Whilst UK pasture and forage can supply many horses with plenty of energy or calories, typically it lacks certain trace minerals including copper, selenium and zinc and conserved forage e.g. hay and haylage also lacks vitamin E. Having a balanced diet is important for long term health and longevity. Zinc for example is important for hoof condition and whilst in the short term your horse may look great on just grass alone, a long term deficiency of zinc can result in poor hoof quality.