With practice, it is easy to learn how to detect whether or not a horse is at its ideal weight. The following tips apply to all horses and ponies.
The best indicator is the ribs. Locate the ribs by feeling them at the level of the point of the shoulder, so that your hand is in the middle of the side of the chest. A horse that is too thin will have “washboard” ribs; that is, there will be dips between the ribs. A horse that is too fat will have a pad over the ribs that prevent you from easily feeling them unless you press into the flesh firmly with your fingertips. A horse that is at its ideal weight will have ribs that are easily felt without pressing hard and are well filled with flesh between them. A thin pad of fat over the ribs is also acceptable and may be desirable for some horses that are hard keepers – especially if they are older horses.
It is essential to feel the ribs rather than just looking at the diameter of the belly. The diameter of the belly is associated more with how much feed is in the gut at that time, or with conformation or pregnancy, than it is with body fat levels. It is important to remember that horses have heavy coats in winter that can hide washboard ribs from view.
The owners were underfeeding because they thought the ponies were fat.
Another common error is to judge a horse’s weight based on looking at the topline. While excessively thin horses will have a gaunt topline, this can also be caused by conformation or decreasing muscle tone, which begins to occur in the early teens.
I suggest that you practice the rib technique with your veterinarian. Soon you will be an expert at determining whether or not your horses are at their ideal weight.