Hoof growth, hoof wear, biomechanics, and trimming (Proceedings)
What we see as the current condition of the claws on any cow is a result of continuous growth, continuous wear, and intermittent trimming. The growth rate of hooves is relatively constant but subject to minor modifications. Nutrition can influence hoof growth rate. Hooves do not grow as fast during starvation as during adequate feeding. Since dairy cattle are never intentionally starved this effect is unimportant. A small variation occurs during the lactation/gestation cycle and with season. The reports on this effect are difficult to interpret since the cattle calved seasonally. In a study we conducted at Cornell on mid-lactation cows, growth rate was influenced by floor surface within the freestall pens. Cows grew hoof faster on concrete than rubber floors. The implication is that hoof growth can respond to environmental conditions by making more hoof when the standing or walking environment is more mechanically insulting. Typical growth rates are about 6 mm of hoof wall per month with variations due to environment less than 10% of control rates. Wear rates are much more variable and depend on the abrasiveness of the walking surface and distance walked. Data is not available to compare the wear rates of dry versus wet hooves on the same walking surfaces.
Why do we see more disease in rear feet than fore? Why is there more disease in lateral rear claws than medial? The fore versus rear argument in dairy cattle has 3 components. The rear limbs of dairy cattle are forced to carry weight in excess of the original design criteria. The wild-type cow which gave us our modern dairy cows never had a large udder, even at calving. As we have selected for more milk production, I do not think we have been able to simultaneously modify the musculoskeletal system to accommodate this extra weight at the rear of the cow. Secondly, the forelimbs are attached to the body by elastic components versus the direct bony connections in the rear limbs. Concussive forces created during locomotion must be absorbed by the digital cushion and the flexion of the hock and stifle. Thirdly, rear feet are always more exposed to the bacteria and moisture of manure and urine. The skin near the hooves is more likely infected with bacteria due to maceration by this moisture and the hoof capsule is softer due to greater hydration. The medial versus lateral argument is potentially more confusing. Lateral claws grow about 10% faster than medial claws and wear about 8% more in freestall housed Holsteins. Thus lateral claws can progressively outgrow medial claws. They are larger even in fetal calves. Larger lateral claws are more heavily loaded than medial claws. Larger loads result in more potential for mechanical insults. Cows may adopt a toed-out posture to help equilibrate the weight between the rear claws when overgrowth and some discomfort occur. This toed-out posture or being "cow hocked" can be used as an indicator of the need to trim an individual cow or by population evaluation to determine when a group or herd needs trimming.
Body weight is supported by the column of digital bones resulting in the load being approximately evenly divided between the eight digits with normal claws and conformation. The third phalanx or P3 is the end of these columns. The load on P3 is supported by several structures of importance in our concern for lameness. There are laminae in the mural corium, that are tightly attached to lateral and cranial portions of P3, and that interdigitate with laminae in the hoof wall. These have less surface area of mutual contact per unit of supported weight than in the horse. Therefore the laminar region of the bovine digit while very important is not as signifigant as in the horse for support. There is also support of P3 by ligaments that suspend the caudal portion of the bone and blend with the interdigital cruciate ligaments axially and with the laminar corium abaxially. The tension of the deep flexor tendon on P3, in addition to fixing the bony column in a nearly vertical formation, pulls the distal tip of P3 ventrally and transfers some weight forward in the claw. Between P3 and the solar corium is a complex arrangement of fat deposits that cushion and distribute weight transferred to the sole. The fat pad is thickest at the heel and plays a dynamic role in cushioning during walking.