Here are some Questions that are asked on the internet, and some of my answers.

Question My horse is gaining weight. She is fed 3x a day. What would be the best way to get her to lose weight? She is lunged for 10 mins day.
AnswerI think I don't have enough information about your horse or its lifestyle to answer this question reliably or safely. General rule make all changes gradually, and cut grain feed down but keep up the fibre, eg hay.
Horses thrive on routine, so don't make any changes you can't keep up.I would advise that a happy fit horse should have plenty of turn out time and as horse's natural food is grass. That is the best food, providing it does not suffer from Laminitis.A horse's digestive system is designed to be eating a little hi fibre food for about 18 to 20 hours a day, be careful not to feed grain then nothing until the next feed. Better to turn out with grass, or if in, a hay net with small holes, that keeps the horse occupied and gives a natural trickle feed.I do hope that there is nothing wrong with your horse, and that you will be able to ride it and both of you have some fun. If the only exercise is lunging for 10 mins it does not need any grain at all. Done correctly lunging is double the exercise for the time, so don't overdo , it can do damage.