Pony Contributions to the Study of Breeding
Based on their 14.2hh height, these Percheron Welsh ponies are likely out of Percheron mares.
As a breeder, I often find scientific reports on the subject of breeding to be interesting. And as a pony breeder, I love hearing about research conducted with the help of ponies. I was thrilled, then, to learn about Walton and Hammond’s work in the 1930s with Shetland Ponies and Shire Horses. I found reference to their work in a couple of modern-day research papers, suggesting that the findings from back then are still relevant. Then I found a few studies that have been conducted since then, also using ponies.
Walton and Hammond used artificial insemination (AI) to cross Shetland mares on Shire stallions as well as the opposite: Shire mares on Shetland stallions. They also had purebred foals for comparison. They were expecting to see that the crossed progeny would be intermediate in size and similar to each other regardless of dam. They were surprised. What they found was that crossbred foals out of the pony mares at three years old were larger than purebred Shetlands, but they were smaller than the comparable crossbreds at the same age out of the Shire mares. At birth the foals born to the Shetland mares were comparable in size to a purebred Shetland foal at birth, while the crossbred foals born to the Shire mare were larger but not as large as a purebred Shire foal. “The cross-foals from the Shire mare were three times the size of the cross-foals from the Shetland mares.” (1)
Through eighteen months, the crossbred foals grew differently depending on who their mother was and as compared to purebred Shetlands. “The foals from the Shire mares grew much less rapidly than pure Shire foals, and the foals from the Shetland mares grew much more rapidly than pure Shetlands.”
A paper published in 1999 was based on work with Polish ponies. In this case embryo transfer was used, and observations were made up to 13 years. The recipient mare was a larger heavier type of equine. From birth up to weaning, the foals born from the larger, heavier recipient mare grew faster and put on more weight than the nearly genetically identical foals born to pony mares. (2)
Another study published in 2004 also used embryo transfer, this time comparing Thoroughbreds carried by Thoroughbreds, ponies carried by ponies, Thoroughbreds carried by ponies, and ponies carried by Thoroughbreds. The first two pairings were conceived by AI and the latter two were ET. “The Thoroughbreds carried by the Pony mares were merely scaled down versions of the Thoroughbred-in-Thoroughbred controls while the Ponies carried by the Thoroughbred mares were scaled up versions of the Pony-in-Pony controls.” (3)
All of these studies show that the maternal/uterine environment plays a role in the size of the foal at birth as well as the foal’s size at maturity. It doesn’t matter whether the fetus/foal is a genetic offspring of the mare that carries it or whether it is genetically unrelated and placed in the maternal environment via ET. I’ve always been told that mothers influence offspring more than fathers, which I have found confusing based on my understanding of genetics. Now I have a new appreciation for how maternal influence happens outside of genetics!
Walton, A. and Hammond, J. “The maternal effects on growth and conformation in Shire horse-Shetland pony crosses,” School of Agriculture, University of Cambridge, Revised 15 February 1938.
Tischner, M. “Maternal influence on pre- and postnatal growth of foals born after embryo transfer,” Journal of Reproduction and fertility. Supplement, 31 Dec 1999
Allen, W.R., et al. “The influence of maternal size on pre- and postnatal growth in the horse,” Reproduction, 2004 Jan.
© Jenifer Morrissey, 2020