Forage Quality - Fiber and Energy

Edward B. Rayburn, Extension Specialist
January 1997

Historically, forages have been called roughage because they contain more fiber than concentrate feeds such as shelled corn. However, forages vary widely in fiber and digestibility. Knowing the fiber content of hay or pasture is the best way to estimate how much of the forage the animal can eat and how much energy the animal can get out of what it eats.

Well-managed pasture and hay can be low in fiber and highly digestible. Late-cut hay is usually high in fiber and low in digestibility. Two types of fiber are measured in many forage-testing laboratories. Each has its own relationship with animal production.

Neutral Detergent Fiber (NDF) - NDF is an estimate of the plant's cell wall content. Some of this fiber is highly digestible. NDF is the best indicator of how much forage an animal will eat. A high-producing dairy cow can eat about 1.1% of her body weight in NDF. As an example, if a grass forage has 50% NDF, a 1,300 pound cow is able to eat about 29 pounds of forage dry matter (1300 x 0.011/0.50=28.6) compared to 36 pounds of a mixed, mostly grass forage containing 40% NDF. A cow can eat more forage if it is low in NDF.

As most farmers know, livestock eat more legume hay than grass hay. This is because legumes are lower in NDF than grasses. This results in 6-10 pounds more milk from a legume-based ration than a grass-based ration having the same balance of energy, protein, and minerals. For growing steers, a pasture containing 30 percent legumes will provide 0.25 to 0.33 pounds more gain each day than the same grass species without legumes fertilized with nitrogen.

Table 1 shows the NDF content of forages and supplemental feeds used in the Northeast. Note that mixed forages that are higher in legume are lower in NDF.

¹ less than 15% legume
² 15-49% legume
³ 50-85% legume
⁴ greater than 85% legume

There is the chance that animals will bloat when grazing pure legume pastures. Also, legumes in pure stands are more subject to damage from insects, diseases, weeds, and winter injury than legumes grown with grasses. A good compromise is to try to keep between 25% and 50% legumes in mixed grass-legume pastures to stimulate animal production and prevent bloat.

Acid Detergent Fiber (ADF) - ADF is a laboratory estimate of the less digestible cellulose and lignin, or "woody" fiber, in the plant. ADF is the best indicator of the fiber requirement for healthy rumen fermentation. Total ration ADF should be greater than 19% for a dairy cow. If it is not, milk butterfat may be depressed. The ADF content of pastures can be low (Table 1). This means that if too much grain is fed to cows on pasture there will not be sufficient ADF in the diet to maintain the milk's butterfat content. In barn feeding situations, grain can constitute up to 60% of the ration dry matter. In a pasture feeding program, feeding grain levels over 40% of the total dry matter intake may depress butterfat.

Energy - There are several systems used to measure the availability of energy from feeds. ADF is the best indicator of total digestible nutrients (TDN) in a forage. However, NDF is the best indicator of net energy lactation (NEL), net energy maintenance (NEm) or gain (NEg) since dry matter intake has a major effect on a forage's net energy content. As the NDF content of the forage decreases, the net energy content increases. Table 2 shows equivalent energy system values for feeds differing in TDN.

Higher forage intake and digestibility allow lower grain feeding rates without reducing milk production or animal growth. Up to 60 pounds of milk per day can be produced by a cow grazing a mixed, mostly legume pasture, without supplemental grain, if no more than 50% of the pasture is utilized. When the price of grain per hundred weight is less than the farm value of milk per hundred weight, it may be better economics to feed grain and graze the pastures closer. For cows producing more than 50-60 pounds of milk, moderate grain feeding is usually profitable when the cow is on pasture.

Pasture and early cut hay can be low-fiber, high-quality feeds. Be aware of the effects of NDF and ADF on the cow's feed intake and production and how forage type affects the pasture's NDF and ADF content. Good managers can improve cow milk production or calf and yearling growth from pastures and hay by using this information to their advantage.

References:

1. Rayburn, E.B. 1994. Forage Quality of Intensive Rotationally Grazed Pastures. Extension Memo, Oct. 1994. West Virginia University Extension Service, PO Box 6108, Morgantown, WV 26506-6108.
2. Sirois, P. 1995. Northeast DHIA Forage Lab Tables of Feed Composition. Northeast DHIA, 730 Warren Rd., Ithaca, NY 14850-1293.
3. National Research Council. 1989. Nutrient Requirements of Horses. National Academy Press, 2101 Constitution Ave., NW, Washington, DC 20418.
4. National Research Council. 1988. Nutrient Requirements of Dairy Cattle. National Academy Press, 2101 Constitution Ave., NW, Washington, DC 20418.
5. National Research Council. 1985. Nutrient Requirements of Sheep. National Academy Press, 2101 Constitution Ave., NW, Washington, DC 20418.
6. National Research Council. 1984. Nutrient Requirements of Beef Cattle. National Academy Press, 2101 Constitution Ave., NW, Washington, DC 20418.
7. National Research Council. 1981. Nutrient Requirements of Goats. National Academy Press, 2101 Constitution Ave., NW, Washington, DC 20418.

Programs and activities offered by the West Virginia University Extension Service are available to all persons without regard to race, color, sex, disability, religion, age, veteran status, sexual orientation, or national origin. Issued in furtherance of Cooperative Extension Work, Acts of Congress of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Robert Maxwell, Director, Cooperative Extension Service, West Virginia University.

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