Where is the research that says certain cannon bone measurements make a better horse?

I continue to search for scientific evidence that a thicker, shorter cannon bone makes for a better horse, because, otherwise, it would seem odd or irresponsible that Connemara societies the world over would be requiring cannon bones of a certain width and length for a Connemara to pass an inspection. At the very least, officials would have determined that thicker, shorter cannon bones are not detrimental to the horses, right?

I spent four hours combing through research papers this week, and I found a lot of interesting information on cannon bones, though none of it tops my review of Milton Hildebrand’s paper in my previous post.

When it comes to cannon bone strength, what seems to matter is the density of the bone itself. Connemara inspections don’t radiograph cannon bones and evaluate density.

Let’s look at some of the research:

A study of horses participating in the 100-mile Tevis Cup endurance ride in 1998 looked at cannon bone circumference compared to performance. It also looked at rider weight compared to horse weight. The study concluded that human bodyweight was not a critical factor in predicting performance and cannon bone circumference did not have a direct effect on performance. The cannon bone circumferences of horses participating in the ride that year ranged from 17.1 to 21.3 centimeters, and the mean was 19.25 centimeters. Those numbers translate to 6.73 to 8.39 inches, and a mean of 7.58 inches. So, some of the horses in the most grueling endurance ride in America have cannon bones a quarter inch smaller than the Connemara minimum requirement. The results of the study were published in the 1999 Proceedings of the Equine Nutrition and Physiology Society and posted online.

A study published in the Journal of Biomechanics in 2007 says that larger cannon bone pieces appear to fail more rapidly than smaller cannon bone pieces. This is due to larger volumes of material tending to have higher probability of containing a flaw. Researchers tested pieces of bone taken from the cannon bone as opposed to whole cannon bones, and the larger pieces failed first. The researchers concluded: “Smaller stressed volumes of cortical bone possess longer fatigue lives than similarly tested larger stressed volumes.” It’s difficult to conclude from this study that larger whole cannon bones would fail first, as researchers did not specifically say this, but that theory can’t be ignored.

Not surprisingly, several studies have looked at thoroughbreds’ legs. Research published in the May 2006 issue of the Journal of Equine Veterinary Science took several measurements, including cannon bone circumference, of 260 yearlings and compared those to career racing performance. The findings suggested that wither height, body height, hip height and heartgirth influenced win percentage, with bigger being better. But the study specifically said that “leg traits were not favorably correlated with any performance parameters.”

A study published in 1990 in Acta Agriculturae Scandinavica that looked at growth in foals took body measurements of 488 Finnhorse foals and then continued to do so at ages 6, 12, 18, 24, 30, 36 and 48 months. The study found that the “sex of a foal primarily influenced cannon bone circumference. In general, males tended to be taller, longer and wider but lighter and leaner than females.” So, a circumference and shortness requirement for cannon bones works against young stallions — and perhaps older ones — who are more likely to have longer and leaner cannon bones.

An abstract of a dissertation presented in December 2004 to the Graduate School of the University of Florida looked at the effects of housing, exercise and diet on bone development of yearling horses. The author said changes in housing, exercise and diet have been found to impact the final quality and quantity of bone. She concluded:

“In all studies, housing significantly influenced the quantity of bone with pasture-housed horses maintaining a higher rate of deposition. Forced exercise did increase the BMC (bone mineral content) of dry lot-housed yearlings, but (the bone mineral content) did not exceed that of their pasture-housed contemporaries. Analyzing the cortices of the third metacarpal (the cannon bone) indicated that pastured yearlings changed the geometry of bone when compared to both dry-lot housed yearlings with or without exercise. It is not known at this time whether quantity of bone or geometry indicates overall strength and ability to withstand strain.”

So, living conditions hinder some horses from building bigger cannon bones, though there is no evidence to suggest that the bigger size influences strength. It just puts those horses at a disadvantage if we’re judging them by their cannon bone circumference.

I’ll throw one bone, excuse the pun, to tall and heavy Connemara owners who want to ride ponies, and that scenario might become more prevalent, given that people continue to get taller but inspectors are requiring Connemaras to stay under 14-2 (I personally think people should stick to riding horses appropriate to their size). One study indicated cannon bone circumference might be a factor in the soreness of light horses having to carry heavy people. Of course, Connemara inspectors don’t want light horses, so this may be irrelevant.

The study specifically looked at whether horse height, cannon bone circumference and loin width can be used as indicators of weight-carrying ability in light horses. Eight horses were evaluated carrying 15, 20, 25 and 30 percent of their body weight and evaluated for muscle soreness and muscle tightness using a subjective scoring system. The study found that horses had a greater change in muscle soreness and muscle tightness when carrying 25 percent of their body weight and a significant change in soreness and tightness scores carrying 30 percent of their body weight. It concluded that the data suggested that horses with wider loin and thicker cannon bone circumference became less sore when carrying heavier weight loads.

The consistent message across many studies was that the makeup of the bone is what matters.

An article titled “Bone Biomechanics” and written by veterinarian John Kohnke of Australia look at the growth of the cannon bone in a young horse.

He says mineralization of skeletal bone starts in the unborn foal during the last three months of gestation as it doubles in size. A newborn’s skeleton has only 17 percent of mature bone mineral content and a yearling has 76 percent.

He says immature bone is made up mainly of cortical (or external) compact bone and porous reinforcing bone within the bone’s internal structure. Mineralization of cortical bone with the deposit of calcium, phosphorus and magnesium, the principal bone minerals, continues as body weight and exercise loading increase throughout the first year. Subchondral bone (which supports the cartilage) and structural shaft bone must continually adapt and strengthen by additional mineralization. This bone also must repair itself after being subjected to loading, compression and concussion during exercise. After exercise, the bones remodel or adapt their internal structure and strength by increasing the cortical cross-section, circumference and mineral density.

The author says a technique of radiograph index can measure the cortical thickness of the front cannon bones to assess remodeling relative to the stage of training in young thoroughbred horses and may be used to predict the extent of bone reaction and shin soreness.

Note that this index looks at the cortical thickness and remodeling of the bone rather than the circumference as a gauge of the quality of the bone.

The one study that didn’t seem to exist was one by any Connemara official looking at any performance or strength ramifications of requiring cannon bones to be between 7 to 8 inches and short.