As a contribution to the series of articles running over the next few issues on starting and producing the young horse, we are summarising here the work of Dr Deb Bennett PhD of the Equine Studies Institute, for the insight that it gives into the skeletal development of the juvenile horse, and the important implications for the age at which we start our horses under saddle.
Dr Deb Bennett is a graduate of the University of Kansas, and was based early in her career at the Smithsonian Institute. She founded the Equine Studies Institute in 1992, and is known as an authority on the classification, evolution, anatomy, and biomechanics of fossil and living horses. She teaches anatomy short courses, and runs horsemanship clinics for riders of every skill level. She has strong – sometimes controversial - opinions on the training of the young horse, (rollkur, for instance, she regards as an abomination), and has written extensively on breeding, the history of equitation, and the evolution of the horse.
Her best known article on the Timing and Rate of Skeletal Maturation in Horses has become known as “the Ranger piece” for the Tennessee Walking Horse, Ranger, that it features. In fact, the catalyst for the original article was the breakdown of several high-profile racehorses on American tracks, including the legendary Barbaro, due to catastrophic fractures.
In examining what happens to 2- and 3-year-old thoroughbreds in training on the track, Dr. Bennett explores the way that bone in the developing horse can be remodelled through fast exercise, impact, and being galloped on hard ground.
She explodes the myth of “pre-conditioning” leading to the development of “super bone” in the immature horse and argues that “no horse, of any breed, in any country,
at any time in history either now or in the past, has ever been physically mature before it is five and a half years old – and that would be small, scrubby, mares living on rough tucker. Healthy, domestically-raised males, and many females, do not mature until they are six.” The piece, however, is not intended as an attack on the racing industry but to provide breeders and owners with a clear explanation of the facts of skeletal development so that they can make the best decision as to when a young horse may be started under saddle.
Dr. Bennett points out that the external appearance of a young horse can, in some breeds, lead to a seeming level of maturity which doesn’t reflect the actual development of the skeletal system. The key statement is “all horses of all breeds mature skeletally at the same rate”.
Skeletal maturity describes the point in the horse’s development when the growth plates have converted from cartilage to bone, fusing the epiphysis or bone end to the diaphysis or bone shaft. So should you wait to break a horse to saddle until this stage? Dr Bennett’s recommendation is “the longer you wait, the safer you’ll be”. She provides a clear overview of the schedule of fusion to enable breeders and producers to make a decision on when to ride a horse based on an understanding of its internal development rather than external appearance. As a generalisation, the stages of converting growth plates to bone proceed “from the hoof upwards”. The growth plate at the top of the coffin bone is fused at birth. So coffin bones get no taller after birth, although their breadth increases as the horse grows.
The last growth plates to fuse are those of the vertebral column; there are 32 vertebrae between the back of the skull and the root of the tail, with several growth plates on each. Those that cap the centrum don’t finally fuse until a horse is anything between 5 ½ to 8 years old, depending on height and gender; male horses are slower to complete this final phase. Within the chain of vertebrae, the last to fully close are those at the base of the neck.
Dr Bennett spells out the implications for the training of the young horse: that structural damage to the horse’s back from riding him too early is more likely than any damage to his legs.
She describes how a horse will brace his legs, the muscles of his topline and his diaphragm to prepare himself to bear the weight of a rider, and the younger your horse, the more likely it is that this behaviour will become habitual.
Essentially he is trying to compensate for the lack of a strongly developed musculature. “Any horse that does not know how to move with his back muscles in release cannot round up”. A horse’s back needs to be free to oscillate both up and down and from side to side in time with the motion of its legs.
Dr Bennett’s writings have obvious implications for the training of young dressage horses and might give reason to reflect on the increasing pressure to start the competition careers of showjumpers and eventers at 4 and 5 years old. The risk must be that a higher number of these horses will suffer injury or break down as a result of early damage to an immature skeletal system.
A Definition of Terms
Diaphysis : bone shaft
Distal : situated away from the point of origin or attachment, further from the centre of the body
Epiphysis : the end part of a long bone, initially growing separately from the shaft, separated from the main body of the bone by a layer of cartilage, and subsequently uniting with the bone through further ossification.
Ossify : turn into bone or bony tissue
Proximal : situated nearer to the centre of the body or the point of attachment
For full details of the Ranger piece, the Equine Studies Institute, and a list of Dr Bennett’s publications: