Excess weight and over-nutrition have a number of potentially negative effects, including:
- Increased stress on the heart and lungs
- Greater risk of laminitis or founder
- Increased risk of developmental orthopedic (bone and joint) problems in young, growing horses
- More strain on feet, joints and limbs
- Worsened symptoms of arthritis
- Less efficient cooling of body temperatures
- Fat build-up around key organs which interferes with normal function
- Reduced reproductive efficiency
- Greater lethargy and more easily fatigued
From the American Association of Equine Practitioners booklet, which also includes very helpful indepth coverage of what to do in order to safeguard your horse's health.