Weight gain in horses, other than while they are growing, happens because they are accumulating adipose tissue (fat) or developing muscle mass. Accumulation of adipose tissue will result in a horse’s body condition score changing, as this score represents fat coverage across the horse’s body. Recall the Henneke system of 1-9, where 1 represents an emaciated horse and 9 represents a grossly obese animal.
It’s not uncommon have a “hard keeper” with a BCS of 4, for example, that would benefit from putting on some weight to increase their score to 5. Similarly, a broodmare’s reproductive efficiency may be improved is she is in slightly higher condition than a non-breeding mare. It is not wise to ever feed a horse to have a body condition score greater than a 6 (or perhaps a 7 for broodmares), as the negative consequences of being overweight or obese include increased risk of metabolic syndrome and laminitis.
To gain weight as fat, a horse needs to take in more calories than he expends. How many calories the horse needs depends on his body weight, metabolism type (i.e. hard keeper vs easy keeper) and workload. Increasing calorie intake can be accomplished easily, and safely, by increasing the amount of forage (hay and/or pasture) the horse consumes, and/or perhaps selecting a more nutrient dense type of forage that would have more calories per unit weight, such as a grass hay with a less mature plant, or with some legumes mixed in.
Horses are limited to some degree in how much they can consume due to gut fill, and some horses simply won’t eat much more hay. In these cases, I look into other types of forage or high fibre feeds, such as hay cubes, haylage, beet pulp or rice bran. Another very cost effective way to increase calorie intake is by adding fat to the diet, because fat has more than three times the calories per unit weight as most forages. Vegetable oil is cheap and palatable, though other types of oil are fine as well, and contain the same amount of calories. Most horses can tolerate up to two cups per day.
There are commercial weight gain/high-fat types of supplements available, that won’t have as much fat in them as straight oil. Oil is 100% fat, while most supplements might be 40-70% fat. Increasing a commercial grain mix can add more calories, but also adds in more vitamins and minerals that might not be needed. Further, the grain/starch intake might become too high and increase the risk of colic. Another trick to increasing calorie intake is to increase the number of meals per day. This way you don’t need to increase the volume of feed at each meal.
Increasing muscle mass requires more than just additional feed. Most muscle growth is not achieved by increasing the number of muscle cells, but rather by increasing their size by “hypertrophy” and number of contractile fibres within the muscle, which is stimulated by exercise. To support this growth, the horse should be in a positive energy balance (consuming ample calories) and be consuming adequate amounts of amino acids, which are the building blocks of protein. It should be noted that most athletic horses are already exceeding their protein requirements simply because their feed intake is high. However, care should be made to ensure that the key amino acids, such as lysine, methionine and threonine are adequate. There are many commercial products available that suggest they can promote muscle growth or add “topline,” however, without sufficient stimulation from exercise they will not do anything – just like I can’t take a supplement to achieve a six-pack in my sleep. Further, there is no evidence that these supplements are any more effective when combined with exercise training than a good diet. Cross-training types of exercises can trigger less used muscles to stimulate growth. These include hills, pole work and gymnastics, swimming, or even the use of some specialized equipment such as the Pessoa lunging system, a chambon, long or side reins – though research into these different methods is conflicting regarding what systems are most effective.
It should also be noted that in some cases, if a horse is struggling to maintain weight, work effort might need to be decreased for a period of time. It may be too difficult for a horse to consume adequate calories if calories are required for both weight gain and for exercise.
Gaining weight is relatively simple in terms of increasing calorie intake, though calorie selection – fat from oils and fibre from forage – may be important regarding overall health. Exercise is required to trigger muscle growth, which needs to be supported by excellent nutrition.
Refeeding Caution for Horses
If you have a horse that has been neglected or starved, that might have a very low body condition (1 or 2) please note that initial refeeding after starvation is a careful process due to the risk of “refeeding syndrome.” After this risk has passed, these horses can be fed to gain weight (as ideally both muscle and fat).