The summer heat often spell problems for horse's hooves. Lots of factors get blamed, but sometimes it's more a matter of guilt by association rather than a true cause.
Some horsemen believe that turning horses out at night in the summer subjects the hooves to too much moisture from dew on the grass. The theory is that the feet soak up all this moisture, swell, then shrink again when brought inside to dry stalls or worked during the day under dry conditions.
However, a study by Kempsom and Campbell of the University of Edinburgh found that sections of hoof wall could be baked in an oven or soaked in water for two weeks and still remain intact and sealed to the extent that the outer layers wouldn't absorb anything past a depth of three to five cell layers. And dry conditions aren't a problem for the healthy feet, either. (Note we said "healthy.")
Heat may have a little to do with it, but it's more heat from exercise than weather. A study presented at the 2001 American Association of Equine Practitioners (AAEP) meeting reported that feet remain abnormally hot on thermographic readings as long as 24 hours after a fast exercise session. Even low-level exercise raises the temperature of the feet.
Extremely high tissue temperatures, high enough to cause cellular damage, have been documented in some tissues (tendon) following exercise. Since the hoof can't be rapidly cooled (it doesn't sweat and has no surface blood vessels), overheating could contribute to some of the hoof-quality problems that develop in summer, like white line separations/weakness, cracking and even sole bruising.
However, the most prominent reason for hoof-quality issues appearing in the summer is that the problem horse doesn't really have healthy feet to begin with. The predisposing causes are there year round, but the horses are simply being worked more in the summer and that seems to bring the problems to the surface.
