Grazing can have a neutral, positive or negative effecton rangeland plants, depending on how it is man-aged. Land owners and managers can better protect

rangeland plants, and, in turn, other rangeland resources,if they understand:

� The effects of grazing and browsing (eating theleaves and young twigs of trees and shrubs) on indi-vidual plants and plant populations.

� The indicators that show which plants are in dangerof overuse by grazing and browsing animals.

� The grazing management practices that help pre-serve the rangeland resource.

Understanding these factors and knowing the availablemanagement options allows landowners and managers tomake better decisions about which actions are best for aparticular site and when to take action. Timely action canpreserve the long-term health of the rangeland as well asthe viability of livestock and wildlife operations.

Interactions between range plants

and range animals

Rangelands are ecosystems that have adapted to with-stand such disturbances as drought, flood, fire, and graz-ing. All disturbances affect plants to some extent, eitherdirectly or indirectly, depending on the timing, intensity,and frequency of the disturbance. Generally, the morediverse the vegetation, the better rangeland can withstanddisturbance.

Rangeland plants provide nutrients—proteins, starchesand sugars—to grazing and browsing livestock andwildlife. These nutrients, or plant foods, are produced byphotosynthesis. Because photosynthesis occurs only ingreen plant tissue and mostly in the leaves, a plant

becomes less able to produce food, at least temporarily,when its leaves are removed (defoliation) by grazing andbrowsing animals.

Products of photosynthesis are just as important toplants as they are to animals. Like all other living things,plants need food to survive and grow. The food that plantsmake for themselves through photosynthesis is used formajor plant functions such as surviving dormancy, grow-ing new roots, growing new leaves in the spring, andreplacing leaves lost to grazing or browsing.

Most native rangelands evolved under grazing.Therefore, rangeland plants have developed the ability towithstand a certain level of grazing or browsing. Althoughgrazing animals do disturb rangeland, research has shownthat rangelands gain few benefits when livestock are total-ly excluded for long periods.

What happens to a plant

after grazing or browsing?

Grazing affects not just the leaves, but also other partsand functions of plants, including the root system, foodproduction after defoliation, and the destination of foodproducts within the plant after defoliation.

Food reserves and the root system When a plant’s leaves are removed, its roots are also

affected. Excessive defoliation makes the root systemsmaller.

Removal of too many leaves has a profound effect onthe root system (Figure 1). Research on grasses hasdemonstrated that when 80 percent of the leaf is removed,the roots stop growing for 12 days. When 90 percent ofthe leaf is removed, the roots stop growing for 18 days.Root growth drops by half when 60 percent of leaf isremoved.

B-611411-01

*Associate Professor and Professor and Extension Range Specialists, The Texas A&M University System

Grazing and Browsing:How Plants are Affected

Robert K. Lyons and C. Wayne Hanselka*

As root growth is reduced or stopped, root volumedecreases (Figure 2). Plants with smaller roots have lessaccess to water and other nutrients in the soil needed tomanufacture food. A smaller root system also makesplants less drought resistant.

Early research demonstrated that roots lose storedfoods after defoliation. These observations led to the con-clusion that the roots and crown of grasses were majorsources of food for the initiation of growth after defolia-tion.

However, recent information indicates that, at least ingrasses, stored foods are not as important in initiating thisgrowth. Although food reserves decline in grass rootsafter defoliation, these reserves do not appear to be sent tothe food-producing parts of the plant.

Recent research indicates that this decline in foodstored in grass roots after defoliation results from a com-bination of:

� Remaining leaves sending less of the food theymanufacture to the roots, and

� Roots themselves using the root food reserves.

In addition, studies involving grass crowns have shownthat this part of the plant stores only about a 3-day supplyof food reserves. This finding indicates that this part ofthe plant does not supply enough food to promote signifi-cant growth after defoliation.

If roots do not contribute stored food to promotegrowth after defoliation, where does the plant get thisfood?

Food production after defoliation Grazing and browsing decrease, at least temporarily, a

plant’s food production by reducing the amount of greenplant material available to produce food. Other factors

affecting food production after grazing or browsinginclude the amount, kind, and age of plant material (leaf,sheath, stem) remaining on the plant.

For example, grass leaf blades, whether mature oryoung, often produce food at a higher rate than leafsheaths (the leaf base enveloping the stem) or stems. Inaddition, young leaves produce food at higher rates thanolder leaves. Therefore, the more leaf material left aftergrazing, the faster grasses recover from grazing.

In many plant species, including some grasses, theleaves on grazed or browsed plants produce food at higherrates than leaves of the same age on plants that have notbeen grazed or browsed. In plants where it occurs, thisprocess happens over several days in leaves remaining ona grazed or browsed plant and in new leaves developingafter grazing or browsing. This process is one way thatsome plants partially cope with grazing or browsing.

Destination of food products after defoliation Plants use the foods they produce for growth and main-

tenance. Any excess food is sent from the food-producingplant parts to other parts both above and below ground,where it is stored.

Once a plant has been defoliated, it may change thedestination of its food products. The destination of thatfood varies with plant species. In some species, more foodis sent to growing shoots and less to roots. This processoccurs for a few days until the food-producing tissues canbe reestablished. In some grass species, more food prod-ucts may even be sent to the more active food-producingleaf blades rather than to less active leaf sheaths.

A plant’s ability to send food products to new shootsafter defoliation can help it quickly reestablish its food-producing parts. Plant species that have this ability arebetter able to tolerate grazing.

20 40 60 80 100

100

80

60

40

20

Leaf volume removed, %

Red

uctio

n in

roo

t gro

wth

, %

Figure 1. The effect of leaf removal on the root growth of agrass. With 80 percent leaf removal, roots stopped growing for12 days; with 90 percent removal, root growth stopped for 18days.

Grass

Roots

Figure 2. Heavy, frequent defoliation stops root growth andreduces the size of the root system. It reduces the plant’s abili-ty to absorb water and other nutrients, thus making the plantless drought resistant and less able to manufacture food.

In investigations of grazing tolerance, researchers com-pared two western grass species that had different levelsof grazing tolerance. They found that after defoliation, thegrazing-tolerant species sent more food products to newleaves and fewer products to the roots. In contrast, thegrazing intolerant species sent large amounts of foodproducts to the root system. This finding helps explainwhy some grasses are better able to resist grazing.

How do plants cope

with grazing and browsing?

The ability of plants to survive grazing or browsing iscalled grazing or browsing resistance. The most grazing-resistant plants are grasses, followed by forbs (herbaceousplants other than grass), deciduous shrubs and trees, andevergreen shrubs and trees.

When a grass seedling develops, it produces a primarytiller, or shoot. This primary tiller has both a main grow-ing point and secondary growing points located at orbelow ground level.

Additional tillers can develop from secondary growingpoints at the base of a tiller. Tillers can also develop frombuds at the nodes of stolons (above-ground lateral stems,such as in buffalograss) or rhizomes (below-ground lateralstems, such as in Johnsongrass) of grasses with thesestructures.

Cool-season grasses begin growth in the fall, maintainsome live basal leaves through winter, and continuegrowth in the spring. Tillers produced in the fall areexposed to cold and can produce seedheads in spring.Tillers initiated in the spring usually do not produce seed-heads.

In comparison, warm-season grasses produce newtillers in late summer and early fall. Although these youngtillers die back when exposed to frost, their buds will pro-duce new tillers the following spring.

Tillers of most grasses live only 1 to 2 years.Individual leaves usually live less than a year and mostonly a few months.

A plant can produce leaves only at an intact growingpoint. As long as that growing point is close to theground, it is protected from being eaten (Figure 3). Atsome point, most grasses elevate at least some of theirgrowing points to produce tillers, or shoots, that haveseedheads.

Tillers stop producing new leaves when a seedheaddevelops from the growing point or when the growingpoint is eaten. Plants then must depend on other tillers tocontinue producing new leaves or wait until basal budsproduce new tillers.

Excessive grazing of a grass plant when its growingpoints are elevated reduces new leaf production, andtherefore, the ability of the plant to produce food and tol-

erate grazing. Destruction of the growing point also pre-vents seed production and production of new seedlings.Grasses should be rested from grazing periodically toallow them to produce leaf material to feed the plant andto allow seed production.

Timing of growing point elevation varies among grassspecies (Table 1). For example, growing points of buffalo-grass and other sod-forming grasses remain close to theground, giving these grasses high grazing resistance.

Little bluestem and sideoats grama keep their growingpoints close to the ground until just before seedheadsemerge. Although this strategy protects growing pointsfrom being eaten for a longer period, these two grassesproduce many tillers with seedheads, which means thatmany growing points are exposed. The combined effect ofdelayed elevation and the production of many tillers withseedheads gives these two grasses moderate grazingresistance.

Yellow indiangrass and switchgrass elevate their grow-ing points above ground level soon after growth begins.This early elevation results in low grazing resistance.

Grasses with low (yellow indiangrass and switchgrass)to moderate (little bluestem and sideoats grama) grazingresistance require more care in grazing management. Thiscare can be accomplished in several ways.

One way to manage these low- to moderate-grazing-resistant grasses is to lower grazing pressure by stockingfewer animals to allow some plants to escape grazing.

12

3 4

56

7

8

3

3

4 5 6 7 8

Figure 3. This illustration represents a grass tiller (or shoot)and its main growing point. On the left are the grass tiller andeight leaves, numbered 1 to 8. On the right is an enlargementof the area near the base of this tiller where the main growingpoint is located. All the leaves shown have developed from thisgrowing point. As long as the growing point is close to theground as shown here, it is safe from being eaten and cancontinue to produce leaves for the life of the tiller (1 to 2years).

Another method is to make sure that pastures with thesegrasses are rested from grazing every 3 or 4 years duringthe growing season to allow the plants to produce seed.

Still another method that has been used successfully isintensive-early stocking. With this approach, grazing ani-mals are stocked at higher than normal numbers for thefirst part of the growing season and then removed frompastures for the rest of the growing season. This approachhas typically been used with stocker (young steer andheifer) operations.

Johnsongrass is an interesting contradiction. Because itproduces strong rhizomes (underground stems), it shouldbe resistant to grazing. However, Johnsongrass also pro-duces a high proportion of reproductive stems, which can-cels the advantage of rhizome production and results inlower grazing resistance.

The growing points of forbs, like those of grasses,remain close to the ground early in the growing season.Forb species that elevate growing points early are lessresistant to grazing.

For woody plants, growing points are elevated aboveground and, therefore, are easily accessible to browsinganimals. If these growing points are removed, lateral budsare stimulated to sprout and produce leaves. However,woody plants replace leaves relatively slowly.

Grazing avoidance and grazing toleranceGrazing resistance can be divided into avoidance and

tolerance (Figure 4). Grazing avoidance mechanismsdecrease the chance that a plant will be grazed orbrowsed. Grazing tolerance mechanisms promote growthafter grazing or browsing.

Grass Species Growing Point Elevation/Reproductive Tiller Ratio Grazing ResistanceBuffalograss Remain close to ground High

Little bluestem Elevation late w/ large number reproductive tillers Moderate

Sideoats grama Elevation late w/ large number reproductive tillers Moderate

Switchgrass Elevation early Low

Yellow indiangrass Elevation early Low

Johnsongrass High proportion of reproductive tillers Low

Grazing Resistance(Allows plants to survive grazing)

Avoidance

Tolerance

Growing points low

Growing pointselevated late

Leafaccessibility

Mechanical deterrents(awns/spines)

Leafpalatability

More vegetative thanreproductive tillers

Short leaves

Leaf closeto ground or stem

High numberof stems

Hairs, wax, silica

Tough leaves

Bad taste

Toxic to animal

High amount ofdead material

Figure 4. Examples of plant grazing-resistance mechanisms.

Table 1. Examples of growing point elevation and grazing resistance for some common range grasses.

Grazing resistance factors can be related to plant anato-my, plant chemistry or plant physiology:

� Anatomical features that help plants resist beinggrazed include leaf accessibility (leaf angle, leaflength), awns or spines, leaf hair and/or wax, toughleaves, grass species with more vegetative stems(fewer growing points exposed) than reproductivestems, and the ability to replace leaves, whichdepends on growing points.

� Chemical factors of grazing resistance include thosecompounds that make plants taste bad, toxic, orhard to digest.

� Physiological factors include sending new foodproducts to new leaves, water-use efficiency, androot growth and function.

Competition and grazing Competition from neighboring plants for soil nutrients

and water affects plant response to defoliation. Studieshave shown that when competition is reduced, leaf growthin defoliated plants can be similar to that in nondefoliatedplants. Competition can be reduced by 1) lowering graz-ing pressure by stocking fewer animals and 2) restingplants from grazing.

If competition is not reduced, new leaf growth may notoccur because of a lack of available nutrients to grow newleaves. Therefore, plants that are grazed severely whileneighboring plants are not grazed or grazed less severelyare at a competitive disadvantage.

Do plants benefit from grazing? It is not clear if plants benefit from being grazed.

Certain species may benefit from grazing but not neces-sarily from being grazed. For example, plants may benefitindirectly from removal of competition or from the cre-ation of a favorable environment for seed germination ordirectly from removal of self-shading or removal of inac-tive leaves.

Some grazed plants experience compensatory photo-synthesis (food production). However, this response doesnot mean that the plants benefit from being grazed, onlythat they have ways to cope with grazing.

Browse management considerations

Browsing animals such as goats and deer prefer certainbrowse species. Preferred species vary with naturalregions (such as the Edwards Plateau, Rio Grande Plain,Trans Pecos, etc.) of Texas. However, Texas kidneywoodand Texas or Spanish oak are examples of highly pre-ferred species; live oak represents a moderately preferredspecies; and ashe juniper (blueberry cedar) and mesquiteare examples of low-preference species.

Without proper management, the more desirablebrowse species can disappear because of these prefer-

ences, while less desirable or undesirable species becomemore abundant. From a livestock perspective, proper man-agement involves controlling browsing livestock numbersand controlling access to browse plants to provide restfrom browsing. From a wildlife standpoint, proper man-agement involves harvesting animals when wildlife censusnumbers and browse use signs indicate a danger to thebrowse resource.

Just as with grasses, browse species can be managed topromote and maintain key species, that is, the preferredplants that make up a significant part of the production ofbrowse available for animals to eat. This task is accom-plished by controlling animal numbers and providing restfrom browsing.

How to determine if the range

is being overused

Managers can use browse indicators to help make man-agement decisions about the browse resource. These indi-cators include degree of use, hedging, and the presence orabsence of seedlings.

Degree of use is the amount of the current season’sgrowth that has been removed by browsing animals. It isbest observed at the end of the growing season in late fallfor deciduous plants and late winter for evergreens. Whendetermining degree of use, consider only current seasongrowth by comparing browsed twigs with unbrowsedtwigs.

Browse use can be divided into three levels of currentseason growth removal: light use is marked by less than40 percent removal; moderate use ranges from 40 to 65percent removal; and heavy use is more than 65 percentremoval.

Moderate use on key browse species is the correctmanagement goal. When use approaches the upper limitof moderate use for key species, browsing pressure shouldbe reduced by 1) resting areas from browsing livestockuse or reducing livestock numbers and/or 2) reducingwildlife numbers.

Hedging is a plant response to browsing marked bytwigs that have many lateral branches. A moderate degreeof hedging is acceptable (Figure 5) because it keepsbrowse material within easy reach of animals and stimu-lates leaf and twig growth.

However, excessive hedging produces short twigs withsmaller than normal leaves and twigs. Eventually, entireplants can die from excessive hedging.

Another indicator of excess browsing pressure is thehedging of low-preference plants such as agarita (Figure6). When animals consume plants they do not normallyeat, it usually means that not enough of their preferredfood is available.

To provide forage, browse plants must be within reachof browsing animals (Figure 7). As hedging increases, thelower branches disappear and a browse line develops. Abrowse line is the height on trees or shrubs below whichthere is little or no browse and above which browse can-not be reached by animals.

Areas where trees or shrubs have a highly developedbrowse line have a park-like appearance. In the earlydevelopment of a browse line, light begins to showthrough the lower vegetation. With continued browsingpressure, a distinct browse line develops (Figure 8).Development of browse lines on low-preference plantssuch as ashe juniper (blueberry cedar) also indicatesexcessive use of the range (Figure 9).

Figure 7. The absence of a browse line on desirable woodyspecies indicates that forage is accessible to animals and thatthe number of animals is probably in balance with the supplyof browse.

Figure 8. A prominent browse line on moderately preferredbrowse species such as live oak is an indication of past overuse.

Figure 9. A prominent browse line on ashe juniper (blueberrycedar), a low-preference plant, is an indication of severe over-use of the browse resource.

Figure 5. A moderate degree of hedging as shown on thisTexas kidneywood plant, a highly desirable browse species, isacceptable.

Figure 6. The hedging on agarita, a low-preference browseplant, indicates excessive use.

The height of browse lines depends on browsing ani-mal species. For example, white-tailed deer usuallybrowse to about 3 to 4 feet, goats to about 4 to 5 feet, andexotic wildlife species to 6 feet and more.

To keep woody plant populations healthy, plants mustbe allowed to reproduce. Therefore, the presence ofseedlings of desirable browse plants is another indicatorthat managers can use to check for range overuse.

Management considerations

Regardless of whether a ranch’s production goal islivestock or wildlife, plants feed these animals and protectthe soil from erosion. A good steward should aim to con-serve the soil and plant resources so that animals are pro-duced in a way that can be sustained over time.

To influence the effect of grazing disturbances onrange plants, managers can control three factors of graz-ing or browsing:

� Intensity refers to the amount of grass or browsethat is eaten. It is the most important factor becauseit affects the amount of leaf available for food pro-duction as well as the amount of root system ingrasses and the production of seed.

� Timing of grazing affects plants more severely atcertain stages of their development. The most criti-cal grazing period is usually from flowering to seedproduction. Although the least critical period is dor-mancy, leaving plant residue is important even dur-ing dormancy. Research and demonstration workhave shown that removing high quantities of forageduring dormancy is almost as detrimental to plantproductivity as during active growth periods.

� Frequency refers to how often plants are grazed orbrowsed. Animals tend to come back to the sameplants to graze or browse during a growing season.If a plant is repeatedly defoliated, it can be weak-ened and may die.

To manage grazing and browsing and protect the rangeresources, managers should:

� Observe the status of and changes in grasses, forbs,and woody species as well as in livestock orwildlife. Make adjustments when either the rangeplants or animals show signs that the range is beingoverused.

� Rest grasses periodically, but not at the same timeevery year. Grasses differ as to when growing pointsare elevated, making it difficult to find one optimumrest period for all species.

� Leave enough residual forage ungrazed to keepplants healthy and to capture rainfall. The best wayto prevent excess rainfall runoff is to maintain ade-quate ground cover. When the range has enoughplant material to promote water infiltration into thesoil, less rainfall is required to produce forage.

� Note when the more palatable key species start toshow overuse. Grazing and browsing animals areselective: They graze or browse the most palatableforage species first and often. If the more palatablespecies are overused and disappear, the plantspecies that survive will be those that can best resistgrazing. Animals often avoid eating plants that areabundant but not palatable; instead, they spend timeand energy searching for plants that are more palat-able but scarce. Therefore, overuse of more-palat-able species can reduce animal performance.

� Adjust livestock and wildlife browsing by reducinganimal numbers and/or resting pastures when younotice more than moderate use or excessive hedgingon desirable brush plants and before the develop-ment of browse lines.

For more information

Briske, D.D. and J.H. Richards. 1994. Physiologicalresponses of individual plants to grazing: current statusand ecological significance, p. 147-176. In: M. Vavra,W.A. Laycock, and R.D. Pieper, (eds.), Ecologicalimplications of herbivory in the West. Society forRange Management, Denver.

Dietz, H.E. 1975. Grass: the stockman’s crop, how to har-vest more of it. Simmental Shield, Special Report.Shield Publishing Co., Inc. Lindsborg, Kan.

Natural Resources Conservation Service. 1994. The useand management of browse in the Edwards Plateau ofTexas. United States Dep. of Agr. Temple, Tex.

Sayre, N.V. 2001. The New Ranch Handbook: a guide torestoring western rangeland. Quivara Coalition, SantaFe, N.M.

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