Zimectrin Gold has an added ingrediant (Praziquantel) to get rid of tape worms.
Make sure you're feeding her enough of the senior feed. Check the bag's feeding recommendations and weigh your scoop. Most senior feeds are meant to be fed in higher amounts, 7-12 lbs a day.
You can make your own senior feed. Doing this allows you to increase or decrease the amount of calories while still insuring she's receiving plenty of vitamins and minerals. I make my own feed this way as I have 8 horses to feed that all need differing amounts.
First, make sure your horse is on good pasture turn out or free choice quality grass hay. If your pasture is lacking, start supplementing with hay. I like to supplement with 10-15 lbs of hay a day if there is any grazing available. Hay is the best way to add calories to your horse's diet.
For "feed", I start with alfalfa pellets. Alfalfa pellets are high in digestible protein, amino acids, and vitamins. To that I add a fat source (flax is my choice) and a vit/mineral supplement. That makes your "feed." Then I add any supplements the horse might need. In your case, I would add a digestive aid (probiotic) to make sure your horse is absorbing all of the nutritents and calories in the feed.
If she likes to bolt her feed down, I would add some water to the mix to prevent choke. Most horses do fine on hay pellets, but I have two that are fast eaters and need a bit of water in theirs to make sure they eat slower

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You can contact
HorseTech.com--Horse Nutrition Supplements--Equine Nutrition on the Net and have them make you an "all in one" supplement. They have a great flax product that they can custom blend with anything in it you might want. They have a new vit/min product called High Point Grass that would be perfect for this type of diet. You can have them add extra flax to that for weight gain. High Point already has yeast and probiotics in it for digestion. They do not charge anything extra for a custom blend, just the cost of the ingrediants.
If you want to save a little money, you can buy everything separate. Most feed stores can order flax meal. My Tractor Supply has Omega Horseshine, which is a good flax product. My Farmers Association has ADM's flax meal in a big 20 lb bucket. Or you can order whole flax seed and either grind it yourself (it spoils, so you'd need to do it daily) or feed it whole.
Good vitamin supplements (some with pro/prebiotics for digestion):
Grand Vite from SmartPak Equine SmartVite Senior Grass Pellets from SmartPak Equine Select II from SmartPak Equine Equi-Base Grass Pellet Quick Links
Good digestive aids:
SmartDigest from SmartPak Equine Ultra-Elite Digest from SmartPak Equine Focus WT from SmartPak Equine Quick Links Quadra Fac 2000
So, my diet for her would look something like this:
- 3 lbs Alfalfa Pellets
- 1 cup Flax Meal
- Vit/Min supplement
- Digestive supplement
After 2 weeks, I would increase to this:
- 5 lbs Alfalfa Pellets
- 2 cups Flax Meal
- Vit/Min supplement
- Digestive supplement
Use a weight tape every week on the same day and at the same time to track her progress. After 30 days, you should see some good improvement. Once she's at a good weight, you can take her back to the first diet above and adjust as needed. My "easy keepers" only get 1 lb of Alfalfa Pellets and 1/2 cup of flax, no digestive aid.