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Texas Agricultural Extension Service ¥ Zerle L. Carpenter, Director ¥ The Texas A&M University System ¥ College Station, Texasand Why What Range Herbivores EatÑand WhyRobert K. Lyons, T.D.A. Forbes, and Rick Machen*Different range animals have different dietsÑsome eat grass, some eat browse (leaves fromwoody plants) and forbs (wildflowers, weeds,etc.), and some eat all three. The differences intheir diets allow many types of range animals tocoexist on the same range. For many years, the major herbivores onTexas ranges were cattle, sheep, goats, deer, andhorses. Recently, however, several new herbi-vore species (such as axis and fallow deer) havebeen introduced to Texas from Asia and Africa,of new species and possible reintroduction ofnative species, it is important to understand theones best fit different range habitats. Although a herbivore is, by definition, a plant-eating animal, herbivores do not eat just anyplant. For example, if a deer, which is adaptedto eat forbs and browse, is forced to eat largeamounts of grass, it will probably not performas well as deer that eat forbs and browse. The type of diet selected by range herbivoresis determined by their mouth parts and theanatomy of their digestive systems. A soundunderstanding of what range herbivores eat andwhy will allow the landowner to use the range-mals to perform better.What Range Herbivores Eat The diets of range herbivores vary among dif-same species by season of the year (Figures 2On an annual basis, bison eat mostly grass, afew forbs, and little browse (Figure 1). Cattle eatless grass, but more forbs and browse thanthat they eat mostly grass and only smallamounts of forbs and browse. Sheep eat lessgrass than either bison or cattle, slightly moreforbs than cattle, and more than three times asmuch browse as cattle.browse and about the same amount of forbs ascattle. Because Spanish goats are more efficientbrowsers than Angora goats, they can maintainmore browse in their diets than Angoras whenbrowse is scarce. Spanish goats are more effi-cient browsers becausethey are taller and can browse at greaterheights.they have less hair to get caught in denserOf the Texas range herbivores, deerÑbothwhite-tailed and muleÑeat the most browse.Although mule deer appear to eat more browsekinds of forage available. Diets often reflectavailability of forage types: for example, deerprefer forbs, but browse is probably a morereadily available food source during tough times. Diets also vary from season to season. Forexample, cattle eat more grass in winter and lessand winter; and more browse in fall and less inand less in winter; and more browse in winteranimals, like bison, are relatively stable acrossby range herbivores are due to both internal(digestive system) and external (such as mouthsize) physical differences among these animals.These physical differences have been used toclassify herbivores into different feeding types. Herbivore Feeding Types Animal digestive systems lack the enzymesrequired to break down or digest the chemicalbonds found in the cell walls of plant materialbecause they have microorganisms in theirdigestive systems that have the chemicals need-ed to digest it. Cellulose is digested by fermenta-tion. Fermentation requires time and a con-ducive environment in the digestive system *Assistant Professor and Extension Range Specialist; AssociateProfessor, Grazing Ecology; Assistant Professor and ExtensionLivestock Specialist, The Texas A&M University System. Browse (7)Forbs (12)Grass (81)SheepBrowse (22)Forbs (17)Grass (61) Browse (43)Grass (45)BisonBrowse (2)Grass (93)White-tailed DeerBrowse (52)Grass (12)Forbs (36)Mule DeerGrass (10)Browse (72)Forbs (18) Browse (4)Grass (90) Browse (50)Grass (18)Forbs (32) Browse (20)Forbs (16)Grass (64) Forbs (6) Figure 1.Average annual diet composition by percent grass, forbs (wildflowers, weeds, etc.) and browse (leaves of woodyplants) 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 United States), horses (western United States),pronghorn antelope (western United States), and elk (western United States) on rangeland (adapted from Vallentine 1990). microorganisms to break down the cellulose. Animals with one simple stomach, like horsesand swine, are called Òmonogastrics.Ó Mostmonogastrics do not use cellulose because theydo not have a specialized part of the digestivesystem where fermentation can take place. Somemonogastrics (like horses, rabbits) have either anand/or cecum where fermentation can takeplace. Monogastrics with an enlargedstomach (like the hippopotamus) arethe front part of the digestivepart of the digestive system are(like the horse, zebra, andRuminants are differ-because they have fourdigestive systemsand because theychew their cud.partments, the aboma-physical restriction tothrough the digestivesystem. For food toleave the rumen, thesmall and heavy, whichrequires rechewing and fermenta-nant species now exist in the world. Mostlarge herbivores on Texas rangelands are rumi-nants (cattle, sheep, goats, and deer). Althoughcamels and llamas chew their cud, they are nottrue ruminants because they lack one of theFeeding Type and Forage Availabilitythe forage available, there are advantages anddisadvantages to being a ruminant or hindgutfermentor. If forage quality is low but forage quantity isabundant, hindgut fermentors have the advan-tage because there are no physical restrictions tofood passage in their digestive systemsÑthisallows food to move through the digestive sys-tem quickly. Consequently, animals with thiskind of digestive system can meet their nutrientneeds by eating large quantities of low-qualityforage. In the same situation, a ruminant animalwould be at a disadvantage because low-qualityforage takes longer to break down, and thephysical restrictions to food passage in theirdigestive systems limit the amount of foragethey can eat.Therefore, awould not below-quality foragethrough its digestivenutrient needs. ty is moderate, a rumi-nant would have theadvantage because thephysical restrictions toin the digestive tract longer,allowing it to be digestedmore completely. a disadvantage ifquality are low.vantage in this situationbecause they do not efficiently digest the forage,which passes rapidly through their digestive sys-tems, and the limited forage supply may notallow them to eat enough to make up for theage supply and the physical restrictions of therumen, ruminants too may not be able to eatenough to meet their nutrient requirements. Browse (1)Forbs (25)Grass (74)SheepBrowse (10)Forbs (38)Grass (52)GoatsBrowse (34)Grass (49)Browse (4)Grass (84)Browse (19)Grass (64)Forbs (17)Grass (53)Browse (33)Forbs (14)Browse (15)Grass (79)Browse (33)Grass (58)Forbs (9)Browse (53)Forbs (10)Grass (37)Forbs (17)Forbs (12) Grass (87)Browse (27)Grass (70)Forbs (3)Browse (53)Forbs (5)Grass (42)Forbs (7) Summer WinterForbs (6) Figure 2.Average seasonal diet composition by percent grass, forbs (wildflowers, weeds, etc.) and browse (leaves of woodyplants) for cattle (Edwards Plateau and South Texas), sheep (Edwards Plateau), and goats (Edwards Plateau), on rangeland(adapted from Vallentine 1990). BisonBrowse (0)Forbs (1)Grass (99)White-tailed DeerBrowse (34)Forbs (52)Grass (14)Mule DeerBrowse (58)Grass (14)Browse (0)Grass (93)Browse (58)Grass (7)Forbs (35)Grass (12)Browse (62)Forbs (26)Browse (56)Grass (12)Forbs (32)Browse (81)Forbs (11)Grass (8)Forbs (28)Forbs (7)Browse (63)Grass (13)Forbs (24)Browse (88)Forbs (6)Grass (6)SpringSummerFallWinter Forbs (1)Grass (99) Grass (93)Forbs (7) Figure 3.Average seasonal diet composition by percent grass, forbs (wildflowers, weeds, etc.) and browse (leaves of woodyplants) for bison (Colorado), white-tailed deer (Edwards Plateau and South Texas), and mule deer (western United States) onrangeland (adapted from Vallentine 1990). In summary, different forage situations placehindgut fermentors and ruminants at relativeadvantages or disadvantages: hindgut fermentorshave an advantage with high forage quantityand low quality; ruminants have an advantagewith low quantity and moderate quality; andboth are at a disadvantage with low quantityand low quality. Not all ruminants are alike. Therefore, thisgroup of herbivores deserves separate attentionbased on research findings of the past few years. Ruminant Feeding Types Until recently, information about ruminantdigestive systems came mostly from research oncattle and sheep and a few goat studies. Otherruminants were assumed to be similar to thesedomestic ruminants. Studies involving Africanruminants with different diets have led to a bet-ter understanding of why these animals eatwhat they do. These studies indicate that dietselection by ruminants is closely related to dif-ferences in the anatomy of their digestive sys-tems, beginning at the mouth and continuing tothe hindgut. These studies have led to a classifi-cation system for ruminant feeding types. requires an understanding of how plant cells areferent plants. Plant cells have a cell wall andcell wall holds the cell together and containscellulose) that must be broken down bymicroorganisms before they can be used byanimals.If broken down, the digestible part of the cellwall provides sugars which can bematerials like starch, protein,sugars, fats, and oils.to break down these materi-als. Grasses, especiallygrass stems, older grassplants, and tropical grass-es, contain large amountsof cell wall material, sothey are difficult to digest.Forbs and woody plantleaves (browse) have thinnercell walls compared to grasses and contain morecell contents, making them easier to digest. overlapping categories. First, browsersly digestible cell contents (forbs and browse).About 40 percent of ruminants worldwide canbe placed in this feeding type. Examples of thisgroup on Texas rangelands include white-tailedand mule deer. grazerscontaining plants like grasses; about 25 percentof all ruminants fall into this category. Texasexamples of this group are cattle, bison, andintermediate feedersits diet among grasses, forbs, and browse overthe year and within seasons. About 35 percentof ruminants can be placed in this group. Texasexamples of this group include pronghorn ante-lope, elk, goats, fallow deer, and nilgai. Table 1 compares parts of the digestive sys-tems of grazers and browsers. These differencesin each category are adapted to use. For eachcomparison, Table 1 also indicates the impor-tance of these differences to the feeding types. Competition Between Ruminant TypesFigure 4 illustrates that many ruminants dobut may, in fact, overlap another category.figure a species name appears, the more grassthat species is expected to eat. On the otherhand, the farther to the left a species nameappears, the more forbs and browse that speciesis expected to eat. Ruminants in the intermediate feeder catego-ry are expected to eat about equal amounts ofgrass and browse and/or forbs,but these animals may over-browsers. For example,nilgai overlap with graz-ers, which indicates theirdiets would be expectedto be more like that ofdeer. The more overlapbetween species, the moresimilar their expected dietsare and the more expectedcompetition for forage.Horses, for example, which Table 1. Comparison of Anatomy of Mouths and Digestive Systems of Browsers and Grazers (adapted from Hofmann 1986,1988). ComparisonBrowsersGrazersSignificanceMouth openinglarge, narrowsmall, wideLarger mouth opening allows stripping of twigs and gnawing of flowers and fruit. LipsflexiblerigidFlexible lips allow more selectivity of plant parts eaten.TongueslenderthickBrowser uses slender tongue with lips to select individual plant parts. Grazers wrap tongue around clumps of forage, not efficient for Taste budsfewmanySmell is probably more important in browser food selection and taste avoidance is probably more important in grazers.TeethsharpflatBrowsers can puncture plant material quickly releasing easily fermented cell contents. Grazers grind food, cell walls freed for microbial digestion. Jaw muscleslightheavyHeavy grazer muscles needed in grinding fibrous plant material. Salivary glandslargesmallBrowsers need more saliva to keep rumen pH from becoming too acidic from fermentation of large quantities of rapidly fermented RumensimplesubdividedAllows 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. smalllargeBrowsers 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 muscleslightheavyHeavy muscles allow grazers to handle larger amounts of forage Rumen papillaecover rumenlower rumenWith an increase in these structures, absorption occurs over a wallgreater portion of the rumen in browsers allowing acids produced during fermentation to exit the rumen quickly and help control Reticulum sizelargesmallSmall size, many and deep subdivisions hold forage in the grazer rumen longer allowing more time for fermentation. subdivisionsfew shallowmany deep OmasumsmalllargeLarger size provides more absorption surface. LiverlargesmallLarger liver is needed to absorb more rapidly fermented cell contents from browser rumens and to detoxify chemicals in browse. Hindgut volumelargesmallLarger volume indicates that hindgut fermentation 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. 7 are non-ruminant grazers, would be very com-petitive with either bison or cattle grazing thesame area because their diets are so similar.Because of their flexible diets, intermediatefeeders are very competitive with both browsersand grazers. The impact of this competition isespecially great for smaller animals. Smaller animals have higher relative nutrientrequirements and must, therefore, consumehigher-quality diets. A small browser with highnutrient requirements and little flexibility in thesource with an extremely flexible and competi- Figure 4.Feeding type classification for domestic livestock and native, Asian, and African wild ruminants. Some speciesoverlap 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 woodyplants) expected in the diet. Intermediate feeders tend to shift their diets among grasses, forbs, and browse over the year andwithin seasons (Adapted from Hofmann 1986,1988; Mungall and Sheffield 1994). tive intermediate feeder. One study illustratingthis point was conducted at the Kerr Wildlife(browsers) and sika deer (intermediate feeders)were placed in an enclosed pasture. At the endof the study, white-tailed deer were nonexistentand sika deer were abundant. When browse andforbs were significantly reduced in the pasture,white-tailed deer had no alternative foragesource. Sika deer, however, wtheir diet to grass and survive.Range herbivores differ widely in the kinds offorages they are adapted to use. These differ-ences are largely based on the anatomy of theanimals. Most of the economically importantrange herbivores in Texas are ruminants. In ruminants, the degree to which an animaling type, which is determined by its digestiveanatomy. The least-adaptable ruminants are thebrowsers and grazers. Between these two groupsare the intermediate feeders, which are extreme-ly flexible in their diets and, therefore, the habi-tats they can use. Although grazers will eatbrowse and browsers will eat grass, they willnot perform well when forced to shift their dietsto these extremes. Understanding these differ-are suitable for which animals can improve thelandownerÕs ability to successfully manage dif-ferent range herbivores. For More InformationSome information in this publication is takenArmstrong, W.E. 1984. How to manage deerhabitat: Edwards Plateau. In: Proceedings ofthe 1984 International Ranchers Roundup. TexasAgricultural Extension Service.Hofmann, R.R. 1986. Morphological evolution-ary adaptations of the ruminant digestive sys-tem. In: A. Dobson and M.J. Dobson (eds.),Aspects of digestive physiology in ruminantsComstock Publishing Associates, CornellUniversity Press, Ithaca, NY. Hofmann, R.R. 1988. Anatomy of the gastroin-testinal tract. In: D.C. Church (ed.), nant animal: digestive physiology and nutritionPrentice Hall, Englewood Cliffs, NJ. Mungall, E.C. and W.J. Sheffield. 1994. on the range. Texas A&M University Press,Vallentine, J.F. 1990. Grazing managementAcademic Press, Inc., San Diego, CA. Educational programs of the Texas Agricultural Extension Service are open to all people without regard to race, color, sex, disability, religion, age Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, andJune 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.RS 2, AS