Forages are the primary energy source for beef cows. From a forage standpoint, as plants mature, fiber increases. Fiber is less digestible than other plant parts and fiber digestibility declines as plants mature. These factors cause the concentration of energy in plants to decline as maturity advances. In addition, as plants mature, the increase in fiber and bulkiness reduces the amount of the forage an animal can consume. As the plant matures, ADF (acid detergent fiber) and NDF (natural detergent fiber) increase. Protein concentration also declines as plants mature. When designing diets using harvested feeds, many rations are balanced using average values and these "book values" often result in over- or under-feeding certain nutrients. More economical and better balanced rations can be formulated using nutrient concentrations determined from feed analysis.

Each sample must represent only one "lot" of forage. A "lot" of forage consists of forage harvested from one field at the same cutting and maturity. All forage from the same "lot" should be similar for: type of plant(s), field (soil type), cutting date, maturity, and variety. Variation in any of these characteristics can cause substantial differences in the nutrient value of the forage.

Forage testing laboratories will not accept a “grab” sample of a baled forage. Sample baled hay after curing (usually 17 to 21 days after baling), using a core sampler or probe. Such an instrument is essential for collecting a representative sample. For large round and square bales, the probe should penetrate at least 18 inches into the bale and have an internal diameter of at least 3/8-inch. If the probe is 18 inches long or longer, 15 large round bales should be adequate if the “lot” size is 30 to 40 bales. Collect one sample from each bale by coring straight in from the center of the end of square bales and from the wrapped circumference of round bales. Place the entire sample into a plastic bag and seal tightly. For loose or compressed hay stacks, use a hay probe at least 24 inches long to collect 15 or more samples from each "lot." Sample loose hay stacks from the top and from the side of the stack. Compressed loaf stacks require six sampling locations: 1) top front, 2) top middle, 3) top rear, 4) lower front side, 5) lower middle side, and 6) lower rear side.

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