The energy provided by your diet is not sufficient. A low energy intake can results in loss of bodyweight and condition, reduce growth rates in young animals and result in poor performance for competitive and racing horses.
Increase the energy content of your diet through increasing use of feeds and oils with a higher digestible energy value. When increasing feed it is important to still maintain adequate fibre intake and to give consideration to the amount of starch given per meal.
Feeds are normally higher in energy than forages with an energy value typically ranging from 10 MJDE (mega joules digestible energy) up to 14 MJDE per kilogram as fed. Dried forages are naturally variable and typically range from 7-9 MJDE on a dry matter basis. The most energy dense nutrient is oils or fats which contain approximately 35 MJDE per kilogram.
If the horse has a good body condition at present and maintains this on the current diet it is important to check that the correct level of exercise has been chosen.