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Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research. (1980)

Chapter: Appendix A: Digestive Physiology of the Horse

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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Page 298
Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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Suggested Citation:"Appendix A: Digestive Physiology of the Horse." National Research Council. 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research.. Washington, DC: The National Academies Press. doi: 10.17226/18642.
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APPENDICES Appendix A The Digestive Physiology of the Horse and Its Interrelationship with Feeding Ecology of the Equidae A report for the National Research Council Committee on Wild and Free-Roaming Horses and Burros by Montague W. Demment Department of Zoology University of Wisconsin, Madison Introduction The overall purpose of this review is to provide an interpretive perspective of the digestive and nutritional characteristics of the horse. This perspective is aimed primarily at understanding the way digestion and nutrition in this species affect its overall ecology, encompassing both its interactions with its food supply via diet and other herbivores, primarily cattle. Although this paper reviews the literature, it does so selectively. The emphasis has been to include key papers that enable digestion to be related to equid ecology. Extensive and recent reviews of horse nutrition and digestion are already available in the literature (Olsson and Ruudvere 1955, Mehren and Phillips 1972, Frape and Boxall 1974, Robinson and Slade 1974, Frape 1975, Hintz 1975, Hintz and Schryver 1973). The horse is a herbivorous animal which extracts substantial energy from the structural carbohydrates of plant material. To accomplish the digestion of these carbohydrates, vertebrates must rely on a symbiotic relationship with bacteria and protozoans (Moir 1968). This digestion occurs by a process of fermenration in the gut. Although the products of fermentation are widespread in the G.I. tract (Elsden et al. 1946), they are usually concentrated in a site within a particular section of the tract. This fermenta- tion site provides not only a protected environment of the proper pH for the microbes, but also the ability to slow down the passage of fibrous particles for more complete fermentation. 259

260 One of the primary functional dichotomies in the classification of herbivores is based on the location of the fermentation site (Janies 1976). The equids ferment forages in the hindgut. The ruminant species are primarily foregut fermentors. Ruminants are so named because they regurgitate forage already ingested and remasticate (i.e. ruminate) the bolus to break down the forage particles for more complete digestion. However a number of foregut fermentors (e.g. colibid monkeys, macrapods, hippo- potamus) do not reuminate. The rumination process is ecologically quite important and should be distinguished as a separate characteristic beyond foregut fermentation. In a confusing bit of well-established terminology, hindgut fermentors are called nonruminants (this nomenclature could easily apply to foregut fermentors as well). I will continue this misnomer for the sake '.of convention. Plant Material, Chemical Classification and Fermentation Plant material is not homogeneous in its response to the digestive systems of animals. Plants are composed of components which differ in the rates they can be digested by animal and microbial enzymes. Therefore, ideally, plant material should be characterized by the component fractions within which the action of digestive enzymes is uniform. To establish relative digestibility would be a case of determining the proportions of the component fractions in the foods. The problem facing nutritional assays is complicated because chemical analyses do not necessarily act on forages to separate nutritionally uniform elements. This problem is particularly acute with the widely used crude-fiber determinations which attempt to separate the digestible and indigestible components of forages. Crude friber recovers mainly cellulose in the indigestible fraction (although it is nutritionally available to herbivores) while losing part of the lignin (truly unavailable) to the digestible fraction. In the first growth of temperate grasses where lignin is positively correlated with cellulose, crude fiber is an adequate predictor of digestibility (Dijkstra 1969); but in tropical grasses, where no such relation- ship exists, crude fiber is a poor index of forage quality (Olubajo et al 1974). The detergent system has been developed to correct, in part, this problem (Van Soest 1967). Because the principal chemical components of plants differ in their solubility at different pHs, successive extractions with solutions of different pH will sequentially remove these components. This method separates plant material into cell constituents and cell-wall fraction, further dividing cell wall into cellulose, hemicellulose and lignin (TableZL) Although these chemical entities are good predictors of digestibility (Mertens 1973), the detergent system does not characterize the digestibility of particular constituents of the forage. Digesti- bility within these fractions can vary with taxon, growing conditions and extent of lignification. However, cell contents

261 can generally be characterized as rapidly digestible while the components of the cell wall, hemicellulose and cellulose, ferment slowly. The major functional division of plant material is between the cell constituents and the cell wall (Table 21). The contents of the cell are the fraction active in plant metabolism and are composed primarily of sugars, proteins, and storage carbohydrates. This fraction can be digested directly by vertebrate enzymes or fermented rapidly by microbes. The cell wall provides the structural matter for the plant. This fraction cannot be degraded by vertebrate digestion but can be hydrolized by bacterial and fungal enzymes (Gibson 1968). Therefore the utilization of the cell wall as a nutrient source is dependent on microbial symbiosis (Hungate 1966). The cell wall while providing the plant with structural support also functions to defend the plant against herbivores. By increasing the proportion of the plant which is slowly digestible or totally resistent to breakdown, the plant decreases its digestibility and palatability. Cellulose and hemicellulose, the principal structural carbohydrates of the cell wall, vary in the extent of their availability for fermentation dependent upon the lignin content of the cell wall (van Soest 1967). The mechanism by which the effect of lignin is exerted is unclear but may be involved with the cross linkages which occur between the lignin and the structural carbohydrates of the cell wall (Van Soest 1977). Kawamura et al. (1971) have shown through histological studies of digesting plant matter that the structual characteristics of the tissue in conjunction with chemical linkages affect the rate of digestion. For this reason characteristic taxonomic ratios of lignin to cellulose and hemicellulose are important nutritive characteristics of forages (Van Soest 1973). Table 22 is a compilation of chemical analyses of plant materials grouped on the basis of their function. Several interesting patterns are apparent. In general there is a positive association between the permanency of a structure and its cell wall and lignin contents. Cell wall and lignin function as both structural support and herbivore defense. Since plants are most likely to put the most energy into the defense of more permanent parts (McKey 1974) and these structure are most likely to be supportive, both function are likely to produce a positive relationship between permanence of the plant part and its cell-wall and lignin content. Herbivores are likely to be sensitive to differences between the functional categories of plant material when making feeding decisions. Different life forms show somewhat different fiber values within functional categories. Grasses have lower fiber content in their stems than trees and shrubs but higher cell-wall and lignin fractions in their leaves. Therefore grasses provide a larger fraction of slowly fermentable material because they have a larger

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264 cell-wall component and a smaller indigestible fraction. For animals specializing in extracting energy by means of microbial fermentation, the grasses hold the greatest potential. Temperate grasses show lower cell-wall and lignin fractions than tropical grasses. Deinum and Dirven (1975) demonstrate that the extent of cell-wall formation of both temperate and tropical grasses was a response to temperature, independent of age after maturity. They also found that the digestibility of the cell wall decreased with age at a greater rate at higher temperatures. This decrease occurred because of the lignification of vascular bundles and sclerenchyma (Deinum 1976). Stems, which have higher proportions of vascular bundles and sclerenchyma relative to parenchyma, .which is not lignified, will show higher lignin values than leaves 'and hence a greater potential for depresssion of digestibility •with age and temperature. The generally lower digestibility of .tropical forages reported by Minson and McLoed (1970) and reviewed by Moore and Mott (1973) is most likely due to the differences in termperature at which they were grown and their greater stem-to-leaf ration. Furthermore tropical grasses show a greater difference between cell-wall and lignin values in the stem and leaves than temperate grasses, while the amount of soluble carbohydrates within the stem is greater in tropical grasses (Deinum and Dirven 1975). Therefore selective grazing would be expected to be more important in tropical than temperate grass- lands. Digestive Process Digestive rates. -- The actual digestibility of a forage is a function of the digestion rate (enzymatic action) acting on a particle for the duration of its retention in the digestion site. Waldo et al. (1969, 1972) proposed a digestion model involving the fiber kenetics of the rumen. They considered the rumen to be filled.with two fractions (pools), one digestible and the other indigestible, whose disappearance from the rumen was a function of the rates of digestion and passage. Plots of the log arithm of residual cell wall as a function of time for in• vitro digestion trials yield linear results when the indigestible~~portion is subtracted from the calculations (Smith et al. 1972). These results (i.e. a constant proportion of the cell wall removed per unit time) indicate first-order kenetics and validate this assumption in the Waldo model. Mertens (1973) found retention time to be the most important factor predicting intake and digestibility in sheep. Intake and retention time are linked. Increased intakes decrease retention time and depress digestibility (Raymond et al. 1959, Pearce and Moir 1964, Moe 1965, Alwash and Thomas 1971 and 1974, Tyrrell and Moe 1975). The type of diet, related to its physical form, often affects retention times in both ruminants (Alwash and Thomas 1974)

265 an nonruminants (Uden 1978). Direct correlation between particle size, retention time, and digestibility have been determined in sheep (Pearce and Moir 1964). The concept that digestion can be characterized by two parameters, digestion rate and retention time, is potentially useful for comparisons of ruminants and nonruminants. Differences in digesti- bility of forage between the two groups can be examined in light of evidence for differences in these two parameters. Fermentation site. -- The more important processes occurring in the fermentation site (either ruminant or nonruminant) are diagrammed in Fig. 9 . The fermentation site, regardless of location, is an ecological system within which there is active competition bet- ween individuals for energy and nutrients (Hungate 1966). Upon entry to the site all digestible fractions of the forage are subjected to microbial digestion. The soluble nutrients (i.e. cell contents) are rapidly fermented and the rate of the digestion of other components is related to their solubility (Hungate 1966). Microbes produce respiratory products, volatile fatty acids (VFA), which are the primary energy source from fermentation for the host. Heat and methane, also products of fermentation, are unavail- able and their energy value is lost to the herbivore. Values for these losses vary depending upon diet but generally range from 10-30% of digestible energy (DE) (Baldwin et al. 1977). Microbial populations within the fermentation site form an ecosystem of populations (McBee 1971). The populations grow at rates proportional to their nutrient supplies (Orskov 1978). They assimilate energy and utilize dietary protein or ammonia as nitrogen sources for growth and reproduction (Hungate 1966). The ability to upgrade ammonium ions to protein nitrogens has the potential to provide urea recycling and microbial protein synthesis to the herbivore. Since microbial bodies are being continually washed from the fermentation site the protein (and other synthesized components such as vitamins) have the potential to be digested and absorbed in their synthesized form. The positioning of the fermentation site along the gut is important to the function of fermentation for the herbivore (Parra 1978). Since the soluble fraction (cell contents) of the forage is directly digestible by gastric enzymes of vertebrates, if this fraction is fermented in the foregut, a sizeable fraction of the energy is lost to heat and methane production. Since little of the soluble nutrients escape fermentation (Bryant 1963, Bryant and Robinson 1963, Nolau et al. 1972), a critical difference between ruminants and nonruminants exists in the importance of the soluble fraction as a dietary component. This point will be discussed in detail below.

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267 Fermentation-site positioning may also determine the relative capacity of ruminants and nonruminants to utilize the synthesized nutrients in the bodies of the microbes. In the foregut fermentors, microbes are subjected to gastric digestion in the abomasum and the resulting components are absorbed in the small intestine. However, for the nonruminants no established mechanism has been identified for the digestion of microbes in the colon and somewhat contradictory evidence exists for the absorption of any digestion products. The utilization of urea and microbial protein is a clear example of the differences in these systems and because of its importance as a limiting nutrient in grassland ecosystems (Sinclair 1977) will be discussed in detail below. The ability of ruminants to utilize low-protein diets has been widely established (see reviews by McDonald 1968, Tillman and Sidher 1969). They show the ability to survive on diets composed entirely of NPN (Loosli et al. 1949, Virtanen 1966). Since there is no evidence that their amino-acid requirements are different from other mammals (Moir 1968), this ability reflects the microbial synthesis of amino acids from NPN source (NAS 1976). In the foregut fermenters, dietary protein is attacked by the rumen microbes for energy (Hungate 1966). The intense competition among the microbes for energy sources is reflected in the low concentrations of free amino acids and peptides in the rumen (Bryant 1963, Hungate 1966). Ammonia, produced partially from hydrolysis of protein, is rapdily incorporated into microbial protein (Bryant and Robinson 1963). Nolan et al. (1972) found that 80% of the N in the bacteria of the rumen was from the ammonia pool. Some of the protein of low solubility does however pass through the rumen without digestion (Chaluysa 1972) and some free ammonia is absorbed through the rumen wall and transported to the liver for amino-acid synthesis or urea formation. The microbes, carrying the preponderance of the dietary protein, are washed from the rumen and their proteins digested and amino acids absorbed in the small intestine (Armstrong and Hutton 1975). Ammonia produced in this process is absorbed into the portal system and returned via the blood as urea to diffuse into the rumen (Houpt 1959) , and to the salivary glands to reenter the rumen via the esophagus (McDonald 1948, Nolan et al. 1972). Microbes can hydrolyze the recycled urea and utilize the ammonia as a nitrogen source (Bryant and Robinson 1963). The recycling system, capable of upgrading endogenous urea to essential amino acids, also has the ability to utilize dietary NPN sources for all amino-acid synthesis. There- fore dietary selection for amino acids is not essential, but selection for nitrogen in any digestible form is critical. Also because rumen microorganisms rapidly assimilate most of the glucose in the rumen, foregut fermenters must rely heavily on deamination of protein in the liver for their blood-sugar supply. Because urea is a product of deamination, without the ability to recycle urea, large nitrogen losses would occur in the urine of ruminants (Moir 1968).

268 Because in the nonruminant dietary proteins are hydrolyzed in the stomach and small intestine by the herbivore's proteolytic enzymes and absorbed directly into the body in the small intestine, the energy losses involved in the microbial breakdown and resynthesis of protein in the rumen are avoided. However because the major fermentation site is posterior to the site of digestion and absorption, the digestion of protein and absorption of amino acids must occur in the colon. The mechanism for the digestion and absorption of amino acids in this organ is not well documented. Furthermore because the fermentation site is located in the hindgut the time for the digestion of the microbes and absorption of their nutrient components is much reduced relative to the ruminant system. Therefore nonruminants would be likely to show higher fecal nitrogen losses than ruminants of the same size. The existence of nitrogen cycling in nonruminants has been demon- strated in man (Walser and Bondenlos 1959) , rats (Rose and Dekker 1956), horses (Houpt and Houpt 1971, Prior et al. 1974) and rabbits (Rogeczi et al. 1965, Houpt 1963). A number of studies have demonstrated the presence of urea nitrogen in the body amino acids (Richards et al. 1967, Snyderman 1967, Grimson 1971, Rogeczi et al. 1965, Davies and Koonberg 1950). Since recycling of nitrogen does occur in nonruminants, the ecological consideration is whether there are quantitative and qualitative differences between the two digestive strategies. Quantitative differences between ruminants and nonruminants would be expected on the basis of the positioning of the fermentation site. The magnitude of the difference should be size specific, decreasing with larger body size as retention times increase. Prior et al. (1974) show similarity in urea retention between horses, cattle, deer and sheep. The horse's large body size compensates for the hindgut fermentation site to produce efficiencies similar to the deer and sheep. Man and rabbits snowed much lower efficiencies. The rabbit's inefficiency may be a function of its size where in man, an omnivore, the absence of a highly developed fermentation site would likely impair the efficient hydrolysis of urea by microbes. Qualitative differences arise if the site of amino acid synthesis is the liver or the microbes of the gut. The range of amino acids that can be synthesized in the liver is limited. Those amino acids required by the animal but not synthesized by the liver must be present in the diet and are referred to 'essential' amino acids. However microbial synthesis can supply the full complement of amino acids. Ample evidence of all the cirtical steps of nitrogen recycling exist for the nonruminants, but except for the horse (where the evidence is by no means unequivocal, see discussion in Robinson and Slade 1974), there was no demonstration of the ability to absorb amino acids from the colon. Work on germ-free animals indicates that urease (the enzyme for hydrolysis of ureau in gut

269 is solely of bacterial origin (Delluna et al. 1968). Therefore the contact of ingesta with a concentrated bacterial population is important for efficient nitrogen recycling (i.e. humans, who lack developed fermentation sites, would not be expected to show high nitrogen recycling capabilities). The administration of antibiotics to the nonruminant reduces or eliminates its ability to retain nitrogen (Wrong et al. 1970, Houpt and Houpt 1971), Once ammonia enters the liver and is dextoxified to urea, it is recycled in the blood to the caecum of the horse (Houpt 1963), G.I. tract in man (Waser and Bodenlos 1959, Richards 1972) and to the salivary glands (Forland et al. 1964). The urea in the gut is hydrolyzed and the free ammonia can either diffuse through the colon wall (Castell and Moore 1971) or be assimilated by the microbes in the cecum (Wootten and Angenzio 1975). The degree to which ammonia is utilized by the microbes is dependent on the energy sources in the lower gut (Mason 1976). The ability to absorb amino acids in the colon has been demonstrated only in the horse (Slade et al. 1970). The ability of mature horses to maintain themselves regardless of the nitrogen source is further indication of the assimilation of microbial amino acids (Robinson and Slade 1974). However the actual mechanism by which the microbial cells are digested in the lower gut has not been described. Several mechanisms have been suggested: the continued activity of pancreatic enzymes in the caecum (Robinson and Slade 1974), autolysis of microbes in the colon (Baker 1942) and the activity of protrolytic bacteria, common in the squid caecum (Kern et al. 1973) . Comparative Anatomy and Digestive Function Size of gut components. -- Since retention of particles within segments of the gut is a function of intake and the volume of the section, the relative size of segments in the G.I. tract indicates the relative capacity for digestion within the segment. On a comparative basis differences in allotment of volume to the segments indicates the digestive strategy of the species. Data on the volume, estimated by contents, is given in Table 24 . Some interesting patterns are apparent. As expected, ruminants devote the majority of the gut volume to the foregut and nonruminants allocate a substantial capacity to the hindgut. Within the non- ruminant the relative volume of the hindgut increases, indicating the growing importance of fermentation with larger body size. However donkeys have, on an absolute scale, substantially larger lower tracts than expected and show greater capacities for fiber digestion than the horse (Wolter and Velandia 1970). The fermentation products, a crude estimate of fermentation rates, show that nonruminants achieve similar concentrations within the fermentation site as ruminants (Table 23). The horse and the cow are similar in body weight and weight of gut contents (Eldsden et al. 1946). The fermentation contents appear to be slightly greater in cows than horses (10.0-17.8% of body weight in the cow, 1l.2% in the horse). Interestingly Parra (1978) found that the weight of fermentation contents had a similar exponent

270 e sis s = s 7. 4- d « *• CN — « ,^ — CU Oi jj1 C Q C -. C — , — C ^ rr N in tf r- ^j ^ .««• 63 0 r^ — v .v f*4 tN r*i , O CJ CO O 0) iH 0 CO ^ fN C 0 ^ = •H XI a •^ tN ^ vC .*1 tN P* CO CO c u *aj o * 4J r-l U-l M r-i • S ^" ^ 5 uO 5 *"• O h Cf l 4- . W. 4J e c U C CO M ^- ^« 3 0) O u c x: CJ -C . 4J ^ CJ 9 Al tM iT1 V J" PJ <a )N r« si ^ 3 LT S n t Ptl 6 0 > 0 - ' ^ ^ w CO c o •H E 4-i 0 - r* o —1 "~ " o P- o^ . *"". w. . . , 0) 4J CO U ^TO"cvtr1— • fM ^ a — c cf l 0;J ^^ c c in C tT. o HI c ^u". OOCr*^. — •H 4J ec "c ^' V = = 5 . n r,' a C O i— i a t 3 rH t0 13 O CO £ ~- c c tN • in « •> 4i h 4J £ P-, 0 C £H 0) r* *r — ,» — ~ K ^ cfl a c b . CJ b 4J O 3 -J Is T1 M M C « C "CO* 4-. n -; n * (*; * Q •« £ § cn M fN — — -^ > w en 01 i j •H •H U j. : s § 0) c tN'* — Q*-lO^Pl v ~ — "" T ^ a CC n) E o "*' v f! f^ rs' ^* *•*" I— 5 « CJ C cv r~ *• " — •— r* flC 53 o — j- w C1 ^ ^ ii ** U-l cr C £ r O c ^ O •-"* ^ r«i .-^ —i ^ rsi 3 - ' o ? = £ a CO •H r - E ^ CO - I - Q. y*^ *** O — ^ ^ g >•1 ^> u *° ^ « O ^j * _ CJ "^3 ^^ — ' ^. . - rsi ^- 3 * 0 rN -M ri C3 r^ = - c c i— i PQ 4J •^ tN V iS I I - I ro • ! ^ 5 (M c r tU rsi 5 E — — ' C 3 - o x1 3^5^ 5 | r § - :" J H — ~ !N~ - <

271 tn •p . 2 •P •r.l t^ \ <t3 rC O •P C i 0) o o O CD *o '4—1 o f0 MH CO V0 CM S*«i •H in o o •a 0 rH f*" O C o •H o p*1 •— i o ^o f* , i r^ ^r cr* CO 3 o • • • tN n vfi c 0 •H c rH d) ,,— x v£ CO ^^ rH 00 5 l o m 0 i* O 0 id E^ c •P 0) s^s o £•, o in X CN tM <N o o; cn w i c rr tN in •-! a 6 en" £n O o 0) 03 •H CO c o •-» rH o i n • • • r-l 1 o TD 3 ro O •p •H CO .p O JH rn l rH fN O O 0) cx O fN e •H l4H CQ o O tN i 0 CO ^^ O T3 fy C £: 3 E^ o t—1 CO ^ i CO 0 fu CO JM CO CO rd H ^ . o -H tt tQ flj tT3 in r* r-i in CD •H CQ •y E r OCX M 1H ro *tr ^>s EH -1 H CO 4H a ^ 0 eIH •p O c 3 >, O 1 1 CO ^C t3 t3 •p rH '3 TJ ._, -H ^ rH w C - W 3 c in <? tD c <n O • •H (3 •— 1 -H .H t3 S Sj _, CO CO 3 ^ O CN C" u v n 8) tr c 03 Q !*1* i^c^N Uf0r^ f0 ^r CU J^CLI CT1O| tr1i;i r^oi c P-J i Q^ Lt r*1 o^r11* o *H r** .P rH A CCrH l-iJJ1 )MiJ1 fflSt Q H w<i-i a:so asx w<^r rH

272 for regression against body weight, as gut-contents weight in ruminants and nonruminants fit the same regression line. The horse, similar in weight, metabolic requirement, and fermentation. site size, shows differences in its ability to digest the fibrous fraction of forages. Similarity in the above parameters affecting digestive capacity implicates the different retention mechanism of the rumen and caecum in producing differences between horses and cows in digestive efficiencies of the cell wall of forages. Mechanisms of retention in the cecum and rumen. -- Uden (1978) did comparative work on the digestive capacities of ruminants and nonruminants. He found that fecal particle size was larger in nonruminants than ruminants. Although this phenomenon is often attributed to the longer retention times of ruminants (see below) there are a series of differences in the two digestive systems which produce different particle size. A critical element of the rumen's function is the selective delay of ingesta. The selective delay occurs by lowering the probability that forage recently ingested will escape the rumen. If a volume is filled with a liquid, in which perfect mixing occurs, there is a certain probability that some intake will be simultaneously lost. Once a particle enters the volume its probability of escape does not change with time. The rumen has a mechanism to link the probability of escape with residence time of a particle. Smith (1968) has demonstrated how particle-size breakdown occurs microbially in the rumen and that particle size is related to passage rate (Fig. 10)• Since forage enters the rumen as large particles and is progressively reduced to small size, the rumen efficiency is produced by linking the probability of passage to residence time via particle size. Although somewhat elective, the caecum behaves more like a perfect volume than the rumen (Uden 1978) and particles of different sizes appear in the colon at similar rates (Argenzio et al. 1974). These results would suggest higher rates of escape for undigested particles, and coupled with differences in transit times, should produce lower digestibilities of the cell-wall fraction of forages in nonruminants. Body size. -- A number of anatomical and physicological factors are related to body size but because they often change with weight at different rates individuals of different sizes are required to be different types of animals. Both metabolic rate and gut capacity change differentially relative to weight. The significiance of this ratio was originally suggested by Short (1963) and has been discussed by Prins and Geelen (1971), Janis (1976), and further developed by Parra (1978). To formulate this relationship more precisely a model is presented.

273 a 0) a) 3 ,C O CO rH 4-i 03 O 0) C > a) it) e x: 3 Pi cn co CU rH x: o p 'H •P S fc O <0 bO > tT3 p CO .H CO r-i 0) .H —' a; £, •P M 03 O O "O a o O 3 x: o •H rH rH 0) CU X > UH 'H 03 a; rH x; eU bO rH P. CU C 0 03 M 'H •H rH .H 03 .P cn bO Pi .P 0) 03 03 bO ax: c cn •P 'H CD t«-l CO rH O CO 03 O 3 0J'rH O Pt .P Pi 03 Pi p o X: T3 0) i— 1 P CO bo a •H N it) 3.H CO 0) CO cn x: *O 1 03 E-i C rH a 0) rH 03 o • , e •H /~N 0) CO 4-, CX3 bO •H tD 03 >i CJ 05 CO Pi 0) rH cn a) CH 03 > cn x: a, i .P 1 — I cy 'H U-, .H N e O .P •H CO c CO ^Xl ^ e 4J 4n O •rl CO 0 P. rH 0) U-, •H CO •p XJ 03 O 03 0) d) H .P rQ ?H a o) O O •a P. C JM/39VSSVd JO 31VH

274 Because basal metabolic rate (Kcal/kg/time) decreases nonlinearly with body weight, the total metabolic requirements of mammalian herbivores, MD (Kcal/day), increases as K MD = 70 W75 (Kleiber 1975) (1) K where W = weight in kg. The use of the exponent .75 is applicable for interspecific comparison across a wide range of body sizes. Differences in this exponent between species (Thonney et al. 1976) may slightly modify specific comparisons I may make, but will not alter the general conclusions. Because Mfi increases with weight, albeit at a decreasing rate, large animals always require more total energy but small animals require more energy relative to their body weight. The capacity of the gut determines, in part, the capacity for digestion in the herbivore. Parra (1978) plotted the wet weight of gut contents of herbivores, both .ruminants and nonruminants, against weights (Fig. 11 ). Demment (submitted) argues that the bias of the measure of capacity by digesta weight is systematically related to body size. He suggests that the exponent should be l.00 instead of the l.09 of Parra's regression. Importantly ruminants and nonruminants show the same gut size relative to weight (Parra 1978). ' If the rate of metabolism determines the nutritional requirement, and the gut size the capacity to process food into nutrients, then the nonlinear response of metabolism coupled with the appro- ximately linear response of gut size produces higher ratios of metabolism to processing capacity in small animals relative to large. If the gut is considered a volume into which mass flows, but behaves like liquid, the problem can be examined using single- pool kinetics (Shipley and Clarkl972). The time a particle spends in the volume, its retention time, T, can be expressed as: T = Y (2) where V is the capacity of the gut (mass) and I is the intake (mass/time). The dry-weight volume of the gut has been approximated as 10% of the wet-weight contents and Parra's fitted equation (Fig. 21 ) has been corrected accordingly: V - .00936 W1.0768 (3) where W is the body weight in kg and V is the GIT capacity in kg dry matter. Kleiber's equation for basal metabolism can be converted to a dry-matter (kg) requirement using 4409 Kcal/kg (N.R.C. 1973). The intake of dry matter (kg/dry) required to balance basal metabolism at a constant digestability is:

275 ^r O CO CO O cc CM o h. O z ro UJ O ^^ CD cc UJ U. O h- CO O o 0 ^}" UJ or o o CJ u. o u. — o UJ in N CO 0 O o • 22 — ; ; ! 1 I 0 0 CD 0 0 o 0 o (0 CM S3!D3dS 1HV JO % CU 03 bO 03 0) rH x: •p M p O O > 3 c 4-i •H C-i bO •H C x: 0) CU cu 3 > SH CO S 0 p A •o IP, C cu *^ g M-i r* 03 0) c oo ^J •H •p r-t i-H .p 3 r-l bO u 0] M-t h 0) = 0 Z cn O 0 ^x 4-i c UH 0) 0) CO (~l CO CU id CU EH XI fj •H 3 0) cn CJ CO •H 0) • 0) cx^ m fn £ en f^ V-x' 0) U bC /•; • H CO •H cn EH ~* 3 CU ^j 3 0 3 C *•"•• 0] • r CTi o >1 c cn CO JC > T3 •H 0) D •H O e N £ Q o 3 •H rQ £•, PI CO ^ »- CU 4-i o 3Z cn CU ^ a. c r* T3 o O c •rH H c a. a (d XI ™* u bO .H UJ •H 0) • 1D x: 3B "Hn^.. Clf Cfi t— 1 MH *o c_, "O cu < CD 0 •a x: 4-1 p "tH •P >• P P 0 rH C E Q cn cu >, e CU rH O U O. x: p C CD <u O rH cn U-i rH •H 0) 03 +j p • oo 3 ifl >, , Mi C_| bO c t. 0) O 0 TD •H 0 p C e DO OJ 0) c •H 0 4) ^-« bO £ A id' H t^ e P I. cu ^^ 0 C CU h cn r^ a) b bO 0 C .x fJ C-t •p cn p C CU 3 0J C + 0, bO •rl 0) OJ a 0 CO IU yj a> oc cu x; 0 a Q) CSJ H «M cn 4H t 1 T> ^ .-H g 3 bo •H ~

276 .0159 W75 (4) D where D is the percentage dry matter removed from the intake and not present in the feces. Substituting equations (3) and (4) into (2) yields: T = .589 D W3268 (5) The rate of passage, Kp (a rate constant), is the reciprocal of the retention time: - 327 W Kp ~' '•'• . S89D Equation 6 states that rates of passage in small animals must be higher than in large animals to meet basal metabolic require- ments on the same quality foods. This relationship suggests that small animals are constrained by rapid passage. To compensate they eat rapidly digesting foods (i.e. large cell-content fractions). Only in the middle body sizes (5-800 kg) is the MR/gut size ratio sufficiently low to allow selective delay of forages. In these body-size ranges, ruminants are dominant in grassland ecosystems (Figl2). Since total metabolic requirement increases with body size, larger animals require more total food than smaller animals. The general relationship of abundance and forage quality is presented in Table 24. These data suggest that high-quality forage is rare and low-quality forage abundant. Therefore as body size increases, diet quality must decrease (Demment 1978). Demment and Van Soest (in prep) argue that the upper limit on ruminant size is set by the rate at which low-quality diets can be reduced in particle size for passage. It is the very mechanism that allows increased efficiency of fiber digestion that limits the range of diets the ruminant can utilize efficiently. Therefore very large herbivores are nonruminants which eat a lower-quality diet than ruminants. The dominance of the perissodactyls during the Eocene and the subsequent reduction in number coincidental with the rise of the ruminants in the Oligocene (Janis 1976) , coupled with the present patterns of body-size distributions of the two groups, strongly argues that competition from the ruminants has been of major importance in shaping equid feeding ecology. The more efficient ruminant system for the digestion of plant cell walls may have displaced the equids as the grasslands of the world became predominant. As argued above, the nonruminant system, designed for passage and not delay, functions effectively in the small and very large body sizes. The evolutionary response of the horse has been to increase its body size through evolutionary time (Simpson 1950), quite possibly in order to

277 1000- RANGE 1N BODY WT. (kg) = 0.683-1384.0 LOG Y = 1.0768 LOG X - 1.0289 r = 0.99 n = 33 100 o» UJ I- o o o 10 O.1 • RUM1NANTS x NON-RUM1NANT HERBV. Figure 12 10 100 BODY WE1GHT (kg) 000 (From Parra 1978). Linear log-log plot of gut-contents wet weight on body-weight yield. The exponent is very close to l.00, indicating an isometric increase in gut size with body weights. There is no significant difference between ruminants and nonruminants in this relationship.

278 have a body size sufficiently large to eat a diet of coarser quality than the ruminants (Bell 1969). Digestive Capacity of Equids Crude-fiber digestion. — The fibrous fraction has been shown to be the most important forage parameter for predicting digestion coefficients for horses (Olsson 1949, Hintz 1969, Fonnesbeck 1969, Vander Noot and Trout 1971). High correlations with crude-fiber values have consistently been found (e.g. Olsson 1949). But Fonnesbeck (1963), evaluating several assays, found Van Soest's methods (1964) of detergent analysis to be the best predictor of digestibility in forages for horses (the inadequacy of crude fiber as a forage measure was discussed under "plant material" above). Olsson (1949) conducted the most extensive review of the literature on digestion trials of horses. He determined the following relationship between the digestion coefficient of organic matter (y) and crude fiber as percent of dry matter (x): y = 99.71 - 3.066x + ,0679x2 - .00056x3, n = 1094. A depression of digestion coefficients with the increase in the fibrous fraction of forages is hardly unexpected nor confined to equids (Baumgardt 1970). However substantial evidence exists to indicate that ruminants show declines in digestion coefficients that are not as rapid as equids with increasing cell wall. Higher digestibilities for cattle than horses have been widely recorded (Axelsson 1949, Hintz 1969, Barsaul and Talapatra 1970, Vander Noot and Gilbreath 1970, Geyer 1974). Several studies have measured the digestibility of chemical components of the forage and found that while equids can digest NFE and often protein as well as ruminants, they consistently show lower digestibilities for the fibrous fraction (Axelsson 1949, Barsaul and Talapatra 1970, Vander Noot and Gilbreath 1970). Hintz et al. (1973) found that, while ponies and sheep had similar dry-matter digesti- bilities, ponies were lower in the digestion of cell wall than the sheep and New World camels. Since the ponies were four times heavier than the sheep, the difference is likely due to body size. Camels and ponies were identical in weight. In the most extensive controlled comparison of horses with cattle (also sheep and swine), Axelsson (1949) calculated regression equations of digestibility against crude fiber for each species (N > 100 for each species). His equations are as follows: horses y = 88.2 - l.25x ruminants y=87.8- .83x swine y = 93.7 - l.60x where y is digestibility of organic matter and x is crude fiber as a percentage of dry matter. The regressions for horses and

279 ruminants are plotted with Olsson's (1949) regression equation in Fig. 13 . Since the real data are not given in either paper, nor plotted as points, it is difficult to determine if the differences in the horse curves are due to regre sion techniques. Intuitively one would expect the differences between horse and ruminants to remain at least constant (Hintz's (1969) evaluation supports this notion) with increasing fiber content. However this relationship is clearly one requiring further investigation. I have added an addendum which addresses this problem. Horses show shorter retention times than do cattle (Tables 25 and 26) If the caecal design alibws the escape of incompletely digested particles, this inefficiency when compared with the rumen should require higher intake rates and faster passage rates (all other factors being the same). The data from Argenzio et al. (1974) emphasizes the retention capabilities of the equid lower tract. The absence of sufficient particulate marker in the fecal collections negates the ability to calculate particulate retention times for these data. Linerode (1966, ref. in Robinson and Slade 1974) suggests that passage through the upper digestive tract may be three times faster in horses than ruminants. The passage rate does not appear to be affected by diet composition (Vander Noot et al. 1967) although sample size is small in this experiment. High individual variability is often the case with digestion trials and passage measurements (Uden 1978) and determination of treatment differences would necessarily require large sample sizes. The physical form of foods (pelleted, ground, chopped, or loose) showed no effect in work by Wolter et al. (1976) while the same workers (1974) found that hay (normal ground, pelleted) took 37, 26 and 31 hours respectively to transit the gut. Haenlein et al. (1966) measured more rapid passage with pelleted and wafered hay than loose hay. Fistulation of horses appears to lengthen retention times and increase digestibility (Pulse et al. 1973). While the importance of caecal retention times are citical to the determination of fiber digestion in the horse, the effect of fistulas and cannulas, necessary for these determinations, must be carefully measured in each experiment. Hindgut fermentation. -- The sequence of digestive enzymes to which a forage is exposed as it transits the gut is different in the cow and horse and has the potential to supply different quantities of nutrients to the herbivore. Initially forages are exposed to gastric enzymes in the horse and microbial fermentation in the cow. The digestion of the soluble carbohydrates and protein is almost complete in the horse (Fonnesbeck 1968, Roberts 1975a,b). Although some cell contents may escape to the lower tract when diets consist of large quantities of concentrates on a wide range of forage/concentrate ratios, these nutrients are directly digested

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283 in the foregut (Hintz et al. 1971). In the rumen the soluble fraction is rapidly fermented and in a sense, this energy is needlessly run through another trophic level where the cost of fermentation (i.e. microbial respiration and methane production) must be subtracted- Therefore, when considering the soluble frac- tion, the difference between DE and ME (metabolizable energy) is much greater in the cow than in the horse. This point will be considered more quantitatively under the intake section below. The fibrous fraction has been largely digested when it escapes the rumen; some fermentation does occur in the lower tract of ruminants. The foregut placement of the rumen greatly enhances the ability of the ruminants to digest and absorb the microbial bodies (discussed above with reference to protein). When the digesta leaves the stomach of the horse it is composed largely of cell wall which is fermented in the hundgut. The relative importance of fermentation in the colon increases as the concen- trate in the diet decreases (Hintz et al. 1971). Therefore under field conditions, as the forage matures, there will be a shift decreasing the amount of energy being absorbed as glucose in the small intestine (because the soluble fraction is decreasing) and increasing the energy derived from VFA in the hindgut. Since the metabolizable energy available from glucose is greater than that from VFA production, for the reason stated above, the maturing of forage causes a decrease in ME not apparent in DE values for the horse. This shift does not occur for cattle since the proportion of a feed fermented will change little with forages. What are the possible effects of the pretreatment of forage in the stomach on the efficiency of hindgut fermentation in the horse? (1) The digestion of carbohydrates, protein and fat precaecally decreases products required for methane production in the lower tract of nonruminants (Maynard et al. 1979, p. 193). In a comparison of species' values from the literature, Galloway et al. (1968) found relatively lower values of. methane concentrations in the hindguts of nonruminants (horses, rats, and humans) than ruminants. Hungate (1966, p. 272) states that methane production correlates inversely with proprionate production. Proprionate production usually increases with the quantity of starch in the diet (Stevens et al. manuscript). Similarly Moe and Tyrrell (1979) have shown that methane production in the rumen of cows is affected more by digestiable cellulose and hemicellulose than by soluble nutrients. (2) Structural carbohydrates ferment more rapidly when pretreated with gastric digestion. Although suggested by a number of researchers (Van Soest pers. comm.; Uden 1978), to my knowledge no work has addressed this problem.

284 (3) The absence of soluble carbohydrates slows digestion of the fiber fraction. Although Kern et al. (1973) have shown that the addition of oats to forage increases the numbers of bacteria in the caecum (presumably by increasing the amount of soluble carbohydrate reaching the lower tract), it did not increase the numbers of cellulolytic bacteria nor increase cellulose digestion. Substantial evidences exists to support the contention that fermentation processes and VFA absorption in the hindgut are similar to the rumen (Stevens et al. manuscript, Argenzio et al. 1974, 1977). However the efficiency of the microbial populations may differ. Alexander (1963) suggested that, because cotton thread digested more rapidly in the caecum of the pony than the rumen of the cow, cellulolytic organisms in the pony were more efficient than their counterparts in the cow. However Koller et al. (1978) commented that cotton thread bears little resemblance to the cell wall of forage plants. To examine the comparative digestive capabilities of cellulolytic microbes in the pony and cow, Koller et al. (1978) compared forage digestibilities using in- vitro and nylon bag techniques. Their results show that ruminal bacterial digest the dry-matter and cell-wall fractions more efficiently than do caecal bacteria when exposure times are constant. They caution that their experimental procedure does not resemble the treatment of forages in the complete animal digestive system, citing keys et al. (1969, 1970) who found indications that hemicellulose digestion in the hindgut may be enhanced by intestinal action on fiber. Evidence exists of differences in passage rates, forage quality reaching the fermentation site and efficiency of digestion within the site (albeit often contradictory). To develop a meaningful understanding of the capabilities of the horse as a herbivore will require a better understanding of the dynamic aspects of fermentation in the lower gut of equids. Intake. -- No single topic has received as much research attention and yielded such equivocal results as intake. Intake and its determinants have been principally studies with ruminants since intake is an important factor affecting production (Reid 1961). This research has been particularly interested in the responses of intake to dietary quality. A number of herbivorous species are capable of increasing intake in response to diets of decreasing caloric density (rats, Peterson and Baumgardt 1971a, b; chickens, Hill and Dansky 1954; pigs, Owen and Ridgmen 1968, sheep, Dinius and Baumgardt 1970, Weston 1966, lambs, Owen et al. 1967; cattle, Montgomery and Baumgardt 1965). But the intake response to compensate for lower caloric densities is limited. Beyond a critical point, intake decreases in response to lower-quality diets (rats, Peterson and Baumgardt 1971a, b; pigs, Owen and Ridgman; sheep, Dinius and Baumgardt 1970, Weston 1966, cattle, Montgomery and Baumgardt 1965). The initial response to decreasing caloric density has been considered a physiological response to balance energy requirements while the depression of intake beyond the k

285 critical point is interpretated as the limits of gastro-intestinal or rumen capacity (Baumgardt 1970, Baile and Forbes 1974). The point where intake compensation ceases appears to be sensitive, independently, to the energy requirements of the animal and its body size (Peterson and Baumgardt 1971b, Owen and Ridgmen 1968). The observation that digesta present in the gut and the rumen is a relatively constant maximal value, regardless of forage, has led to the concept of fill models in ruminant studies (Blaxter et al. 1961, Compling et al. 1961). Rumen fill modeling has become quite sophisticated in these 20 years (c.f. Ellis 1978). However the achievement of the unified predictive model of intake is still elusive. From the simple concept that the rumen volume limits M intake (Adolph 194*7), these models have developed into formulations of the.dynamics of rumen fill. All the processes likely to play a role in influencing fill have been implicated as determinants of intake (digestibility, Crampton 1957; passage rate, Compling et al. 1962; particle break- down, Weston 1966; rate of disappearance of particles in the rumen, Blaxter et al. 1956; rumen volume, .Compling and Balch 1961, Egan 1972; volumetric qualities of the forage, Van Soest 1966). What is quite clear from this literature is that intake is a response to a complex set of variables and that the importance of a particular parameter in determing intake varies depending on the conditions of the animal, the experiment and the forage (Baumgardt 1970). No comparable models nor similar research has been conducted on equids, to my knowledge. The traditional argument has been that nonruminants can respond to a greater degree than ruminants by increasing intake on low-quality diets (Bell 1969, Janis 1976). The ruminant appears to be limited in the rate at which cell wall can be broken down to relieve rumen fill. Welch and Smith (1969, 1970) have found strong correlations (r = .99, .94) between cell-wall intake and rumination time. Cell-wall measures have always shown strong correlations with intake (Mertens 1973), but recent work has shown that cell-wall content and intake is only weakly correlated with bulk volume (P. Vander Aar, unpublished data). These facts suggest that the cell wall of low-quality forages may limit the intake of ruminants by restricting particle- size reduction via rumination (Demment and Van Soest, manuscript). Nonruminants cannot compensate with intake for low-energy diets in an unrestricted response. In fact data on the rat (Peterson and Baumgardt 1971b) show a critical point in diet dilution beyond which compensation does not maintain constant energy intake. The response curve looks similar to those measured for ruminants (Baumgardt 1970). However the complication of body size and physiological state make it difficult to compare the points at which compensation response terminates with those of ruminants.

286 The addendum to this review shows the relationship between TDN of forages for cattle and horses and their crude-fiber values (taken from the NRC requirements for the two species). While the trends in nutrient availability are clear, they are fairly useless in determining energy levels available to an animal on the range. It is critical to any prediction of intake or possible reproductive capacity to know the degree to which a species can respond to low- quality forages by increasing its intake. In the rat, lactating females showed higher intakes on all diets than males even beyond the critical point. This result shows that physiological states can influence performance for animals of approximately similar gut sizes (Peterson and Baumgardt 1971b). This point is especially pertinent when the assumed adaptation of nonruminants is the capability of large increases in intake to compensate for decreasing nutrient density in its foods. Since, as discussed earlier, digestibility is determiend by retention time, which itself is a function of intake, calculations of intake directly from digestibility values are crude. A focus of future horse research should involve the intake-forage-quality problem. Energy Requirements and Intake Calculations In only one study among the recent work on the energy requirements of horses have measures been made independent of energy intake. The one exception is Wooden et al. (1971) where heat production was calculated, but discarded due to disagreement with Kleiber's (1975) formulation. Results from the NRC (1973), Stillion and Nelson (1972), and Wolfram et al. 1977 are all based on DE values which are not indicators of ME since equations for this conversion are not available in the literature. Wooden et al. (1970) used the ME values for cattle which are most likely very crude approxi- mations. Although .75 exponent of body weight is widely cited, it is not documented for horses. Thonney et al. (1976) has shown that metabolism is often a function of exponents significantly different from 175 power of body weight within a species. These comments aside, the NRC (1973) recommendation for maintenance is DE (kcal/day) = 155 W75 where W is weight in kg. This value is considerably higher than Knox et al. (1970) have suggested (114 kcal DE/W^gVday) , or Hoffman et al. (1967) adovcated (112-128 kcal DE/Wkg.75/day). On the other band, Wolfram et al. (1977) measured values of 176 kcal DE/Wkg' /day, substantially greater than the NRC requirement. These differences, however discomforting, are likely to be of minor importance in the face of estimating the energy requirements of exercise from qualitative data (e.g. Hintz et al. 1971). Maintenance metabolism for mature bulls (calculated by regression of NRC data, 1976 is DE (kcal/day) = 194.7 W.75. In a comparison of 500 kg animals, a horse would require 16.4 (Meal/day) and cattle 20.6 (Meal/day) at maintenance. The 4.2 (Meal/day) or 20.4% difference in DE is presumably fermentation losses. For cattle

287 ME as a percentage of DE varies linerarly from 80% at 50% digestibility, to 88% at 80% digestibility by the equation: ME (Meal/kg DM) = -.45 + l.01 DE (Meal/kg DM) (Moe and Tyrrell, 1976) It is useful to combine this information with data on forages to be able to predict intake for a particular animal on a given forage quality. To do this I have plotted a linear regression of TDN (%DM) against crude fiber (%DM) from data in the NRC (1973, 1976) requirements for horses and cattle (see addendum and Fig. 14 ). These equations have the form: TDN = f - dCf where TDJJ is TDM (%DM), f is 90.0 for cattle, 9l.0 for horses, d is l.023 for cattle, l.27 for horses and Cf is crude fiber (%DM). Since DE = K TDN (NRC 1979) where DE is DE (Meal/kg DM), K is 4.409 and ME can be expressed by equation of Moe and Tyrrell (1976) for cattle and for horses (ME = DE), and Mt = aw.75 where Mt is total ME requirement (Mcal/day/animal), then maintenance intake for animals of different weights can be derived by substi- tution : . I = M± ME where I is intake (kg DM/day/animal). By substitution, I - aw'75 (Cattle) b + (c K (f - d Cf)) where b is -.45 and c is l.01 (i.e., constants from the Moe and Tyrrell equation), and I = aw' , nn (Horses) K (f - d Cf) ' The values of intake calculated from these equations are presented in Fig.15 for horses and cattle of different weights. Intake for horses is consistently lower than for cattle on diets of low crude fiber. As this fiber fraction increases, horses approach cattle intake levels and then exceed them above -40% crude fiber. This pattern reflects the loss of DE to fermentation in the cow dominating the lower overall DE values for horses. As the fiber fraction increases the depression in digestibility in the horse eventaully exceeds the fermentation losses in cattle.

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290 Since no estimate of fermentation losses in the horse have been made by the NRC (1978) (i.e. ME = DE), the equation for horse should be used, along with these comparisons, with caution. It would seem highly unlikely that caecal microbes do not respire or produce some nonmetabolizable byproducts. Addendum I was unable to complete this analysis in time to include the results in the appropriate position within the text (under Digestive Capacity of Equids). These data are values from the NRC (1973, 1976) for TDN (% DM) as a function of crude fiber (% DM). These regressions show a linear response, very similar (especially for the horse) to those measured by Axelsson (1949). There is no indication of the curvilinear response reported by Olsson (1949). These data show that the TDN in forages decreases more rapidly for horses than cattle. The equations presented here are incorporated with others relating TDN, DE and ME to predict intake of the two species (see Energy requirements and intake calculations), References Cited Alexander, F. and E. M. Davies. 1963. Production and fermentation of lactate by bacteria in the alimentary canal of the horse and pig. J. Comp. Path. 73:1-12. Alwash, A. H. and P. C. Thomas. 1974. Effect of the size of hay particles on digestion in the sheep. J. Sci. Food Agr."25:139-146. Argenzio, R. and H. F. Hintz. 197l. Effect of diet on glucose metabolism in ponies. J. Anim. Sci. 33:226. Argenzio, R. A., J. E. Lowe, D. W. Pickard and C. E. Stevens. 1974. Digesta passage and water exchange in the equine large intestine. Am. J. Physicol. 226(5):1035-1042. Argenzio, R. A., M. Southwcrth, J. E. Lowe, and C. E. Stevens. 1977. Interrelationship of Na, HCO~ and volatile fatty acid transport in the equine large intestine. Am. J. Physiol. 233:E469-478. Armstrong, D. G. and K. Hutton. 1975. Fate of nitrogenous compounds entering the small intestine. In: I. W. McDonald & A.C.I. Warner (ed). Digestion and metabolism in the ruminant. University of New England Publ. Unit, Australia. Axelsson, J. 1949. The ability of cattle, sheep, horses and swine to digest the nutrients of the feed stuffs. Ann. Roy. Agri. Col. Swed. 16:84-100. Baile, C. A. and J. M. Forbes. 1974. Control of feed intake and regulation of energy balance in ruminants. Physiol. Reviews 54:160-214.

291 Baker, F. 1942. Microbial synthesis and autolysis in the digestive tract of herbivores. Nature 149:582. Baldwin, R. C., L. J. Koong and M. J. Ulyatt. 1977. The formation and utilization of fermentation end-products: mathematical models in microbial ecology of gut. eds. R.T.J. Clarke and T. Bauchop, p. 347-39l. Barsaul, C.S. and S. K. Talapatra. 1970. A comparative study on the determination of digestibility coefficients of feeding- stuffs by different species of farm animals. Indian Vet. J. 47:348-355. Bauman, D. E., C. L. Davis, R. A. Frobish and D. S. Sachan. 197l. Evaluation of the PEG method in determining rumen fluid volume in dairy cows fed different diets. J. Dairy Sci. 57:929-934. Baumgardt, B.R. 1970. Control of feed intake in the regulation of energy balance. pp. 235-253. In• Physiology of digestion and metabolism in the ruminant. Proc. Intern. Symp. 3rd. (ed.) A. T. Phillipson. Orcil Press, Cambridge, England. Bell, R. H. V. 1969. The use of the herb layer by grazing ungulates in the Serengeti.. In A. Watson (ed.), Animal populations in relation to their food resources. Symp. Brit. Ecol. Soc. Blackwell, Oxford, p. 111-128. Blaxter, K. L., N. McC. Graham and F. W. Wainman. 1956. Some observations on the digestibility of food by sheep and on related problems. Brit. J. Nutr. 10:69-7l. Blaxter, K. L., F. W. Wainman and R. S. Wilson. 1961. The ' regulation of food intake by sheep. Anim. Prod. 3:51-62. Browning, B. L. Mew York. 1975. The chemistry of wood. Krieger, Huntington, Bryant, M. P. 1963. Symposium on microbial digestion in ruminants USDA Agric. Res. Serv. Spec. Rep. 44-92 pp. 1-15, Washington, D.C. Bryant, M. P. and I. M. Robinson. 1963. Apparent incorporation of ammonia and amino acid carbon during growth of selected species of ruminal bacteria. J. Dairy Sci. 46:150-154. Galloway, D. H. 1968. Gas in the alimentary canal. pp. 2839- 2859. Iii Handbook of Physiology. Section 6: Alimentary Canal. Vol. V, ed. C. F. Code and W. Heidel. Am. Physiol. Soc. Washington, D.C. Camping, R. C. And C. C. Balch. 196l. Factors affecting the voluntary intake of foods by cows. l. Preliminary observations on the effect, on voluntary intake of hay, of changes in the amount of the reticulc-ruminal contents. Brit. J. Nutr. 15:523-530.

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