All forage plants are composed of cells having fibrous cell walls for support and protection. Contained within the cells are several soluble compounds, most of which are highly digestible (Fig. 1). Since cell wall material is the primary constituent of forages, one of the main objectives of forage analysis is to characterize the cell wall fiber.
Plant fiber has three major components: cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are digestible to some extent by ruminants. Ruminants can convert these fiber components to energy because the rumen provides the correct environment for bacteria and other microorganisms that actually break down the fiber. Lignin is indigestible, and thus cannot be used by ruminants for energy.
Both grasses and legumes have two main plant parts, leaf and stem. As a structural component of the plant, stems typically contain more fiber for support. Leaves, on the other hand, provide a means for capture and utilization of energy from sunlight and tend to be lower in fiber content than stems. Given the large difference between the digestible fiber of stems and leaves, the proportion of leaf to stem in a given forage plant relates directly to its forage quality.
Appraisal of a forage based on sight, smell, and touch can provide some general information, but chemical analyses are needed to asses the economic potential of the forage.
At a recent forage meeting, approximately 80 forage producers and industry people were asked to rank four bales of hay by a visual appraisal of their forage quality. The hay ranged from pure alfalfa to an alfalfa-grass mix. An objective quality evaluation of the same bales, based on Relative Feed Value (RFV), found considerable differences among them. There was no consistent pattern in the ratings by individuals but, in fact, the bale judged best on the basis of appearance had the lowest RFV of the four. Clearly, objective forage analysis is required.
The Van Soest Fiber Analysis System separates feeds into distinct fractions that relate to their nutritive value. Neutral detergent fiber (NDF) consists of the total fiber in the forage and relates negatively to forage intake by ruminants. Acid detergent fiber (ADF) is composed of highly indigestible fiber and relates negatively to forage digestibility. Total nitrogen concentration in the forage (usually expressed as crude protein) is also a useful measure, since adequate intake of nitrogen is essential for animal productivity.
Forage laboratories analyze samples for NDF, ADF, and total nitrogen. It is also possible to accurately estimate these components using near infrared reflectance spectroscopy (NIRS). Other estimates of forage quality, such as total digestible nutrients (TDN), net energy of lactation (NEL), and relative feed value (RFV) are derived from mathematical manipulations of NDF and ADF values.