Horizontal fissures result from disruption of horn production at the dermis beneath the coronary band, leading to a defect in the integrity of the wall. These fissures run parallel to the coronary band. The defect varies in severity from a shallow groove (hardship groove) to a complete fracture (fissure) of the wall. A comparable anomaly is seen as a band of horn differing in appearance from the remainder of the claw. One form of the band is seen in animals stressed after weaning (weaning groove) or during a period of nutritional deprivation. The fissure moves distally as the claw grows, and the distal portion becomes progressively more mobile (thimble) until it fractures, leaving a “broken toe.” A series of grooves can destabilize the vertical strength of the dorsal wall, causing it to bend (buckled toe).
The horizontal groove or fissure is an important indicator of metabolic disturbance. The date on which the causal insult occurred can be calculated by measuring the distance from the hair line to the fissure and dividing that number by the growth rate of the claw. In mature dairy cows, the rate of growth of the wall measured along the dorsal flexure of the claw is ~0.5 cm/mo. Growth rates are more rapid in young animals, in animals on intensive feed, and during the summer months.