Hoof wall cracks are divided into two categories – vertical (sand cracks) and horizontal. If a crack goes clear through the hoof horn, it causes pain and lameness.
Paul Greenough, DVM and professor emeritus in Saskatchewan’s Western College of Veterinary Medicine (WCVM), has studied hoof cracks for many years. In that time, he’s examined the feet of thousands of cattle in western Canada and says dry conditions are probably the biggest predisposing factor.
Meanwhile, Jan Shearer, Iowa State University professor and Extension veterinarian, says vertical wall cracks are much more common in beef cattle than dairy cattle, but horizontal cracks are common in all cattle.
Cracks that run parallel to the coronary band often represent a benign physiological change that creates disruption of hoof-horn formation, Greenough says. Rings around the hoof are normal because the hoof wall grows at different rates during various seasons and physiological reasons, however the growth rings occasionally become cracks.
“The hoof horn is like a history book,” Greenough says. “You can see what happened to the animal at any given time during the past 12 months. Some growth lines become deep horizontal grooves, which means something happened to that cow. If a cow had a retained placenta, difficult calving, or a long period of really bad weather or some other stress, you may see a deep groove in the hoof wall.”
Shearer adds that every time a cow calves, she undergoes a period where hoof growth slows or stops.
“A physiological change is affected by hormones and metabolism as she goes from non-lactating to lactating,” he says. “Interruption of hoof-horn growth creates a horizontal groove, or a growth arrest line, which is associated with calving,” he says.
Seasonal changes are another factor in growth rate. This may be partly nutritional (when feed is plentiful, with green grass), or due to longer hours of daylight. “Growth rate is affected by physiological factors. Some of these things aren’t well understood, but people need to realize that not all horizontal rings are indicative of cattle disease,” Shearer explains.
Some rings, for instance, are associated with extreme changes in nutrient level, balance or availability. “With horizontal growth arrest lines, we sometimes see a true hardship groove – an extremely significant or deep horizontal groove – which we associate with something beyond normal physiological changes,” Shearer says.
Greenough says a nutritional glitch, disease or some type of shock causes the horn to stop growing. When horn formation resumes, a full-thickness crack sometimes develops.
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“Severe disruptions in hoof-horn formation create ridges and grooves,” Shearer says. In the most extreme cases where the fissure is deep enough to create a full thickness defect in the wall, this lesion is called a ‘thimble’.”
When a deep crack gets down to the level of the corium – the sensitive “quick” underneath – the fragment below the crack may become separated from the hoof wall above it. “As this fragment (thimble) is growing down the foot, a deep crack may become painful because every time that fragment moves, it pinches the underlying corium tissues,” Shearer says. The fragment must be removed or the cow will be in constant pain and won’t travel, he adds.
Fewer than 1% of dairy cows develop vertical wall cracks (sand cracks), but up to 64.5% of beef cows get vertical cracks, Shearer says. More than 80% of these cracks occur on the outside claw of the front foot. The inside claw is slightly larger, and bears most of the weight, but there is more movement and strain on the outer claw.
Greenough says 27% of all mature beef cattle in Saskatchewan have a sand crack; incidence can be as high as 65% in some pedigree herds. These cracks also are more common in heavy and older animals. And, size of the foot in relation to the weight of the animal plays a role.
“We don’t know why most cracks appear on the outside claw of the front foot, but this may be due to the weight and conformation of those particular animals that puts additional stress on that claw. There is a genetic component, but actual causes are far from clear; there are probably several contributing factors,” Greenough says.
“Some people think cracks are related to the fact that front feet have a steeper angle than the hind foot and are subjected to more stress. Yet dairy cows have the same steep angle in the front feet, and don’t get vertical cracks,” Shearer adds.
Based on the numbers, there seems to be a breed predisposition to wall cracks, Shearer says. For instance, some family lines, in any breed, have more tendency to crack. And, in certain herds, the cows that develop vertical cracks are often related.
“Other factors associated with vertical cracks include vitamin and trace mineral deficiencies,” Shearer says.
Dry, brittle feet may be prone to cracking. “We see brittle feet in dry regions,” Greenough says. “The moisture content of hoof horn is probably a factor. The outer layer of the hoof is waterproof, but this protective layer wears off in sandy environments allowing moisture to evaporate through the hoof wall.”
Chris Clark, DVM and an associate professor of large animal medicine at WCVM, studied the causes of hoof cracks for his master’s degree project.
“We didn’t come up with complete answers, but did discover that the water content of hoof horn varies considerably throughout the year on western Canadian prairies. In summer, the moisture content of the hoof is close to optimal. But as cattle go through winter, the hoof dries out,” he explains.
In the cold, dry climate, feet are not exposed to water, because snow is dry, not wet, he explains.
“Humidity is extremely low here in winter. We took hoof samples of cattle in February, when it’s typically -30°C. As the hoof wall dries out, it becomes brittle and less pliable. Heavier cattle probably have more force on their feet, and brittle feet may crack,” Clark says.
One factor he investigated was whether long toes made claws more susceptible to cracking, due to added strain.
“We found bigger feet were more prone to cracking, rather than a longer toe. We examined hooves from cows at slaughterhouses – cows that did and did not have sand cracks – and did biomechanical testing of the hoof; we were unable to find a difference. Many theories have been proposed regarding causes, but my final conclusion was that drying was the biggest risk factor,” Clark says.