The stage of forage plants at cutting is the most important factor affecting the nutritional content of hay. Nutritional quality drops as forage plants mature. Grasses mature faster than legumes such as alfalfa and clover. Thus, the nutritional quality of grasses drops faster. If you are going to buy mature hay, it will be better the more legumes it has in it.

Indicators of maturity include stem size and softness as well as the presence of seed heads or flowers. If you see plenty of mature grass seed heads when you open a bale, it will be low in nutrient content. Depending on the type of horses you want to feed, this may or may not be a problem, says Maribel Fernandez, regional educator with the University of Minnesota Extension Service.

"Animals with high nutrient requirements need to get more concentrated nutrients from hay. Alfalfa cut in the flower stage that has kept most of its leaves is still a very good option, as long as it's not moldy. Mature idle horses will actually be better off with more mature hay, even if it's a grass mix," she said.

The other big factor affecting hay quality is the weather just before and during cutting, and also during drying and baling.

Hay that received light rain right after cutting has lost some of its nutrients, says Fernandez. This hay can be used for animals with low nutrient requirements, such as idle adult horses that aren't pregnant or lactating.

Reject hay that feels damp, warm or steamy. And be especially careful not to buy hay that's moldy. "Hay should smell sweet and not musty, dirty or moldy," she says. "Buy hay that has been in a well-protected shed or barn for at least two weeks. Before you load bales, open one or two and look and smell for mold. If hay is dry and not moldy, you can expect that it won't start molding at your place."

Fernandez says hay that has conditioners such as propionic acid or calcium carbonate is okay for horses, and those materials prevent mold. She also suggests spraying diluted molasses on hay once it's in the feeder if you need to make it more appealing to horses.

Leaves on forage plants make up 80 percent of the nutritional value of hay, according to Fernandez. Leaf retention is highest when handling and weathering are minimized.

"If you need to meet high nutritional requirements for your horse, the best strategy is to use a lab nutrient analysis of the hay," says Fernandez. "This is particularly important if you buy a truckload of hay to use throughout a period of time. It's a good idea to analyze each new load of forage, even if you always buy from the same farm. Ask for an equine test, since horses have different digestive systems than cattle, and use forages differently."