I have a five-month-old Thoroughbred/Quarter Horse filly that weighs about 600 lb (275 kg). She is still nursing her mother, and they share a five-acre pasture with poor-quality, drought-stricken grass. The pair has unlimited access to grass hay, and they share 1 lb (0.45 kg) of a broodmare feed twice daily with one-third quart beet pulp and molasses added at each feeding. They have unlimited cobalt salt at their disposal. This filly has no swelling in any joints; however, her ankles look square and I'm concerned it's a nutritional issue.
We appreciate your concern for healthy growth of your foal. The boxy look to the joints is probably physitis, an inflammation of the growth plate. While not every foal develops physitis, it is a common occurrence among young horses and rarely causes any permanent damage. If your foal has no swelling and is not lame, the chances that it will outgrow the physitis without intervention are very good. It may have developed from rapid growth, whether that was spurred by genetic potential or excess calories from the dam’s milk or the supplemental grain cannot be ascertained. If you see a worsening of the condition, more drastic steps may be necessary.
Severe cases of physitis may be identified in fast-growing foals, particularly foals that are consuming large amounts of grain (2 lb or 1 kg per day is not a large amount), whose dams are particularly prolific milk producers, or those with poor conformation. Some foals that become sore from physitis shift their weight from leg to leg to alleviate pain, which can cause them to develop angular limb deformities. Foals that are sore should be confined to limit running and play, activities that would exacerbate pain. The calories in the diet should be restricted and the foal should be fed a ration balancer to supply the correct amounts of protein, vitamins, and minerals without too many calories. This article on Equinews.com explains the many uses of balancer pellets.
Right now, since you are planning on weaning, you may see an improvement in your foal without any other changes in the diet because calorie restriction will occur without access to milk. If your foal worsens, you may want to consider putting the foal on a balancer pellet with free-choice grass hay and pasture.
If at any time the filly becomes sore, please contact your veterinarian for a thorough examination.