What else do you plan to feed with it? I do recommend giving her access to quality pasture or forage, testing that, and then balancing her diet from there.
I'm going to play devil's advocate here. Why it has more Vitamin A and D than Vitamin E is kind of strange, unless the Vitamin A and D are not readily bioavailable. That would be odd - Vitamin D3 typically is listed as the most bioavailable form of D there is. Dr. Kellon states that Vitamin A is likely to be the most over-supplemented vitamin, and oversupplementing is usually what causes Vitamin A toxicity. So let's use that as a starting point. I understand that part of the intention of this feed is to support healthy bone and muscle growth, but if I'm correct here, it goes about that in an awkward fashion. For example, Purina recommends balancing this feed with forage, but forage is a very easy source of Vitamin A (green forage and grass having the most concentration.) Similar with Vitamin D - sunlight and forage often provide the horse with sufficient. And yet Vitamin E is essential! https://www.horsejournals.com/horse-...icities-horses
Assuming that Vitamin A is the limiting factor, let's do the math. If anyone feels this is incorrect or that I've made a mistake, speak up. I'm still new to feed calculations.
650 lbs = 294.835kg.
(Going off of NRC requirements) Vitamin A: It is recommended that horses up to age of 2 to have 45 IU per kg of body weight in Vitamin A. This is likely to be the upper limit for a young horse. If we go by this, your filly can safely consume up to 13267.575 IU of Vitamin A per day. The feed lists that it contains 11330 IU Vitamin A per kg of feed. If we're limiting the amount of supplement that your 650 lb filly gets by intake of Vitamin A, then she may safely consume 1.174 kg (2.57 lbs) of this feed, and you would have to monitor her other food sources that contain Vitamin A. Keep in mind that a ~7" fresh carrot probably contains around 8500 IU Vitamin A. So to round this off nicely, I'd feed about 2.5 lbs, split into 2 feedings of 1.25 lbs each.
Because Purina only recommends feeding up to .5% body weight of this feed, they claim that you should not feed more than 1.47kg (3.25 lbs) per day, for a 650 lb young horse. I don't know if I'd be splitting hairs by saying this, but I would still not feed much more than 2.5 lbs daily, especially if you plan to also feed forage or give access to green pasture.
Once your horse is over 2 years old, 30 IU Vitamin A per kg of body weight is more appropriate, according to NRC guidelines.
A note about "Guaranteed analysis": it means "these are the minimum amounts that are guaranteed in this batch of product." It says nothing about maximum amounts. Only calcium is listed with a minimum and maximum. This requires us to take further caution against feeding too much. Like I said, I'd stick with 2.5 lbs. If you want to be more certain of the amounts and proportions you're getting in a single supplement feed, you're looking for a 'typical' or 'average' analysis. You can probably request this information from Purina, via E-mail.
This feed seems to offer a good coverage of trace minerals, but it doesn't list them in ppm. So I have no idea how much of each is in it. You'd want to keep an eye on iron (Ferric oxide) and manganese, and make sure there's plenty of zinc and copper in comparison. Iron and manganese can block the absorption of other minerals, typically zinc and copper. That's why I mention it.