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Home , American Bison, Grazing (behaviour), Hindgut fermentation, Ruminant

B-6037

What Range Eat— and Why

Texas Agricultural Extension Service • Zerle L. Carpenter, Director • The A&M University System • College Station, Texas What Range Herbivores Eat—and Why Robert K. Lyons, T.D.A. Forbes, and Rick Machen*

Different range have different diets— eat about equal amounts of grass and some eat grass, some eat browse ( from browse and about the same amount of forbs as woody ) and forbs (wildflowers, weeds, . Because Spanish goats are more efficient etc.), and some eat all three. The differences in browsers than Angora goats, they can maintain their diets allow many types of range animals to more browse in their diets than Angoras when coexist on the same range. browse is scarce. Spanish goats are more effi- For many , the major herbivores on cient browsers because Texas ranges were cattle, , goats, , and ■ they are taller and can browse at greater . Recently, however, several new herbi- heights. vore (such as axis and fallow deer) have ■ they have less hair to get caught in denser been introduced to Texas from Asia and Africa, brush. and there is some isolated interest in reintroduc- ing the American . With the introduction Of the Texas range herbivores, deer—both of new species and possible reintroduction of white-tailed and mule—eat the most browse. native species, it is important to understand the Although appear to eat more browse diets of different animals to determine which and less forbs than white-tailed deer (Figures 1 ones best fit different range habitats. & 3), these differences are probably due to the kinds of forage available. Diets often reflect Although a is, by definition, a - availability of forage types: for example, deer , herbivores do not eat just any prefer forbs, but browse is probably a more plant. For example, if a deer, which is adapted readily available food source during tough times. to eat forbs and browse, is forced to eat large amounts of grass, it will probably not perform Diets also vary from season to season. For as well as deer that eat forbs and browse. example, cattle eat more grass in winter and less in spring; more forbs in spring and less in fall The type of diet selected by range herbivores and winter; and more browse in fall and less in is determined by their mouth parts and the spring (Figure 2). In comparison, white-tailed anatomy of their digestive systems. A sound deer consume more or less the same amount of understanding of what range herbivores eat and grass across all seasons; more forbs in spring why will allow the landowner to use the range- and less in winter; and more browse in winter land resource more wisely and enable the ani- and less in spring (Figure 3). The diets of some mals to perform better. animals, like bison, are relatively stable across seasons (Figure 3). What Range Herbivores Eat Differences in the types of forages consumed The diets of range herbivores vary among dif- by range herbivores are due to both internal ferent species (Figure 1, page 2) and within the (digestive system) and external (such as mouth same species by season of the (Figures 2 size) physical differences among these animals. and 3, pages 4 and 5). These physical differences have been used to On an annual basis, bison eat mostly grass, a classify herbivores into different feeding types. few forbs, and little browse (Figure 1). Cattle eat less grass, but more forbs and browse than Herbivore Feeding Types bison. Horses are similar to bison and cattle in Animal digestive systems lack the that they eat mostly grass and only small required to break down or digest the chemical amounts of forbs and browse. Sheep eat less bonds found in the cell walls of plant material grass than either bison or cattle, slightly more (). Animals that use cellulose can do so forbs than cattle, and more than three times as because they have in their much browse as cattle. digestive systems that have the chemicals need- ed to digest it. Cellulose is digested by fermenta- *Assistant Professor and Extension Range Specialist; Associate tion. requires time and a con- Professor, ; Assistant Professor and Extension ducive environment in the digestive system Specialist, The Texas A&M University System. Cattle Sheep Goats

Browse (7) Browse (22) Browse (43) Forbs (12) Grass (45)

Forbs (17) Grass (81) Grass (61) Forbs (12)

Bison White-tailed Deer Mule Deer

Forbs (5) Browse (2) Browse (52) Grass (12) Browse (72) Grass (10) Forbs (18)

Grass (93) Forbs (36)

Horses

Browse (4) Forbs (6) Browse (50) Grass (18) Browse (20)

Forbs (16)

Grass (64)

Grass (90) Forbs (32)

Figure 1. Average annual diet composition by percent grass, forbs (wildflowers, weeds, etc.) and browse (leaves of woody plants) for cattle (Edwards Plateau and South Texas), sheep (Edwards Plateau), goats (Edwards Plateau), bison (Colorado), white-tailed deer (Edwards Plateau and South Texas), mule deer (western ), horses (western United States), pronghorn antelope (western United States), and elk (western United States) on rangeland (adapted from Vallentine 1990).

2 where food can be held long enough for the Feeding Type and Forage Availability microorganisms to break down the cellulose. Depending on the quality and quantity of the forage available, there are advantages and disadvantages to being a or hindgut Animals with one simple , like horses fermentor. and swine, are called “monogastrics.” Most monogastrics do not use cellulose because they If forage quality is low but forage quantity is do not have a specialized part of the digestive abundant, hindgut fermentors have the advan- system where fermentation can take place. Some tage because there are no physical restrictions to monogastrics (like horses, ) have either an food passage in their digestive systems—this enlarged stomach or areas in the allows food to move through the digestive sys- and/or where fermentation can take tem quickly. Consequently, animals with this place. Monogastrics with an enlarged kind of digestive system can meet their nutrient stomach (like the ) are needs by eating large quantities of low-quality called “foregut fermentors” forage. In the same situation, a ruminant animal because fermentation occurs in would be at a disadvantage because low-quality the front part of the digestive forage takes longer to break down, and the system. Monogastrics in which physical restrictions to food passage in their fermentation occurs in the rear digestive systems limit the amount of forage part of the digestive system are they can eat. called “hindgut fermentors” Therefore, a (like the , , and ruminant animal ). would not be able to get enough low-quality forage Ruminants are differ- through its digestive ent from monogastrics system to meet its because they have four nutrient needs. compartments in the If forage quantity is front part of their limited and forage quali- digestive systems ty is moderate, a rumi- and because they nant would have the chew their . advantage because the One of these com- physical restrictions to partments, the aboma- food passage hold forage sum, is the same as the in the digestive tract longer, stomach. allowing it to be digested The creates a more completely. physical restriction to Both hindgut fer- the passage of food mentors and rumi- through the digestive nants could be at system. For food to a disadvantage if leave the rumen, the both forage food particles must be quantity and small and heavy, which quality are low. requires rechewing and fermenta- Hindgut fermen- tion time in the rumen. About 155 rumi- tors are at a disad- nant species now exist in the world. Most vantage in this situation large herbivores on Texas rangelands are rumi- because they do not efficiently digest the forage, nants (cattle, sheep, goats, and deer). Although which passes rapidly through their digestive sys- and chew their cud, they are not tems, and the limited forage supply may not true ruminants because they lack one of the allow them to eat enough to make up for the four compartments of a ruminant stomach. incomplete . Because of the limited for- age supply and the physical restrictions of the rumen, ruminants too may not be able to eat enough to meet their nutrient requirements.

3 Cattle Sheep Goats

Browse (1) Forbs (25) Browse (10) Browse (34) Grass (49) Forbs (38) Spring Forbs (17) Grass (74) Grass (52)

Browse (4) Forbs (12) Browse (19) Browse (33)

Forbs (17) Summer

Grass (84) Grass (64) Forbs (14) Grass (53)

Browse (15) Browse (33) Browse (53) Grass (37) Forbs (6) Fall Forbs (9) Grass (79) Grass (58) Forbs (10)

Browse (6) Forbs (7) Browse (27) Browse (53) Grass (42)

Forbs (3) Winter

Grass (87) Grass (70) Forbs (5)

Figure 2. Average seasonal diet composition by percent grass, forbs (wildflowers, weeds, etc.) and browse (leaves of woody plants) for cattle (Edwards Plateau and South Texas), sheep (Edwards Plateau), and goats (Edwards Plateau), on rangeland (adapted from Vallentine 1990).

4 Bison White-tailed Deer Mule Deer

Browse (0) Grass (14) Forbs (1) Browse (34) Browse (58) Grass (14) Spring

Grass (99) Forbs (52) Forbs (28)

Browse (0) Forbs (7) Browse (58) Grass (7) Browse (62) Grass (12) Summer

Grass (93) Forbs (35) Forbs (26)

Forbs (11) Browse (0) Grass (12) Grass (8) Forbs (1) Browse (56) Fall

Browse (81) Grass (99) Forbs (32)

Forbs (6) Browse (0) Grass (13) Forbs (7) Browse (63) Grass (6) Winter

Grass (93) Forbs (24) Browse (88)

Figure 3. Average seasonal diet composition by percent grass, forbs (wildflowers, weeds, etc.) and browse (leaves of woody plants) for bison (Colorado), white-tailed deer (Edwards Plateau and South Texas), and mule deer (western United States) on rangeland (adapted from Vallentine 1990).

5 In summary, different forage situations place cell walls compared to grasses and contain more hindgut fermentors and ruminants at relative cell contents, making them easier to digest. advantages or disadvantages: hindgut fermentors The ruminant feeding types incorporate three have an advantage with high forage quantity overlapping categories. First, browsers are ani- and low quality; ruminants have an advantage mals that eat plants and plant parts high in easi- with low quantity and moderate quality; and ly digestible cell contents (forbs and browse). both are at a disadvantage with low quantity About 40 percent of ruminants worldwide can and low quality. be placed in this feeding type. Examples of this Not all ruminants are alike. Therefore, this group on Texas rangelands include white-tailed group of herbivores deserves separate attention and mule deer. based on research findings of the past few years. A second group, grazers, depends on fiber- containing plants like grasses; about 25 percent Ruminant Feeding Types of all ruminants fall into this category. Texas Until recently, information about ruminant examples of this group are cattle, bison, and digestive systems came mostly from research on . cattle and sheep and a few studies. Other A third group, intermediate feeders, shifts ruminants were assumed to be similar to these its diet among grasses, forbs, and browse over domestic ruminants. Studies involving African the year and within seasons. About 35 percent ruminants with different diets have led to a bet- of ruminants can be placed in this group. Texas ter understanding of why these animals eat examples of this group include pronghorn ante- what they do. These studies indicate that diet lope, elk, goats, fallow deer, and . selection by ruminants is closely related to dif- Table 1 compares parts of the digestive sys- ferences in the anatomy of their digestive sys- tems of grazers and browsers. These differences tems, beginning at the mouth and continuing to determine the kinds of forage that animals with- the hindgut. These studies have led to a classifi- in each category are adapted to use. For each cation system for ruminant feeding types. comparison, Table 1 also indicates the impor- Understanding this feeding type classification tance of these differences to the feeding types. requires an understanding of how plant cells are constructed and the kinds of cells found in dif- Competition Between Ruminant Types ferent plants. Plant cells have a cell wall and Figure 4 illustrates that many ruminants do material (the cell contents) inside the cell. The not fit completely within these three categories cell wall holds the cell together and contains but may, in fact, overlap another category. fiber which includes: Within Figure 4, the farther to the right of the ■ Chemical compounds (cellulose and hemi- figure a species name appears, the more grass cellulose) that must be broken down by that species is expected to eat. On the other microorganisms before they can be used by hand, the farther to the left a species name animals. appears, the more forbs and browse that species is expected to eat. ■ Compounds that cannot be digested (lignin). Ruminants in the intermediate feeder catego- If broken down, the digestible part of the cell ry are expected to eat about equal amounts of wall provides which can be grass and browse and/or forbs, used for animal nutrition. Cell but these animals may over- contents contain easily digestible lap either grazers or materials like , , browsers. For example, sugars, , and oils. nilgai overlap with graz- Microorganisms are not needed ers, which indicates their to break down these materi- diets would be expected als. Grasses, especially to be more like that of grass stems, older grass cattle than white-tailed plants, and tropical grass- deer. The more overlap es, contain large amounts between species, the more of cell wall material, so similar their expected diets they are difficult to digest. are and the more expected Forbs and woody plant competition for forage. leaves (browse) have thinner Horses, for example, which

6 Table 1. Comparison of Anatomy of Mouths and Digestive Systems of Browsers and Grazers (adapted from Hofmann 1986,1988). Comparison Browsers Grazers Significance Mouth opening large, narrow small, wide Larger mouth opening allows stripping of twigs and gnawing of flowers and . Lips flexible rigid Flexible lips allow more selectivity of plant parts eaten. Tongue slender thick Browser uses slender tongue with lips to select individual plant parts. Grazers wrap tongue around clumps of forage, not efficient for individual selection. Taste buds few many Smell is probably more important in browser food selection and taste avoidance is probably more important in grazers. Teeth sharp flat Browsers can puncture plant material quickly releasing easily fermented cell contents. Grazers grind food, cell walls freed for microbial digestion. Jaw muscles light heavy Heavy grazer muscles needed in grinding fibrous plant material. Salivary glands large small Browsers need more to keep rumen pH from becoming too acidic from fermentation of large quantities of rapidly fermented cell contents. Rumen simple subdivided Allows food in the browser rumen to leave rapidly, a disadvantage on high fiber forages like grass which require more fermentation time. Grazers are able to hold food in rumen longer allowing high fiber forages more time to ferment. small large Browsers cannot hold large quantities of food. Grazers can store larger quantities of forage in the rumen which is an advantage with slower fermenting high fiber forages. Rumen muscles light heavy Heavy muscles allow grazers to handle larger amounts of forage held in rumen. Rumen papillae cover rumen lower rumen With an increase in these structures, absorption occurs over a wall greater portion of the rumen in browsers allowing acids produced during fermentation to exit the rumen quickly and help control rumen pH. Reticulum size large small Small size, many and deep subdivisions hold forage in the grazer rumen longer allowing more time for fermentation. subdivisions few shallow many deep small large Larger size provides more absorption surface. large small Larger liver is needed to absorb more rapidly fermented cell contents from browser and to detoxify chemicals in browse. Hindgut volume large small Larger volume indicates that is more important in browsers. Less-digestible plant material which quickly exits the browser rumen and undergoes additional fermentation in the hindgut providing additional energy. are non-ruminant grazers, would be very com- Smaller animals have higher relative nutrient petitive with either bison or cattle grazing the requirements and must, therefore, consume same area because their diets are so similar. higher-quality diets. A small browser with high Because of their flexible diets, intermediate nutrient requirements and little flexibility in the feeders are very competitive with both browsers diet to which it can adapt faces potential prob- and grazers. The impact of this competition is lems when it shares the same habitat and food especially great for smaller animals. source with an extremely flexible and competi-

7 Figure 4. Feeding type classification for domestic livestock and native, Asian, and African wild ruminants. Some species overlap feeding types. The farther to the right a species name appears within a column, the more grass expected in the diet. The farther to the left a species name appears, the more forbs (wildflowers, weed, etc.) and browse (leaves form woody plants) expected in the diet. Intermediate feeders tend to shift their diets among grasses, forbs, and browse over the year and within seasons (Adapted from Hofmann 1986,1988; Mungall and Sheffield 1994).

8 tive intermediate feeder. One study illustrating ences in feeding types and which food sources this point was conducted at the Kerr Wildlife are suitable for which animals can improve the Area (Armstrong 1984). White-tailed deer landowner’s ability to successfully manage dif- (browsers) and (intermediate feeders) ferent range herbivores. were placed in an enclosed . At the end of the study, white-tailed deer were nonexistent For More Information and sika deer were abundant. When browse and Some information in this publication is taken forbs were significantly reduced in the pasture, from these sources: white-tailed deer had no alternative forage source. Sika deer, however, were able to shift Armstrong, W.E. 1984. How to manage deer their diet to grass and survive. habitat: Edwards Plateau. In: Proceedings of the 1984 International Ranchers Roundup. Texas Conclusions Agricultural Extension Service. Range herbivores differ widely in the kinds of Hofmann, R.R. 1986. Morphological evolution- forages they are adapted to use. These differ- ary adaptations of the ruminant digestive sys- ences are largely based on the anatomy of the tem. In: A. Dobson and M.J. Dobson (eds.), animals. Most of the economically important Aspects of digestive physiology in ruminants. range herbivores in Texas are ruminants. Comstock Publishing Associates, Cornell University Press, Ithaca, NY. In ruminants, the degree to which an animal can adapt to different diets is related to its feed- Hofmann, R.R. 1988. Anatomy of the gastroin- ing type, which is determined by its digestive testinal tract. In: D.C. Church (ed.), The rumi- anatomy. The least-adaptable ruminants are the nant animal: digestive physiology and nutrition. browsers and grazers. Between these two groups Prentice Hall, Englewood Cliffs, NJ. are the intermediate feeders, which are extreme- Mungall, E.C. and W.J. Sheffield. 1994. Exotics ly flexible in their diets and, therefore, the habi- on the range. Texas A&M University Press, tats they can use. Although grazers will eat College Station. browse and browsers will eat grass, they will Vallentine, J.F. 1990. Grazing management. not perform well when forced to shift their diets Academic Press, Inc., San Diego, CA. to these extremes. Understanding these differ-

9 Educational programs of the Texas Agricultural Extension Service are open to all people without regard to race, color, sex, disability, religion, age or national origin. Issued in furtherance of Cooperative Extension Work in and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Zerle L. Carpenter, Director, Texas Agricultural Extension Service, The Texas A&M University System. 3MÐ4-96, New RS 2, AS