It might be as simple as just giving your horse a couple of scoops of soaked beet pulp and/or a couple of pounds of oats. Is your horse’s appetite hearty? Is he stressed? Sometimes picky eaters or horses who have trouble chewing hay can benefit from beet pulp — they seem to love it. Beet pulp contains about the same calories per pound dry weight as legume hay, and is 10% protein.
Be sure your horse does not have a medical or dental problem. For example, you wouldn’t want to start feeding lots of concentrated feeds to a horse with ulcers. If the winters are severe, the horse’s need for calories increases, and the best way to meet the need is with more hay if he will eat it. You didn’t say how long you’ve owned your horse, to compare to previous seasons. Your TB is too young to be experiencing geriatric issues with feed digestibility, so the next thing is to evaluate your horse’s current ration. Forage is the most important part of your horse’s diet, so look at that first. If you can, while your horse is still on hay and not grazing pasture, weigh his hay if you are serving hay daily. Weigh your horse too, so you have a way to establish fed rations as a percentage of bodyweight. If you find that your horse is eating a good amount of hay, say 2.5% of bodyweight per day and is still thin, be aware that some hay has more calories. Hay cut before it matures has more calories than mature hay. Immature legume (alfalfa, clover) forage hay has about a third more calories than mature cool-season grass hay. The big difference between grass and legume hay is protein and calcium levels. By having your hay tested (check with your vet, feed co-op or university extension office for sources of hay testing) you will not only learn the energy content, but the protein content as well, which can help you decide if you should add just fats and carbohydrates for more calories or if you need to add protein too. Where I live, it costs less than $20.00 for a hay test.
Your horse would benefit from being on free-choice hay if possible. If your horse is getting as much good quality hay as he will eat, it is not necessarily a bad thing to see a healthy horse come through a hard winter without gaining weight. Now that spring is here, if you still need to get more calories in him, you could give him a calorie-dense packaged feed usually labeled as a competition or performance feed. These feeds will have higher fat content than the basic feeds as well as complete proteins, vitamins and minerals. Here are some product names — Purina Ultium Competition formula (12.4% fat), Nutrena Pro Force Fuel (13%), Triple Crown Training Formula (13%), Progressive ProElite HF (12%), Buckeye Cadence Ultra (14%) or Buckeye Trifecta (12%).
If you decide to mix your own feed from scratch, you might combine oats, soybeans or soybean meal, and beet pulp, and add a fat supplement such as Purina Amplify pellets, Buckeye Ultimate Finish, or vegetable oil. Fat has about twice the calories per unit weight as carbohydrates or protein. If you go with the oil instead of a pelleted fat supplement, add a pelleted ration balancer Such as Buckeye Grow ’N Win so you are sure your horse is getting the vitamins and minerals that may be lacking otherwise. Look at the SmartPak line of products too, like SmartGain, as they include ingredients for digestive health.
Before making any changes, get your vet’s opinion, and then introduce the changes gradually. You want slow, steady gains. Good Luck!