While it's true that rapidly growing weanlings have special nutritional needs to keep them growing well, with healthy bones, joints and muscles/tendons, feeding weanlings correctly doesn't have to be complicated.
Feeding recommendations for young horses for the last 20 years or more have focused heavily on feeding grain and pushing for rapid growth so that yearlings are most attractive to buyers. These recommendations were also based on an assumption that horses under a year of age are not very good at digesting hays and grasses.
Common sense alone would tell you that wild horses grow and develop just fine on diets that don't include any grain whatsoever. But their rate of growth is slower, and they aren't being deprived of the nutritional benefits of their dam's milk at an early age. Research in recent years has shown a clear connection between rapid growth from heavy grain feeding and bone, joint and tendon/ligament problems in growing horses. The job now is to shift the focus away from too rapid growth and "fattening" young horses, to providing adequate but not excessive calories and at the same time meeting the critical mineral and protein needs of a growing horse.
Creep feeding is the practice of allowing foals to have to grains in an area that they can access easily but the mares can't. The feed may be put at the end of a chute too narrow for a mare to enter, or in an area with a bar too low for her to go under. Creep-feeding bins are also available with bars/slats across the top that are spaced too closely together for a full-sized horse's muzzle to fit through.
The idea is to allow the foal to develop a taste for the feed he will receive after weaning, introduce his digestive tract to solid food and keep an eye on how much he takes in as a guide to when he is ready to be weaned. These sound like good points, but there are some things to consider.
Foals begin to pick at hay and grass, and nibble at what's in Mom's dish, when only a day or two old. As time goes on, the foal relies less on milk and more on solid foods. While it's a good idea to have the foal's digestive tract accustomed to the diet he will be eating after weaning, it's also important not to overdo it. Unless the mare chases him off, he's also stealing grain from her.
Early orthopedic problems are much more likely to be related to too much feeding - specifically concentrated calories in grains - than they are to too little. This often begins even before the foal is formally weaned. Feeding the mare from a trough that the foal can easily access is just as good a way to introduce him to solid feed.
Fortunately, lactating mares and weanlings have very similar requirements in terms of the mineral density in their diet (grams or mg of minerals per calorie). As long as the mare's diet (grain and hay) is properly balanced and fortified, it will be fine for the baby as well. Don't worry about feeding him any specific amount until he's weaned, or you could end up with a foal that's too fat or growing too fast. Take your cues instead from the body condition of the mare and foal.
If the mare is losing weight while the baby is bulking up, odds are he's eating a good bit of the provided food and you need to cut back on how much they share while putting a bucket with additional food for her in a spot he can't reach. If the mare is refusing to let the foal share, you'll have to go to providing a safe place for him to eat as well.
Calorie and mineral requirements per pound of body weight are similar enough that once allowances are made for the nutrition the foal is getting from nursing, you can just provide him with a scaled-down version of the mare's total diet. Start by offering a 3-month old foal about 20% as much hay and grain as the mare is getting. Increase this only as necessary to keep the foal in a good body condition. As long as he's eating readily, he'll adjust to a full solid diet just fine at weaning time.
Feeding your weanling correctly means balancing calorie needs against mineral needs. Mineral needs are very similar between individuals, while calorie needs will vary depending on how rapidly the foal is growing, and also the breed.
Start with the hay/pasture. You're most likely to provide a balanced intake of minerals if you feed a mixture of different grasses or hays, or grass/hay with no more than 10% to 20% alfalfa or clover.
Mineral levels in hays and grasses vary widely across the country. To properly balance this portion of the diet, you'll need to consult with a local equine nutritionist or your local agricultural extension agent. Tell them where your pasture is located, or where the hay was grown, and that you're feeding a weanling horse. Ask what minerals need to be added, and how much. Remember, it doesn't matter if the grain portion is mineral-fortified and well-balanced. You still have to balance the hay.
A good starting point is to plan to feed 50:50 hay/pasture and concentrate. A horse that is growing at a safe, moderate rate and will mature to weigh about 1,100 pounds will weigh between 450 and 500 pounds at 6 months. On a 50:50 hay and concentrate diet, he will need to eat between 10 and 15 lbs./day total, so five to 7.5 pounds each of hay (or fresh grass equivalent) and concentrate. (The DOD sidebar on page 49 lists some key mineral levels to look for in your bagged feed.)
The other important consideration is protein, both percentage of protein and of lysine. The diet should average about 14% protein, 0.3% lysine. Quality pastures early in the growing season contain as much as 20%, or more, protein, so a 10% to 12% protein feed is fine with these. High-quality grass hays, or 20% alfalfa, 80% grass hays, typically run 11% to 12% protein and require a 16% protein concentrate. If your pasture quality is poor, or hay is lower in protein than that, you can make up the difference by supplying a half-pound to a pound a day of a 30% protein and mineral supplement, like Triple Crown 30 pellets, or Uckele Milk and Grow.
It takes a little care in selecting appropriate products to feed your weanling and keep the total balance of minerals correct. But once you have this diet in place and understand how to troubleshoot for under or overweight problems, it will serve you well for your foal's first year and get you off to a solid start in building a strong, sound young horse.