Since hay is just dried grass, it may seem like switching from pasture to hay isn’t an issue. On some levels this is true but there are important differences between hay and live grasses that have an impact on how you supplement your horse’s diet.
Grass is about 80% water; hay 10%. Water intake improves digestion and reduces risk of impaction. It also makes the forage easier to chew. To help ensure your horses take in sufficient extra water to counteract the difference between pasture and hay be sure to feed generous amounts of salt, at least 1 oz/day for a 1000 pound horse. Mix this into bucket meals or dissolve in water and spray on the hay. You can’t rely on free choice intake to fulfill optimal salt intakes.
Live grasses are a rich source of vitamins C and E. When grass is cut, dried and baled into hays, these levels drop precipitously. Horses can synthesize enough vitamin C to avoid full blown deficiency states (scurvy) but probably not enough for optimal immune function and connective tissue/tendon health. Vitamin E always has to come from the diet. For peak health on hay, consider 5000 mg of vitamin C and 2000 IU of vitamin E for a 1000 pound horse.
Hay is the major source of vitamin A in the form of carotene, which the horse’s body converts to active vitamin A. The levels of vitamin A steadily decrease over time and may also drop in suboptimal drying conditions for the hay. As a rule of thumb, if the hay is not a nice green color or is over 9 to 12 months old, you need to supplement vitamin A. Start at 20,000 IU/day and increase to 40,000 IU/day for very poor quality hay.
Pasture is the horse’s major source of essential fatty acids, fats that the body cannot manufacture and are essential for bodily functions and the immune system. These include the omega-3 and omega-6 fatty acids. The natural ratio in fresh grass is about 4:1 omega-3 to omega-6. When grass is cut and baled, the fragile omega-3s die off quickly. Grains and vegetable oils common in the equine diet are also low in omega-3 and high in omega-6. To correct this, feed 4 to 8 oz of ground flax seed/day.
Evolution has equipped the horse to survive in harsh and extreme conditions but we all know how peak grazing season puts the bloom onto a horse’s condition. You can minimize the negative effects of no grazing by supplementing key nutrients missing from hays.