6 recipient draught mares, purebred saddlebred embryos into a further 8 draught mares and pure-bred saddlebred embryos into 6 pony mares. The resulting foals were measured carefully as they developed.

The results were fascinating. The pure-bred saddlebred foals transferred into large draught mares were no bigger at birth than those carried by saddlebred dams, but grew much faster up to weaning. After weaning, the youngsters carried by their biological mothers caught up.

In contrast, pony embryos transferred into draught mares were more than 50% heavier at birth than those carried by their biological mothers. They were still much bigger and stronger at 18 months of age.

The saddlebred embryos transferred into small pony mares resulted in foals that were much smaller and lighter at birth. After weaning, they caught up with siblings carried by their biological mothers.

It seems that an embryo from an average-sized horse put into a pony will suffer disadvantages up to weaning, but can then catch up with naturally bred horses of the same breed. But a similar embryo transferred into a big draught mare will gain a lifelong size advantage.