I came across this article on what to feed your horse. It is fairly in depth, but I have summarized some of it below: Legume Hay (Alfalfa Hay is one type): This type of hay is high in nutrients and is not recommended for leisure horses. Legume hay is a good choice for broodmares, growing foals, or horses that need to eat lots of calories. It’s also an excellent option for working horses who need more energy and nutrition from their feed than just grass can offer. Not many people in our area have horses used for these purposes.
Our Alfalfa Hay is about a 50/50 mixture of alfalfa and orchard grass. It is ideal for feeding horses that rodeo hard, or carry heavy packs long distance. Horses that are fed this hay consistently will tend to gain weight, especially if they are not worked hard on a regular basis.
Grass Hay (Ours is mostly Orchard Grass): Orchard Grass is high in protein (10-12%) and contains balanced calcium and phosphorus levels. Unless an animal is growing, lactating, or being used as a work horse, this is the type of hay they should be fed. Grass hay quality can vary greatly, especially in our area. Some ranchers prefer to take one cutting, so they wait to harvest until the yield is the highest (late July). At this point the hay will consist of stems, grass heads, and lesser amounts of the more easily palatable leaf of the plant. Grass hay also needs to be drier when it is baled to prevent discoloring and preserve nutrients. Drying the grass longer tends to sun-bleach the hay, making appear lighter in color. Because orchard grass has bunching tendencies, there can be room for annual and perennial weeds to sprout if not managed properly.
Our Grass Hay is harvested in early to mid June and again in mid to late summer. This allows for a consistent sized bale with even amounts of nutrients and a large proportion of the more palatable leaf (fewer heads and stems). We strive to minimize weeds and rework fields as they mature or become infiltrated with other plants.