Before increasing calories (the only way to add weight), do a health check.
How are your horse's teeth? We will learn more about teeth in January, but until then, you should have an experienced horse person (preferably your vet or an equine dentist) check to see that the horse's teeth don't have any hooks and that they're wearing flat. If the teeth aren't wearing correctly your horse can't chew their food well, which is the first step of digestion! If you ever see balls of hay built up on the sides of their mouth or they drop a lot of their food they really need to be checked. Remember that the better they digest, the more they get from that expensive feed you give them...
Do they have parasites? All horses do, but if they get out of control, the worms can eat more than their share and cause health problems. Check into a dewormer program or send in a sample to be tested.
Are they deficient in any vitamins/minerals? If you're noticing something like long hair or odd behavior in combination with being thin, talk to your vet and have them test your horse's blood to ensure they aren't missing any vital nutrients.
If all systems are a go, here are some recommendations for adding weight:
1) Add another meal of hay to your horse's ration or feed it free choice.
Start with the basics and add hay first rather than concentrates. We learned about their small stomachs, so feeding small amounts more frequently can help. Another option is adding in a meal of beet pulp (or adding it to your current grain ration). Adding in a complete feed (a feed that has hay built in) as an additional meal can add quite a few calories. Senior feeds are generally safe to feed to horses that are 4 years or older and they're often created to be easily chewed and digested.
2) Switch to a higher calorie feed.
Try to find a feed that will provide the most calories. COB doesn't have near as much fat (and isn't as balanced nutritionally) as a commercial grain ration or complete feed. Feeds that provide calories from fat are safer than those that have calories from sugars, so be sure to read those labels!
3) Add oil or a fat supplement to the existing ration.
Corn and other vegetables add calories from fat to your horse's diet. Pelleted fat supplements are also available.
Make sure that you always adjust your horse's feed slowly, over 7-10 days, gradually increasing the amounts.
It can be a challenge to maintain a horse's weight, let alone help them gain. Take pictures or use a weight tape to help you determine if they're gaining weight because they can often hide those extra pounds on their big bodies. If you have questions, you can post them here or email me at firstname.lastname@example.org.