The energy provided by your diet is above what is required. Excess energy intake will increase bodyweight which if leading to obesity will increase risk of health problems including laminitis. Excess energy is equally detrimental for young growing horses where a sudden growth rate will increase the risk of developmental orthopaedic disease. A steady and consistent growth rate is the best practice and monitoring average daily gain (ADG) of bodyweight is advised. Excess energy is also a concern for competition or racing horses where an athletic bodyweight is required to maximise performance and reduce excess weight bearing on limbs.
Decrease the energy content of your diet. This may mean reducing the amount of feed and oils given as these have a higher energy value. 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 reducing the feed creates an unbalanced diet that is insufficient for vitamins and minerals consider using a feed balancer or supplement to provide the necessary nutrients. Balancers and supplements are low in energy and ideal for helping to manage energy intake.
Maintaining adequate fibre intake is important when reducing energy intake. Consider restricting pasture access through shorter periods of turnout or using well grazed paddocks and substituting the diet with lower value dried forages.
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.