I love this thread! I wish I had thought of this a year and a half ago! I had these ridiculous lists of things I wanted to learn how to do. I've lucked out, have a great barn owner, who takes great care of the horses, but I had always hoped to one day move them "home". So I really jumped in feet first and soaked up everything I could!
For me, the biggest growth curve was hay - how much do they really need and how often and how many medical supplies do I really need.
We've got two large draft horses. In my mind, big horses need big piles of hay. While they do have 24/7 access to a round bale towards the back of their pasture, they do have hay racks in their stalls for the rainy days, or when it's just too darn cold. I packed those darn racks so full that they couldn't even get the hay out to eat it. Clearly, my two don't need as much hay as they get. They got fat on air. I wish that excuse worked on me too :p
So lesson one, bigger horses don't necessarily have bigger appetites. Loretta whose about 2200lbs and 17.3h (yeah, shes a tad chubby) eats 4 qts of grain per day, 2 in the morning and 2 at night. Grain is given 50% in her corner feeder and 50% in a foraging toy so that she goes out and roots around for it. Duke is slightly smaller, weighs slightly less, and eats slightly more, but isn't as round (although last winter he did look pregnant). My 2 horses eat two times LESS than the 800lb pony. Go figure.
Medical supplies - well I think I bought everything I ever came across for equine first aid. I've got 2 different kinds of wrap, waterproof rap, diapers and ducktape in case anyone ever gets an abscess. Iodine, and betadine scrubs, I've got barbicide to clean combs, picks, and brushes, I've got blue kote, wonder dust, liquid stitches (which I have had to use on myself, twice), soaking boots (in 3 sizes), wormer, backup wormer, backup to my backup wormer, bute, buteless bute or whatever its called, 3 different wound sprays, 5 different types of thrush treatment, farrier supplies (like a file in case we break a nail :p) I literally have an entire tack locker FULL of just first aid supplies. And while I admit that they have come in handy, the majority of them have never been used (thankfully) One of the barn owners horses got her leg tangled up in a fence and ended up with a pretty gnarly degloving wound. Fortunately, it happened just as I was arriving, my medical training kicked in and I ran to my locker, gloved up, and grabbed cotton and bandaging to get her leg covered and stop the bleeding. Now, I also keep spare clothes in there (because I ended up being covered in blood, and I drove home looking like I was fleeing a crime scene.) If you board a horse, I think its important to know where the first aid kit is. We have a community first aid kit for minor bumps, scrapes, etc. Everyone knows that I am fully stocked for a horse zombie apocalypse, and knows I am always willing to share my medical supplies when needed. Sometimes a wound is beyond wonder dust, and if it is, feel free to use what ya need.
Other important things include how clean does a horse stall really need to be (in my mind for the first few months, the answer was spotless). How to NOT waste shavings. Figuring out what method works best for a particular horse in terms of keeping the stall clean. Our drafts don't do well with pine shavings. They get packed into their dinner plate sized hooves, so we use pine pellets (which we find are more absorbent) with a very thin layer of shavings around the perimeter, which is basically just a soft pillow for their head if they were to ever lay down. We have their stalls done twice per day, my two are in and out all day. They have the tiny barn at the back of the property and are in and out as they please. We "fluff" stalls twice daily becasuse its rare that they make a mess, but hey, I'm there, so i might as well clean them.
Pastures and picked daily, and the barn owner brings the tractor in and back drags after mudseason to level the ground back out. We learned last year draft horses, grass seed and rain do not mix, and despite the attempt, they pretty much pummeled all of the grass that grew after a few monsoon-like days. This year, we are going to try planting now, re-seeding in the spring once the ground thaws out. Maine weather + long winters + wet ground + draft horses = mud MUD and more mud.
I think it's also important for people to understand how to worm their horses. We rotate wormers like we rotate seasons. My barn owner gives everone the option, she can worm your horses, or you can do it yourself. I do it myself, and I rotate (following her schedule). Everyone else uses one wormer throughout the entire year, and usually the cheapest one. When you are spending a small fortune on horse care, why cheap out on wormer? Go figure.
Oh, and newbies need a hay lesson (aside from quantity). I had no clue what the difference between first cut, second cut, and mulch hay was. If you haven't grown up on a farm, or with farm animals, or are a city gril (like me) you have no clue that first cut is usually dryer, and may have a higer mix of leaves/twigs/burs or whatever. Second cut is much greener, sweeter smelling, and more rich. It's important to understand who gets what kind of hay, and why. My two can eat anything, but others need second cut only, and in small amounts, others need first cut, soaked and sorted (to pick out anything odd). And mulch hay, is good for sitting on, covering grass seed, or great for chickens to peck through! Or for halloween decorations!
Although I could write a book (and I already have in your post, sorry!) about equine nutrition and what I've learned over the year, I think barn owners should try to educate people whenever possible. Especially when they are clueless, like me. I went in to it thinking I would just keep them on whatever food they were eating, and I am glad I didn't. I am glad I did research, I am glad I aksed professionals, and I am glad I got input from owner draft owners who didn't tell me I was starving my two horses by cutting down the quantity of grain.