Barley provides more digestible energy and total available nutrients than oats, but it doesn't quite reach the levels of maize. It is less likely to trigger "hot" behaviour. It's considered a good feed for putting condition on a horse, but it's certainly not the perfect feed. It has a poor phosphorous/calcium ratio (maize and oats aren't great either, but barley is the worst of the three). It also lacks vitamins A and D. All cereals require a balancer as the extra phosphorus reduces the ability to absorb calcium.