Overeating on foods rich in carbohydrate or rapidly fermentable fibre (cereals, coarse mixes, rapidly growing or fertilised grass) can be the easiest way to trigger laminitis in horses.
Obesity is still considered the most common cause of laminitis and as an owner you should always aim to keep your horse or pony at no greater than condition score 3. This means they should not have a fat deposit along their crest or at the tail head, around the sheath or udders or over the loins. You should be able to feel their ribs easily by running your hand along their side. In fit or finer horses, you could also just be able to make out the outline on the last couple of ribs.
If it is established that your horse or pony is overweight, one of the first things you can address is the amount of rich food and grazing that is available. Where possible, limiting the grass intake by using a suitable grazing muzzle or providing limit access to grazing is an important place to start, ideally, in a paddock that hasn’t been fertilised. The use of a muzzle will restrict the amount of grass the horse can consume per hour, but will still allow them to graze and consume forage which is vital to the health of the horse.
Where it is not possible to use a muzzle, you may want to consider stabling or arena turnout (or turnout in a bare field). You will have to provide an alternative form of ample forage for your horse or risk other health issues arising (such as colic or ulcers). This could be second cut hay, high fibre haylage, or hay replacement chaff. Whichever you choose, it is vital to ensure enough is provided throughout the day and at intervals to ensure the horse never goes longer than 3-4 hours without access.
As a guide, this is the absolute minimum amount of forage per 100kg of body weight your horse should be getting for each of the different forage options:
Hay - 1.22kg
Haylage - 2.5kg
Chaff - 1.18kg
Even overweight horses still need their essential vitamins and minerals, every day. Try to avoid compound feeds with high feeding rates. Even if the sugar & starch levels are low, by the time the recommended daily amount has been fed, a lot of unrequired sugar & starch has been ingested.
Instead look at feed balancers. A good feed balancer will provide the horses essential vitamins and minerals in around 100gms of balancer per 100kg of bodyweight. (500gms of balancer vs 3kg of high fibre cubes).
Once the diet has been addressed, have a look at your horse’s exercise regime. If there is no present signs of laminitis and the horse is capable, exercise will always be the best way to reduce excessive weight. It doesn’t have to be ridden work, even a 30-minute lunge session, anywhere that is safe and suitable, will contribute greatly to reducing that waistline.