Senior feeds are generally made so they can be fed as the sole ration. Look at the label or the back of the bag and read what it recommends you to feed. If it has an amount for a 900 lb horse (or whatever weight she should be), I'd put her on it and let her have free choice grass hay, but only AFTER she eats the ration of Senior. If she weighs 800 lbs and you want her at 1000 lbs, then feed for the 1000 lbs or even for 1100 or 1200 to get the weight on her then cut back when she picks up. I believe Nutrena puts pre & Probiotics in the feed to aid with senior digestion too, that's frequently an issue with older horses because they a) can't chew as well as they need to and b) they can no longer digest as efficiently as the once did. 1 cup (if you are using a standard measuring cup, it won't even weigh out at 8 oz, or 1/2 lb), of alfalfa pellets is pretty useless and a waste of money IMO. I think you'd find if you just fed the senior plus hay for something to chew on, you could cut the other stuff out and she'd gain the weight. IF you feed according to the directions on the package. K.I.S.S. Frequently works best for seniors.