One of my father’s colloquiums was describing something, or occasionally someone, that was not straight as “crooked as a dog’s hind leg.” I am not sure why he chose the dog as the example species, because many other species stand with flexion in their stifle and hock joints, like the bovine, for example. But, I suspect he learned it from someone else. Cows do stand with considerable set in the stifle and somewhat less of an angle in the hock. When I first started practice years ago, more cows had too much hock angle, making them subject to lameness and mobility problems that often led to early culling. Dairy genetics and farmers have done a great job correcting that problem and I almost never see it anymore.
    However, starting 10 or 15 years ago, I started seeing something else that is now common and an even more serious problem: legs that are too straight. Typically, the picture is like this: The farm has beautiful, sound cows and does a great job raising heifers, typically calving in well-grown animals that are more than 85% of their adult weight at 22 or 23 months of age. Heifer calves look great, and there is no evidence of straight hocks. However, around the time of first breeding, some animals begin to show lack of angle in their hocks, and usually a month of two later, the straight legs become obvious. Often, the animals eventually stand with one leg off the ground and held backward. Many will make it through first calving, but often get culled early in their first lactation. Some get culled before they calve because they have so much difficulty walking. Occasionally, there seems to be problems with the stifle joint; these animals push their knees outward and have a very stiff-legged walk. We used to see just an occasional animal, but now we see herd outbreaks where a significant number of animals, perhaps 10% or more of the annual replacements, are culled due to straight rear legs. This is a significant problem because raising heifers is expensive and farmers now typically try to raise only about as many as they need.
    What causes this? I asked just about everyone I know who might have some knowledge, and the consensus was genetics. Apparently, there is nothing in genetic evaluations that penalize a cow for having legs that are too straight, only too soft. I do not doubt this to be true, but I do not believe this is the ultimate cause. In other species, including dogs, horses and humans, rapid growth is thought to result in abnormalities of bone that can result in limb deformities. For example, in pigs and horses  there is a condition called osteochondrosis, or osteochondrosis dissecans. According to the American College of Veterinary Surgeons, it is a “relatively common developmental disease that affects the cartilage and bones in the joints of horses” and is caused by: rapid growth and large body size; nutrition: diets very high in energy or with low copper levels; genetics: risk may be partially inherited; hormonal imbalances: insulin and thyroid hormones; and trauma and (normal) exercise.
    In a recent conversation with Dr. Nigel Cook, of the University of Wisconsin College of Veterinary Medicine, he indicated that he too has seen a significant increase in straight-legged dairy heifers, and he is convinced that while “we should not let the geneticists completely off the hook yet,” the major cause is probably rapid growth rate. He suggested that the mechanism may be premature growth plate closure in the long bones of affected animals.
    His theory bears merit because dairy producers have been successful and growing heifers to adequate calving weights much faster than in the past. Rapid growth is thought to be a primary cause of similar problems in other species. The actual mechanism is unknown in those species but probably involves failure of cartilage to grow correctly and eventually to ossify or become bone. Sometimes loose flaps of cartilage appear in the joints. So, most likely, rapid growth is involved in the condition we observe in dairy heifers. Unfortunately, this condition has received little attention in the dairy industry, so no research is being done. As more producers do a better job of growing heifers, more cases will probably be reported, and hopefully, efforts will be made to find causes and ways to prevent this problem.
    For now, we do not have an answer. However, I am interested in obtaining any observations from farmers or veterinarians regarding this condition, not so much if you have an occasional heifer with the problem but if you have seen significant numbers of affected animals within your herd. If you have such experience, email me at bennettnvac@gmail.com with your observations and ideas. We do not have a solution yet, but we can work toward finding one.
    Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.