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Commodities: Field Crops: Forages

Common Terms Used in Animal Feeding and Nutrition

Glossary: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


Ration: Refers to the 24-hour feed allowance for an individual animal.

Relative Feed Value (RFV): This is a forage quality term that is used to rank feeds, especially forages, according to their overall nutritive value. This ranking is made relative to the typical nutritive value of full bloom alfalfa hay. Full bloom alfalfa hay, containing 41% ADF and 53% NDF on a dry matter basis, has an RFV of 100 and is considered to provide the average score. Though RFV has no units, it compares the potential of two or more like forages on the basis of energy intake. Thus, it serves as an index of forage quality for comparing forage lots. For example, forages with RFV greater than 100 are of higher quality than full bloom alfalfa hay, and forages with a value lower than 100 are of lower value than full bloom alfalfa. Such a single suitable parameter is useful for practical pricing and marketing of the forages.

The RFV is calculated based on the two laboratory determined parameters: NDF and ADF. NDF is used as an indicator of forage intake and ADF is used as an indicator of digestibility. Thus together, ADF and NDF estimate intake potential and digestibility, and they are used to calculate RFV as:

  • RFV = DDM (% of DM) × DMI (% of BW) ÷ 1.29

Where, DDM (digestible dry matter) and DMI (dry matter intake) can be calculated from ADF and NDF as:

    • DDM (% of DM) = 88.9 – 0.78 × ADF (% of DM)
    • DMI (% of BW) = 120 ÷ NDF (% of DM)

Due to the inherent variability of measuring ADF and NDF, absolute RFV numbers are not recommended for making direct comparisons or pricing of forages. Rather a range of RFV values (±5 points of the target) is a more reasonable way to classify a forage (e.g., if a RFV of 140 is desired, any forage with an RFV of 135 to 145 should be considered to have an equivalent value).

One of the limitations of the RFV system is that it assumes constant relationships between NDF and intake, and between ADF and digestibility. However, two forages can have identical NDF levels but very different digestibilities and, therefore, intakes. This often results in the RFV of high quality forages being underestimated because their intake is underestimated.

Relative Forage Quality (RFQ): a forage quality term that is similar to RFV in that it is used to rank forages according to their relative nutritive value. RFQ shares many of the properties of RFV (i.e., its basis of comparison is 100, the typical nutritive value of full bloom alfalfa hay; it has no units; it compares the potential of two or more like forages on the basis of energy intake; it serves as a useful index of forage quality for comparing forage lots; and it is very useful for practical pricing and marketing of forage lots). Unlike RFV, however, RFQ takes into account digestible fiber (Moore and Undersander, 2002a, 2002b).

RFQ is based on intake and true TDN instead of DDM. This makes RFQ a better predictor of forage quality than RFV. This is because RFQ accounts for NDF digestibility (NDFD) and the contribution of other nutrient fractions when calculating TDN, rather than calculating DDM based merely on ADF. RFQ is calculated as:

  • RFQ = DMI (% of BW) × (TDN (% of DM) ÷ 1.23

The above equation for RFQ includes the adjustment factor 1.23 which allows the RFQ to retain the value of 100 for full bloom alfalfa (similar to RFV), which serves as the base value.

The equations used to calculate DMI and TDN for legumes and legume/grass mixtures are specific to those forages and are different from those used to calculate DMI and TDN for warm and cool season grasses. Proper identification of forage type will therefore be essential before RFQ calculation. The two recommended equations for DMI and TDN calculations depending on whether or not the primary forage is legume or grass are explained in the definition of “Dry Matter Intake” and “Total Digestible Nutrients” in this publication. For more information on RFQ, visit the RFQ information page (direct link: on the UGA Forages website (

Roughage: Bulky and coarse feed high in fiber (greater than 18% crude fiber) but lower in energy than most concentrates is considered to be roughage. For example, forage, hay, silage, and haylage are sometimes called roughage.

Rumen: The foregut (or forestomach) of ruminant animals such as cattle, sheep, and goats. The rumen is a large, hollow muscular organ that is the site of most of the fiber digestion that occurs in ruminant animals. This digestion is largely performed by microorganisms (bacteria, protozoa, and fungi) that inhabit the rumen.

Ruminal Microbes: The whole community of the microorganisms present in the rumen of ruminant animals. They accomplish the digestion or fermentation of feed. An estimated 150 billion microorganisms per teaspoon are present in the contents of the rumen. This microbial community consists of bacteria, protozoa, and fungi.

Ruminants: class of animals that have multiple organs working together to accomplish digestion. The tract consists of the reticulum (involved in rumination and in passage from the rumen to the omasum), rumen (large compartment used for fermentation), omasum (once called the manyplies and removes excess liquid and nutrients moving out of the reticulo-omasal orifice), and abomasum (acid-pepsin digestion similar to a monogastic). By comparison, monogastric animals (e.g., swine and humans) have a simple or single chambered stomach that utilizes an acid-pepsin digestion to extract nutrition from the ingested food. 

Rumen Degradable Protein (RDP): RDP is also known as Degradable Intake Protein (DIP). See DIP for more detail.

Rumen Undegradable Protein (RUP): RUP is another name of by-pass protein, escape protein or undegradable intake protein (UIP). See by-pass protein for more detail.