Mark S. Edwards, Ph.D.
Zoological Society of San Diego, P.O. Box 120551, San Diego, CA 92112-0551



The co-evolution of plants and the animals that eat them has led to adaptations of both the consumer and the plant. Mammals that consume plant materials rich in structural carbohydrates are commonly referred to as herbivores. The ability of animals to use the structural carbohydrates in plants as food is dependent upon the capacity of gastrointestinal microbes to degrade them and of the ability of the host to use the end-products of microbial fermentation and the microorganisms themselves (Van Soest, 1994).


Mammalian Adaptations to Herbivory

Structural polymers are the most abundant sources of energy in plants (Parra, 1978). However, vegetative plant parts are relatively low in digestible nutrient concentrations. The structural carbohydrates which make up plant cell walls, include cellulose, hemicelluloses, and related compounds (lignins) that are resistant to digestion by mammalian enzymes (Moir, 1968; Van Soest, 1977).

Mammalian digestive enzymes can hydrolyze a 1-4 and a 1-6 glycosidic linkages in starches and sugars. However, enzymes required to hydrolyze the ß 1-4 glycosidic linkages between individual glucose units of cellulose and hemicellulose are lacking in the digestive secretions of almost all animals (Van Soest, 1982). Mammalian herbivores have developed a wide variety of physical adaptations which promote, either through symbiotic microbial fermentation or mechanical action, the destruction of the structural and chemical defenses of the plants upon which they feed.

Ruminants and ruminant-like animals have a large sacculated foregut that is the primary site of microbial activity. Within the foregut are anaerobic cellulolytic bacteria, and other microbial symbionts which produce enzymes that degrade plant cell walls and promote access to cellular contents. This type of gastrointestinal tract permits microbial fermentation to precede gastric and enzymatic digestion. Moir (1968) listed a number of animals which possess a sacculated foregut-type digestive tract including macropod marsupials, some edentates, hippopotami, camels, and colobine monkeys.

The process of rumination (regurgitation and re-mastication of digesta) is an important mechanism that assists in reducing the protection afforded cell contents by lignin. The additional physical processing by rumination helps reduce the particle size of lignified plant tissues, creating “openings” that permit access for microbial symbionts. However, not all species that have adaptations for foregut fermentation ruminate. Thus, the type of plant fiber and extent of lignification may have different effects on these species than on the more advanced ruminants.

Symbiotic microorganisms occupy enlarged areas distal to the gastric stomach in animals with hindgut fermentation (e.g., elephants, equids, tapirs, rhinoceros). These animals are unable to quantitatively recover as high a proportion of the nutrients produced by fermentation as are foregut fermenters. This decreased return on microbial fermentation may be due, in part, to a faster rate of digesta passage through the lower gut, thus limiting the time available for absorption.


Feeding Strategy in Relation to Captive Diets

Hofmann (1973) grouped East African ruminants into three major classes based upon gastrointestinal anatomy and feeding habits. Herbivores designated as “concentrate selectors” or “browsers” consume plant materials that are relatively low in plant cell walls. These concentrate selectors do not use large amounts of dietary plant fiber effectively and thus selectively feed upon those plants parts that are higher in readily fermentable carbohydrates. Herbivores which consume plants that are relatively higher in cell wall components, typically grasses, are classified as “bulk and roughage eaters”. The third class consists of “intermediate feeders” that are more adaptable to changing habitats and vegetation than most of the concentrate selectors and bulk and roughage feeders. Although there is recent dispute over the nutritional and physiological significance of these divisions (Robbins et al., 1995), the general categories provide a starting point, in terms of captive diet formulation.

Hoofstock diets typically consist of varying proportions of hay, pelleted feed, browse, and commercially available produce. The type of hay included in the captive herbivores diet is usually chosen based on the feeding strategy categories mentioned above. For example, a highly selective browser (e.g., gerenuk) would most appropriately be offered a forage with a high leaf:stem ratio, such as alfalfa. Bulk or roughage feeders (e.g., hippopotamuses) should be fed good quality grass hay.

Pelleted feeds are usually formulated to be either complete feeds (with and ADF concentration of 15% or more), which do not require much supplementation with hay, or concentrates (with an ADF concentration of 9% or less), which must be fed in limited amounts and with a substantial quantity of hay to maintain normal fermentation (Oftedal et al., 1996).


Forage Evaluation

Dried forages (e.g., hay) and/or pasture typically comprise the largest component of the captive herbivores diet. As plants, these feeds will also be the most variable in relation to nutrient composition. For example, the nutrient quality of hay varies more than any other harvested crop in North America (Edwards, 1991).

Identifying good quality forages is as important as identifying balanced complete feeds (Edwards, 1991). The primary factors which affect the quality of hay and acceptance by animals are 1) plant species in the forage; 2) stage of maturity; 3) chemical composition; 4) leaf:stem ratio; 5) presence of foreign material; 6) harvest and storage conditions; 7) physical form; and 8) antiquality constituents (Edwards, 1991; Taylor, 1995b).

Although the experienced hay buyer may be able to judge hay with organoleptic methods, the best method is to probe a representative number of bales and have a sample chemically analyzed (Taylor, 1995a). A minimum of one sample is required for every 10-12 bales in the load (Wakefield et al., 1957). Two methods of analysis are used to evaluate hay quality: wet chemistry and Near Infrared Spectroscopy (NIRS). Chemical analysis combining proximate and Van Soest fiber fractionation (i.e., NDF, ADF, ADL) will produce the most reliable estimates of the nutrient composition of the hay. NIRS is a computer based system, and as a result, is only as strong as the database with which the results are compared.


Literature Cited

Edwards, M.S. 1991. Selection methods and quality evaluation of forages used in captive animal diets. In: Regional Conference Proceedings of the American Association of Zoological Parks and Aquariums. Pp. 771-777. Wheeling, WV.

Hofmann, R.R. 1973. The Ruminant Stomach: Stomach structure and feeding habits of East African game ruminants. East African Literature Bureau, Nairobi, Kenya.

Moir, R.J. 1968. Ruminant digestion and evolution. In: Handbook of Physiology. Pp. 2673-2694. American Physiology Society. Washington, DC.

Oftedal, O.T., D.J. Baer, and M.E. Allen. 1996. The Feeding and Nutrition of Herbivores. In: Wild Mammals in Captivity: Principles and Techniques. Pp. 129-138. The University of Chicago Press. Chicago, IL.

Parra, R. 1978. Comparison of foregut and hindgut fermentation in herbivores. In: The Ecology of Arboreal Folivores (G.G. Montgomery, ed.). Pp. 205-229. Smithsonian Institution Press. Washington, DC.

Robbins, C. T., D. E. Spalinger and W. van Hoven. 1995. Adaptation of ruminants to browse and grass diets: are anatomical-based browser grazer interpretations valid? Oecologia 103: 208-213.

Taylor, R. W. 1995a. Hay sampling and grading. Cooperative Extension Bulletin AF-16, University of Delaware.

Taylor, R. W. 1995b. How to judge hay quality visually. Cooperative Extension Bulletin AF-15. University of Delaware.

Van Soest, P.J. 1977. Plant fiber and its role in herbivore nutrition. Cornell Veterinarian 67:307-326.

Van Soest, P.J. 1994. Nutritional Ecology of the Ruminant. Cornell University Press. Ithaca, NY.

Wakefield, R.C., R.A. Briggs, G.H. Ahlgren, R.B. Randle, and W.H. Hosterman. 1957. New Jersey Agriculture Experiment Station Bulletin #784.