The general storage period for them is 60-90 days. If the heads shatter, the value of the hay is nothing. Grain hays should be used within a month or you need to have lots of hungry cats around to keep the rodent population down. Remember, the grain head is the only part of value in a grain hay. Prebloom and bloom stages are the best quality.
Moisture content is very important to the quality.
As all of the plant is edible, you do get more for your money. With some of the legume hays, the estrogen content can be important. If you are having breeding problems, consult with the extension service or local Agricultural Ministry, or school. If you use alfalfa on a regular basis, you might want to consider supplementing the diet with a calcium rich food, such as calf manna. All feed should be used within 90 days as a good rule of thumb.