You might think that the ideal weight for a horse, like the perception of beauty, lies in the eye of the beholder. Some people like the slim silhouette of thoroughbreds and some like their equines with the solid quarter horse appearance (even if they are not quarter horses). So the perfect look for one breed will be different for another.
No matter the breed, however; horses should not be underweight, and there are some well-established standards in the industry. The most profound example of poor weight is the horse with protruding ribs and hip bones. However, most horses that have weight loss are more subtle and often it is difficult to say for sure if they're too thin. Even the use of a weight tape can be misleading, although if you get a good baseline measurement when the horse is normal, the trends you measure will generally be an accurate reflection of weight gain or loss.
How to Examine Your Horse
The best way to determine whether your horse is the proper weight is to carefully examine the ribs, back, withers and shoulders of your horse. If your horse is completely normal, the ribs will not be visually distinguishable, but you can just feel them. The back will be almost flat on top. The withers will be slightly rounded. The shoulder and neck muscles will blend together smoothly, and the tail head will have a light spongy layer of fat around it.
When a horse becomes thin, the ribs become obvious without touching them. The withers, shoulder and back are initially unaffected with weight loss of less than 50 pounds (for a 1,000-pound horse). When the problem gets worse, the back starts to slope off the topline, and the spinous processes on the topline and the ribs are easy to feel. The tailhead becomes more prominent and fleshy. At this point the horse has lost up to 100 pounds. With more advanced weight loss (more than 100 pounds), the withers, shoulder and neck appear thin.
The shoulders of a horse with this level of weight loss will protrude, and he will have a thin neck and prominent topline where you can see the individual bones in the spine along the back. As the problem advances and the body fat and muscles get broken down, he will look "sunken," particularly in the face and neck.
Even at the early stage, when the ribs become visible, weight loss may indicate a medical problem, rather than a dietary one. Check with your veterinarian to make sure the horse is generally healthy and can eat comfortably.
It is important to consider the demeanor of a horse in assessing weight. In general, nervous horses tend to be thinner than other horses. Those who paw, crib, or pace, burn off an amazing number of calories. You may have difficulty keeping weight on those horses. Horses kept outdoors with no shelter year-round may have a tough time keeping weight on. They burn calories staying warm in the winter and fending off flies in the summer. In many states, keeping a horse outside with no shelter is illegal for that reason. And a horse that is not wormed regularly also will have trouble retaining weight and will be more prone to digestive problems, such as colic.
Guidelines for Your Horse's Diet
If those conditions are not a factor, and your horse is still thin, here are a few guidelines for adjusting his diet:
The most important thing is not to expect instant results. It usually takes more than a month for change to become apparent. Be patient and persistent, but be careful and consistent, and your horse will bloom.