Weanlings have intensive nutritional needs and you shouldn’t start feeding them like a smaller version of an adult.
Nutrient dense diets are those that have high levels of protein/amino acids and minerals per calorie. As you might expect, mineral requirements are extremely high during periods of rapid growth. At 4 months, the horse has higher daily total mineral needs than they do as a yearling despite having lower daily calorie needs. If you really think about this, it is immediately clear that trying to feed weanlings the same diet being fed adults is going to be severely inadequate.
Calories: Calories are actually the easiest part of feeding weanlings. In fact, most are too heavy and this has been linked to developmental orthopedic disease. A 6 month-old weanling requires 7% fewer calories than he will at maintenance at this full adult weight. If feeding him 93% of the adult diet, he will also get 7% less protein and minerals.
Minerals: The foal’s body can’t create the minerals it needs for growth and stores at birth are minimal to none. This is where the needs of the weanling and those of the adult show the greatest difference. For example, the 6 month-old weanling needs almost twice as much calcium and phosphorus as he will when an adult. Obviously 93% of the adult diet won’t get the job done. The weanling may be falling short by as much as 20 grams of calcium.
Protein: While calorie requirements were lower, protein needs are 7% higher and lysine 10% higher. If you are feeding the adult diet at the 7% reduction, the gap gets wider. For a horse that will mature to 50o kg this amounts to a deficit of 90 grams of protein overall and 4 grams of lysine *if* the adult diet was adequate for lysine (many are not).
The Solution: What to do about this? You can scrap the idea of feeding your regular adult diet entirely and go with a specialty mare and foal feed according to directions. If you do that though, the diet can be 50 to 60% grain based with much of your protein and minerals tied to grain calories.
It is well known that overfeeding in general is linked to early orthopedic problems across the board and high grain feeding rates put some horses at higher risk for osteochondrosis. It also used to be believed that weanlings had to have a high percentage of grain in their diet because they couldn’t handle a high fiber diet as well as an adult. Recent research has proven that false.
Going back then to the adult diet with modest levels of grain/concentrates, how can it be fortified for the weanling? Assuming the adult diet meets minimum protein and mineral requirements, look for a supplement with about 25% protein, lysine minimum 1.5% and 5% calcium with a balanced mineral profile. Feed 1 pound per day of this.
If you are already feeding supplemental minerals and don’t need to add more, it’s very useful to have an unsupplemented high protein source. Look for 40+% protein, at least 2% lysine and a mixture of milk/whey protein with vegetable sources. Feed 1/2 lb/day. If total protein is adequate but all or most from hay with unknown lysine content, supplement with an amino acid supplement containing 10 grams lysine and 2 grams threonine per dose.
Finally, for fall and over the winter with no pasture available you need to think about essential fatty acids. Omega-3 fatty acids are fragile and largely destroyed when hay cures and during storage. Adequate supply is required by the eyes, heart and may even influence disposition. Flax and Chia are good sources, 4 to 6 ounces/day.
Tweaking your diet to fill weanling needs is not terribly difficult or expensive but the pay back in terms of growth, health and soundness can be enormous.
Eleanor Kellon, VMD