The proper nutrition for young horses is very important to their growth and overall well-being. Young horses that are in training can be an extra challenge because they are still growing building bone and muscle and they need the extra nutrition for exercise.
With young horses in training it's a good to have hay in front of them all day long to help them buffer the acid that they produce otherwise they may develop ulcers from stress. Giving them a good pro or prebiotic would be beneficial because of the stress that is caused during training.
Young horses in training usually require grain supplementation, but don't overfeed them as you don't want them getting fat or developing DOD (developmental orthopedic disease). Youngsters that are overweight could develop bone and joint problems as well as contracted tendons plus they run the risk of health problems later on in life.
Keep their grain meals small to avoid blood sugar spikes. Feeding them grain three times a day would be the ideal situation. If you notice any signs of joint problems reduce their grain intake a bit to slow down the growth rate.
Don't make the mistake of thinking that your young horse will get all the nutrients it needs from grain as the hay is very important. Know the nutritional value of your hay and then you can adjust with grain and other supplements to balance the ration.
Feeding them straight alfalfa is not a good idea as it is very high in protein which runs between 15-18% or higher. Alfalfa is high in calcium which suppresses magnesium and it causes an imbalanced calcium to phosphorus ratio of 5:1.
Horses that are low in magnesium will become flighty, nervous, cranky, tight, and sore. Magnesium is an important mineral because it is a major mineral in bone and is important for muscle contraction, relaxation and nerve transmission.
Feeding too much alfalfa can cause enterolith stones which can cause colic and blockages and lead to death of the horse.
With young horses make sure you feed a balanced ration to help maintain a steady growth rate. Each horse is an individual therefore feed will need to be adjusted accordingly as some will maintain their weight better than others.
A good habit to get into is weighing their hay and grain and then keeping records which will help you to adjust their feed up or down.