Horses that are not in work have much lower requirements than those that are working, growing, or reproducing. The nutrition supplied by good-quality pasture grass and/or hay goes a long way to meeting the needs of the horse, especially energy and protein requirements. However, good-quality forages may lack certain trace minerals, and dried forages may be too low in vitamins. 

Concentrates are designed to provide nutrients missing in the forage. However, when you feed a concentrate feed well below the recommended feeding rate, the horse will not receive all the nutrients that the feed is designed to deliver. If you look at the tag or bag it will give a recommended minimum, which for a typical senior would be around six lb (~2.7 kg) per day.

To illustrate what happens when you feed below the recommended amount, like 0.5 lb (~250 g) a day, let’s look at one mineral, selenium. If the feed is fortified with 0.5 mg/kg and is fed as recommended at six pounds (2.72 kg) per day, it will supply 1.36 mg of selenium; however, when you feed only 0.5 pound (~250 g) of the same feed, it will only supply 0.12 mg, which is below the requirement of the horse. The recommendation from Kentucky Equine Research (KER) would be 1.87 mg/d. If the forage is high or adequate in selenium, this would not be an issue, but in many parts of the country selenium is deficient in forages. This trend continues for other nutrients, and this is why the feeds are formulated with a certain balance of nutrients. 

Many horses in this country that do not need the abundance of calories that would be supplied by feeding the minimum recommended feeding rate. For these horses, there are “low-intake” feeds or “ration balancers,” which will supply the nutrients with a much lower recommended feeding rate. Typically these low-intake feeds are designed to be fed at 1–2 lb (0.5-1 kg) per day. Balancers are particularly helpful when the protein, calcium, or phosphorus content of the forage is questionable, as well as when trace mineral and vitamin levels are low. The other option is to add a vitamin/mineral supplement to the feed to top off the mineral and vitamin content of the ration, which could be added to the small amount of senior feed you are giving and not cause any significant increase in calories.