Much like humans, horses can develop health problems if they are too fat or too thin. A horse that is too fat runs the risk of a laminitic incident, developing insulin resistance, and having various levels of pain in his joints due to arthritic changes taking place from the extra weight. All of these issues can be life threatening and potentially life ending.
At the opposite end of the scale, a too-thin horse faces incredible complications as well. When a horse is starving, it metabolizes muscle fiber, weakening his body even more. Organ failure is often the result, ending unfortunately in a rather painful death.
How do you tell if your horse is at an optimum weight? Stand back and take a good, hard look at your horse's body. You are looking to see if the following points are visible, or covered up. The biggest and easiest place to being is the barrel. Are the ribs of your horse covered? Is the belly large and protruding, with ribs showing? Is the back bone (spine) visible, or covered?
Look towards the front of the horse; are the shoulder blades protruding or covered in a layer of muscle? Shift your gaze to the rear of the horse. Look at the top of the rump, downwards. You are checking the spine and hips specifically; are they covered with muscle, or are they protruding? If it helps, write down what you see, and take pictures of each area.
Next, place your hand on the horse in the areas mentioned above. Can you easily palpate (feel) ribs? Does the skin feel firm under your hand, or does your hand sink into it like it would sink into bread dough (very soft and non-resistant feeling.) Feel up along the spine; can you feel it? Is it surrounded by firm muscle? Or is the flesh around it soft and give easily under your fingers?
Move towards the shoulder; do you easily feel bone, or can you feel firm muscle? Go back to the hips. Does the rump have a rounded appearance that feels firm? Or is it flat and slack, with not much feel to it at all?
The ideal is to feel a bit of rib at the start of the rib cage, and firm muscle in the shoulders, back and rump. The body should look well covered, with no obvious signs of bone showing. There should not be large fat deposits anywhere. The neck should be firm and symmetrical, without a large fat deposit overlaying the crest (or top of neck).
By keeping your horse at an optimum weight you will lower the cost of medical bills and improve the lifespan of the animal. If you care about your horse, care enough to keep him at an optimal weight. Your horse will reward you with years of friendship.