If alfalfa is cut past maturity, the leaves tend to fall off the stalks, so the hay the horse is getting tends to be a bit to rough for their systems. In some areas of the U.S. the alfalfa hay is straight alfalfa with no other grass mixed in. This can be very high in protein and calcium and if you are concerned, it is best to have it analysed for nutritional values. Adjustments to what is being fed to these horses may have to be made so as to not stresstheir systems with an overdose of specific nutrients like protein or calcium. Speaking of protein, this often seems to be what horse owners base their idea of feeding on.
Horses have fairly low protein requirements in comparison with say a dog or cat.
Protein in the diet is used by the body to replace cells that have been lost or damaged. Young horses need a bit higher percentage of protein since they are experiencing rapid growth and are making a myriad of new cells daily. Mature horses tend to need feeds more for energy, thus it may be wiser to view the feeds you are planning on using as energy replacers. We think too often of how we utilize what we eat. They need and make use of the grasses and fiber they eat and this is the most important part of their diet.