How long does hay "keep"

Hello, So I got an offer to buy 238 Bales of hay @ 1.50 regular price is 3.00. I have a place to keep it but I don't know how long it will last? It will be kept in a dry area covered!

I will be feeding just one horse so it will take a while to go through that many bails (I think)????

That will work out to roughly 1 1/2 years worth of hay for one horse. As long as it was put up right and you can keep it protected from the elements, it will be fine when you feed that last bale. What does deteriorate over time in hay is the vitamin content especially Vit A. Simply add a good vit/mineral supplement.

Recommend a supplement? Also should I cover it or not?

Properly put up hay stored in excellent conditions does not have a shelf life. Well, the longest I have heard is 50 years thus far.
The outside of the stack will oxidize and deterioate, but only on the outside 1-2 inches. The inside of the stack will be nearly the same as the day it was baled.

I had a vet once tell me that after a year the hay loses it's nutritional value...

Had a vet tell me one time that a particular girl was pretty. He sure wasn't right about that. When a vet speaks out of their area of expertise, don't believe everyting they say;.)

I'm a fan of ADM's Growstrong minerals. It has the highest value of vit A (600,000 IU/#) and a 2oz portion only runs $.13. I would not cover the hay if it's stored inside. In a humid climate, you will get condensation under the tarp and get spoilage. If your barn is metal, don't stack it up against the wall as well for the same reason. Stack it on pallets and leave some air space all around the stack.

I have half in barn in a loft which would not be covered, the other half is under a lean too, about 4 feet back from the roof. That was the amount I had a question about?

Storage is the most important factor (assuming high quality, properly cured, etc). As mentioned, in hot (90+), humid (60%+) summers like we have, if you don't have lots of good ventilation, it will start to mold in less than a week from just condensation/dew/humidity. If stored well, it lasts for a very long time (we've fed 2 year old hay that our mares cleaned right up).

Ya I will keep half in the barn which has good airflow and the other half will be under a lean too like I said, all will have good airflow. The question was should I partially cover the stuff that's under the lean too to insure it doesnt get wet?