At 37 it is highly likely that your horse doesn't have many premolars and molars left and that what may be there aren't really effective for chewing because they don't have another tooth opposite. So using beet pulp, alfalfa pellets or other feedstuffs that aren't designed specifically for a horse who's chewing capability is limited or non-existent is going to provide little help. At this point, you need to be providing a good complete senior feed for your horse. These products are designed specifically for horses who aren't capable of chewing foods well and the pellets are processed to take that fact into consideration so that the horse can digest them anyway. So, choose a good senior diet and start by feeding it according to the label and the adjusting up or down based upon your horse's weight. And be aware that when you look at the label it's going to recommend that you feed alot of the feed per day but remember that this is the equivalent of feeding all of his hay and concentrates in one feed. My 34 year old POA gets 10 lbs a day in the winter and 6 lbs a day in the spring and summer when pasture is good.
Good pasture can still often be made use of by these guys because grass is easier to digest than hays or concentrate feeds, but it's likely not providing all the required nutrition. So you can likely feed less in the summer but in the winter you will need to feed according to the label. Hay is more of a pacifier to keep a horse with dental issues feeling content but it's not going to be of much use because chewing is so very important to digestion of hay.
Along with a good quality senior feed fed according to the label, you can also add up to 2 cups of vegetable oil a day for additional calories in a very easy to digest form. But remember, vegetable oil cannot take the place of the balanced and nutritionally complete diet. You have to be meeting your horse's protein, vitamin and mineral needs through the appropriate type and amount of feed first.
Licensed Veterinary Technician