Along with all the comments on this thread that make sense (try figuring out which one applies to your horse ), I read some other interesting "my horse won't eat this great hay" things on another forum.
Below are some of the reasons these folks have come up with:
1. Using BIOSOLIDS in place of traditional fertilizer on fields
http://www.nebiosolids.org/uploads/NEBRABiosolidsIntroV.05.pdf This link is a 48 page .pdf file; I can't get it to open
However, someone on the other forum (strictly eastern seaboard by the way) commented the bio fertilizer makes grass/hay greener and grows at a quicker rate..so farmers get MORE hay in less time.
2. It may depend on what kind of building they use for storing the hay. It was stated that sometimes horses don't initially like hay which has been stored in a cattle barn.
3. Another person had gotten hay from a different supplier. When the local guy she normally bought from deliver a load, she told him he could have the hay her horse would not eat for his cows. He agreed the hay was beautiful and nothing wrong with it except when he smelled it he could tell it was treated with something to promote drying; something he did not do
4. If you have clover in the hay be sure there is no mildew on the hay leaves. One lady had some hay one year that was very nice hay but the clover flowers ended up not being dry at the time of baling. Over a period of time the leafy hay developed layer of mildew that took a long time to detect. She didn't notice it at first because her loft is dark. Once out in the daylight it could be seen better.
Based on all these comments - buying hay is getting to be tricky business.
I try to buy by the season but I couldn't do that in 2012, due to one of my horse's big founder and injury event.
I also did not buy from my regular hay guy last year because he did something wrong with the weed killer and lost his entire first cut - can you say ~1,000 bales? It was safe to feed to cows but our county Ag person told him not to sell it for horse hay.
The bulk of horse owners on this forum do not have hungry horses, so those same horses all feel they have the right to be picky. It's a good thing none of them have thumbs or they would be wrestling our cell phones from us and calling the Horse Abuse Hotline when we try to feed them something they don't want
Anyway, there's some eye-opening reasons, above, as to why horses may not want to eat their hay.