Size vs. Genetics - The root cause of foaling risk?

There is a debate about the root cause of the high mortality rates for miniature horses foaling in the USA.  While most agree that there are issues related to the small size of miniature horses, experts are spilt on the root cause of the high risk of miniature horse foaling in the USA.  Some believe that small size is the root cause while other believe that the USA miniature horses experience a higher incidence of foal death because of a poor genetic history.

Size Issues

According to some experts, a sad side-effect of breeding horses exclusively for size are the inevitable manifestations of equine dwarfism and the accompanying genetic anomalies (abortion, dystocia and dwarfism) that are well-know in animal husbandry.  According to Barbara Ashby in an article in Miniature Horse World Magazine:

"Any breed of animals that attempts to reduce size has to accept dwarfism as the unwanted by-product.  We as breeders of miniature horses can do everything possible to minimize the possibility but it will always be with us".

Many breeders will not risk breeding a tiny mare, and vets acknowledge that dwarfism and foaling problems are more prevalent in the smaller class "A" miniature horse mares (those less than 34 inches tall).

Dr. Ron Friedman notes in an article published in Northwest Miniature Horse Club Newsletter, July-Aug 1996, Vol. 1, no. 67, that the smaller miniature horses have far more pre-natal issues than the larger miniature horses:

"Type A mares also have more dystocia and foals with congenital problems (abnormalities developed during the pregnancy) like dwarfism and jaw malalignment."

In an excellent article in TheHorse.com titled Smaller Horses, Bigger Reproductive Problems, Marcia King notes the size risks: (emphasis added)

"Similar to their full-sized cousins, miniature horses risk fewer reproduction issues through good breeding and management  techniques, including screening breeding prospects against undesirable characteristics (such as limb abnormalities or dwarfism), teasing properly, maintaining health management and veterinary care throughout the breeding and pregnancy, and having experienced people attending the foaling. Still, dystocia and abortion are more common in minis than in full-size horses due to foal size and fetal abnormalities.

Unique to minis, size matters. For example, a small mare bred to a medium or large stallion can result in a fetus too large for the mare to deliver normally. Breeding young mares to larger stallions also can result in foals which are too large for the mare to deliver normally (which happens more often in minis than in regular-sized horses anyway). Notes Philippe Vervaet, DVM, in the department of gynecology and obstetrics at the University of Ghent in Belgium, "Ten to 15% of the miniature horses here are foaling with an oversize foal. In half of the cases, it is due to the owner who bred a mare that is too young and the pelvic diameter was still too small."

If you plan to breed a miniature horse that is less than 34 inches tall, you may want to carefully research it's pedigree to search for dwarf horse ancestors or invest in foreign bloodlines. 

Genetic Issues

Heuristic and antidotal evidence suggests that foreign miniature horse breeds (e.g. UK Shetland Pony) who did not deliberately introduce dwarfism in their bloodlines experience far less dwarfism, dystocia and abortions.

Some breeders claim that dwarf horses were deliberately used in USA breeding programs and others note that some widely-bred miniature horse stallions had obvious equine dwarfism characteristics.  Bond Tiny Tim (only 19” tall, AMHA Registration number R 00015P) was said to have suffered from numerous dwarfism-related health issues, yet he was bred extensively, passing-on his potentially crippling genetic mutations to thousands of his descendents throughout the USA. One miniature horse publication titled Breed Standard comes to the same conclusion:

"In the past century, dwarf horses were purposely used in breeding programs because of their diminutive size (even compared to the Miniature!).  Thus, the Dwarf gene is floating around in many bloodlines today, and it is difficult to locate as the gene has not been identified by geneticists."

Click here to learn about genetics in miniature horses

Conclusions

Regardless of the root cause, it is clear that there are variations in foaling mortality between USA breeding programs.  

J. C. Williams, owner of the Del Tera Miniature Horse Farm in Inman South Carolina, claimed that breeders have a huge variation in the rates of miniature horse breeding death, with some low-investment breeders loosing up to one-third of their mare and foals each year.

Vets claim that high-investment miniature horse breeders experience a much lower mortality because of the availability of medical intervention and hospital c-sections.  Remember, the risks can be high, but much of the mortality risk can be reduced by investing in a horse birthing facilities with 24-hour vet and surgical availability. 

• Click here to read about hospitalization for miniature horse parturition
   
• Click here to read about equine c-sections for miniature horses.

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