Mare dominance leads to severe injury

by Sally Colby
A well-trained horse willingly submits to rules established by its handler, but groups of pastured horses live by a completely different set of rules — rules they define and defend.
Mares are often likely the ones to set the tone in a group of horses. One mare is usually dominant, and the others will figure out their roles and submit. The dominant mare controls the other horses with body language: ears pinned, bold, head-down strides toward other horses, and if necessary, kicking. Any mare in the herd can be dominant, even a young mare.
I was aware of the herd positions of each of my three mares. They spend most of their time in the pasture, and I have plenty of time to observe their behavior. A young mare that was born and raised here was obviously in the dominant position, probably because her mother had been the lead mare. The other two mares, Penny and Dakota are both older, yet accepted their roles in the herd hierarchy. Of those two mares, Dakota is especially laid back and didn’t always respond immediately to the dominant mare’s signals to steer clear.
When Dakota didn’t come in from the pasture one morning at feeding time, I knew something wasn’t right. I looked all over the pasture for her and eventually found her standing at the bottom of the field, head down and stock-still. Although it was still dark, I didn’t see any obvious injuries, so I led her to the barn for a closer look. The dominant mare tailed nearby and trotted circles around us, and when I stopped at the gate to open it, the dominant mare positioned herself right behind Dakota, wheeled around and planted at least four double-barrel kicks to Dakota’s rear end. Other than some obvious kick marks, slight swelling of her right hindquarter area and signs of pain, I couldn’t find any wounds, so I gave Dakota some bute (phenylbutazone, an anti-inflammatory drug) for pain and put her in a stall so that I could observe her. By afternoon, her right rear end was swollen and was reluctant to walk. I immediately called the vet.
By the time the vet arrived several hours later, Dakota’s right side was even more swollen. After providing history, including Dakota’s role as the lowest ranking herd member, the vet did a complete exam — including testing her hooves for an abscess. I had theorized that perhaps Dakota had an abscess brewing and was lame enough that she couldn’t move away to escape the wrath of the dominant mare. Dakota could barely lift her right hind leg and seemed somewhat touchy, but the vet found no evidence of an abscess.
After what seemed like forever, the vet finally said that he suspected either lymphangitis (a severe inflammation of the lymphatic system) or a broken leg; neither of which was good news. Dakota was so swollen that the vet couldn’t x-ray her, and she was in no shape to travel to a veterinary hospital. The vet administered several antibiotics, suggested a schedule for bute and ordered stall rest until he returned in five days. I bedded her stall minimally to make it less appealing for her to lie down or roll in case she did have a fracture.
By the time the vet returned, the swelling had extended to Dakota’s left side and she was developing significant belly edema. Her udder was swollen and hard, and she was still reluctant to move. We continued to treat her with antibiotics, bute and stall rest, and I hand-walked her briefly twice a day. I noticed some serous fluid coming from several swollen areas, so the vet collected fluid for culture.
After another week, Dakota was more willing to move, and less overall swelling revealed a very obvious seroma on her stifle. A seroma is a large, obvious swollen area filled with light yellow serous fluid. In contrast, a hematoma is filled with blood. Seromas can result from a direct blow to the body, and in Dakota’s case, it was clear that the seroma was a result of her being kicked by the dominant mare. The seroma, which extended to her inner stifle and udder area, had probably been there since the initial incident in which she was injured, but the overall swelling disguised it.
Although it’s often best to allow a seroma to reduce and resolve on its own, the vet decided to excise Dakota’s seroma to release the fluid. There’s a risk of infection when a seroma is drained, but Dakota had been on several powerful antibiotics for more than a week, which minimized that risk. After the procedure, I kept the incision open for about a week and massaged the seroma from the top downward to release any remaining fluid. The vet advised me that it would be several months before we’d know how sound Dakota would be.
Once it appeared that Dakota was progressing with healing, we had to make a decision about the overly dominant mare that did the damage. I wanted Dakota to return to the pasture and not be threatened by a horse that clearly did not like her. The dominant mare is now living on another farm, and Dakota and her long-time pasture mate Penny are living without the stress of having to deal with a mare that took her role as lead m