From AAEP’s Ask the Vet: Thoroughbred with No Appetite
I have a TB mare off the track, she is 16 yrs old now, and I’m having a difficult time keeping weight on her. She looked her best about 2 years ago. She could care less about food, it takes her all day to finish her breakfast, then it is time for dinner. She has been wormed and teeth floated and up to date on vaccinations. I tried SmartGain 4 and mare calm for about a year, and now she is on a different regimen. Still could use 100 lbs.
I'm going to give you the same advice I gave the other person with a thin, picky eater: invest in a HayGain Steamer! You've already tried a supplement with fenugreek in it to tempt her with a maple/vanilla aroma (the SmartGain 4). How about a weight gain supplement that's purely fat, or a weight gain supplement that's purely protein (amino acids)? I find that horses respond very individually to different brands within the same category (weight gain) so don't hesitate to experiment with some other products.
Another suggestion is to play with her forage. Interestingly enough, turns out easy keepers LOSE weight with small hole hay nets while hard keepers, like yours, actually GAIN weight. There's something about a constant source of forage trickling through their digestive system that seems to regulate or normalize horses' weight. If you haven't tried alfalfa hay, cubes, or pellets with her, they're also a great way to increase appetite and add weight to horses.
As the previous question and answer mentioned, soaked beet pulp is another way to encourage horses to eat and to get calories into them. Beet pulp is a complex carbohydrate or fiber that is fermented by the good bugs in the hind gut or large intestine (cecum and colon) into readily available energy/calories.
Research suggests the fermentation process might be enhanced with the addition of active live yeast such as Saccharomyces cereviseae. That is, supporting her with a digestive supplement may help her get more nutrition out of the food she already consumes. Besides yeast, you'll also find probiotics, prebiotics, enzymes and other ingredients in this particular category of supplement.
I rarely recommend adding grain or increasing the level you're already feeding, but consider switching to a more calorie-dense product so the food she does take in has more “bang for the buck.