Use FeedXL. Was INVALUABLE when I was in this situation. https://feedxl.com/tools-for-precision-nutrition/
I’d kept horses for decades without being in charge of diet, so it was daunting. With FeedXL you can play around with numbers and also input pricing to make sure you’re covering your nutritional bases without paying too much.
For example - I was buying the premixed Smartpak stuff, but FeedXL showed me I really only needed to buy a mega bag of each isolated mineral/etc. It was better for my horse (they were getting the perfect levels of each nutrient) and it was much easier on the wallet.
I got my hay tested bc that was the only way to know it’s actual nutritional content, my home state of NC’s extension will do it for $10! https://harnett.ces.ncsu.edu/hay-eva…n-and-testing/
Edited to add - utilize the nutritionists for each brand, but don’t feel bad doing your own research. The Nutrena feed that my rep recommended for my body scale 2 rehab horse ended up being off the charts on some things that were negatively affecting her comeback. Once I started balancing things on FeedXL, her demeanor changed and I could tell I was on the right track.