The hoof is a dynamic structure that undergoes continuous changes throughout a horses’ lifetime. Those changes, however, hadn’t been completely document until recently, when researchers from the United Kingdom studied alterations in hoof wall structure and in the hoof capsule, hoof shape changes, and hoof color changes. Additionally, they documented the rapid speed of hoof renewal hoof growth in foals, weanlings, and yearlings.
Simon Curtis, PhD, BSc(Hons), FWCF, who served as principal researcher in the study series, is a farrier based in Newmarket, England. He presented the research findings to an audience of farriers and veterinarians at the 2017 International Hoof-Care Summit, held Jan. 24-27 in Cincinnati, Ohio.
During his presentation, Curtis shared information about the hoof’s development through the various stages in the young horse. We see the importance of the hoof and its ability to function immediately when a foal stands within an hour of birth, then (in the wild) travels 10 to 20 miles per day as the herd searches for food and water, he said.
Hoof Wall Morphology
Curtis’ first study looked at the epidermal (inner hoof wall) width and the density of the hoof wall in cadaver foals from the age of a 10-month fetus through a 4-month-old foal. Curtis took samples from the hoof wall on the medial (inside of the leg), lateral (outside of the leg), and dorsal (front of the hoof) aspects and found the hoof tissue was symmetrical on the medial and lateral sides, but that the dorsum was thicker. In mature horses, typically the medial hoof wall is thicker.
It has been assumed that the epidermal layer of the hoof wall becomes thicker as the foal ages, but it was not known how this thickening took place, Curtis said. His team showed that the hoof wall thickens as foals age in three ways: The horn tubule density decreased as the gap between horn tubules increased, the diameter of the tubules increased, and there were more horn tubules within the hoof wall in older foals.
Hoof-Pastern Axis, Phalangeal Axis, and Hoof Wall Angle
In another study, Curtis’ team looked at changes in the pastern angle and the dorsal hoof wall angle in foals to determine if what farriers see on the surface mirrors what’s happening on the inside as it does in mature horses. He also wanted to determine the typical dorsal hoof wall angle for pediatric foals. The team radiographed 22 Thoroughbred foals and measured the hoof-pastern axis (the difference in alignment of the hoof and pastern) and the phalangeal axis (the difference between the angle of the pedal bone and the pastern bones).
All of the foals in the study had a broken-back or negative phalangeal axis, and the hoof-pastern axis was positive (broken-forward). This means that, unlike in mature horses, what we see on the outside is different than the internal structures. This means that by eight to 11 months of age, the hoof-pastern axis still cannot be confidently used for conformational assessment, although alignment was improving.
The hoof-pastern axis was straight toward the end of the study for many of the foals. These findings verified that the phalangeal axis does not match the hoof-pastern axis as it does in mature horses.
Meanwhile, the team also recorded a decline in the dorsal hoof wall angle of 5.5° from an angle of 59.5°, which Curtis noted is a remarkable change. Mature horses tend to have a dorsal hoof wall angle just below 50°. Because of this, he emphasized that defining the club foot as one with a dorsal hoof wall angle of 60° was not a good definition, especially for Thoroughbred foals. The researchers noted no change in hoof shape in young foals.
Based on findings in adult horses, the team also expected in horses with one hoof steeper than the other, the steep hoof would be narrower than the shallow hoof. However, they did not see this correlation in foals.
Hoof Wall Growth and Renewal
Growth rings occur for a variety of reasons, such as illness, a change in environment or nutrition, and lameness. As the foal hoof grows downward, the foal hoof crease (which forms at birth in the hoof wall) moves down the hoof wall, which helps farriers note changes since birth. One of these changes can include a change in hoof color, as Curtis and his team noted.
The team document hoof renewal time for the foals based on the foal hoof crease and how fast it grew out, which took about 145 days (+/- 15 days). The researchers also measured this in weanlings/yearlings, with 283 days (+/- 29 days) result. In mature horses, hoof-renewal rate is 337 days. This documented that while hoof renewal starts out at a rapid rate, it slows as the horse ages.
They also measured hoof growth rate in foals, which was around 0.43 millimeters per day. Weanlings had a growth rate of 0.28 millimeters per day, while mature horses have been previously measured to have a growth rate at 0.2 millimeters per day.
Curtis attributes this rapid growth to foals having to immediately stand up and follow their dams, and if the hoof wall does not grow quickly, the foal’s hooves would wear down too quickly from travel.
“The rate of growth is important when it comes to the horse’s ability to overcome excessive wear and its ability to replace damaged horn,” said Curtis, noting that the foal also needs to shed hoof quickly in order to allow the growing distal phalanx to expand.
“The hoof wall structure has been described as a miracle of bioengineering, and it is a miracle really,” Curtis concluded. “We ought to celebrate just how successful the hoof is.”