Too much soy is not unhealthy for the horse. The excess protein that the horse cannot use as an energy source is partly excreted by the kidneys as ammonia. This is generally not unhealthy for the horse, but not good for the environment and very inefficient. In some diseases, however, attention should be paid to the horse's protein intake. A veterinarian can best explain this.
The protein requirement of a horse varies if the horse performs additional services, such as a sport horse or a breeding mare.