Check out these tips on buying horse hay:

Evaluate hay content: grass vs. legume (eg. alfalfa). Grass hay is more easily digestible. Pure alfalfa may have more protein than a horse needs.
Nutrition: the amount of energy is based on plant maturity. Consider an equine hay test.
Touch: softness vs. roughness. Horses mouths are sensitive.
Smell: a sweet smell is indicative of higher sugar content.
Color: don’t dismiss hay solely on its greenness, but bleached color may indicate exposure to sun or rain.
Crop: 1st, 2nd, and 3rd cut hay may have differences in nutrient content. 1st crop hay likely has more digestible fiber, but the most important factor to consider is maturity of plant when it was cut.
Mold: if it has mold it may old get worse. Sometimes hay is treated with proprionic acid before baling to prevent mold.

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Tips on Buying a Horse Hay, Krishona Martinson, University of Minnesota