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Effects of selective defoliation and height ofdefoliation on tiller dynamics and herbage yieldof Themeda triandra in a semi-arid rangeland inZimbabweS. Dube & V.E.E. GwarazimbaPublished online: 12 Nov 2009.

To cite this article: S. Dube & V.E.E. Gwarazimba (2000) Effects of selective defoliation and height of defoliation ontiller dynamics and herbage yield of Themeda triandra in a semi-arid rangeland in Zimbabwe, African Journal of Range &Forage Science, 17:1-3, 52-59, DOI: 10.2989/10220110009485739

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52 African Journal of Range & Forage Science 17(1,2&3):52-59

Effects of selective defoliation and height of defoliation on tiller dynamics andherbage yield of Themeda tr;andra in a semi-arid rangeland in Zimbabwe

S. Dube1,3 and V.E.E. Gwarazimba2

IMatOpoS Research Station, Private Bag K 5137, Bulawayo, Zimbabwe2Department of Biological Sciences, University of Zimbabwe, Box MP 167, Mount Pleasant, Harare, Zimbabwe

Received 13August 1999; Accepted 21May 2000

Abstract

Understanding the impacts of intensity and selection ofgrazing on the performance of grasses is important inthe management of grazing areas. It is especiallyimportant in semi-arid environments where. apart frommoisture. the levels at which grasses are utilized has amajor influence on their persistence in the environment.The effects of selective defoliation and height ofdefoliation (5 cm and 10 cm stubble heights) on theperformance of the grass, Themeda triandra, wereinvestigated in a field experiment for two growingseasons. Performance was measured as tiller produc-tion, rate of production. tiller mortality. herbage yieldand quality. Tiller production was greater (81 tillersper plant) under non-selective defoliation than underselective defoliation (49 tillers per plant) in the 1995/1996 season. Tiller mortality was higher (66.45%)under heavy selective defoliation than under non-selective defoliation (21.98%). Herbage yield, apartfrom the control treatment, was high (13.6 gper plant)under light non-selective defoliation. Heavy selectivedefoliation reduced the nutrient levels (e.g. levels ofsoluble carbohydrates under heavy selective defoliationwere 6 g k~J glucose compared to 20 g k~J glucoseunder light non-selective defoliation).

3Correspondence address: MAWRD FSRE Unit, Pri-vate Bag 5556, Oshakati, Namibia; E-mail:dubeskha@mweb.com.na

Additional index words: Crude protein, heavy defolia-tion, light defoliation, soluble carbohydrates, tillermortality.

Introduction

Rangelands are the major feed resource for livestock andwildlife in semi-arid ecosystems. They are characterisedby a nearly continuous herbage layer dominated byperennial grasses and sedges (Sarmiento 1992). Twothirds of the semi-arid rangelands are estiItlated to besuitable only for grazing. Selective grazing by mostdomestic animals and wildlife occurs to varying degreesfor palatable species, certain areas and leaf materials,thereby increasing pressure on the more palatablespecies such as Themeda triandra and Heteropogoncontortus. This may result in the over-utilisation of these

species and a reduction in regrowth vigour leading tolocal extinction (O'Connor 1991; Veenendaal et al.1996) and an increase in the composition of less-palatable and less-preferred species such as Cymbo-pogan plurinodis.

The differential defoliation of plants by animalsinduces shifts in species composition (Noy-Meir et al.1989). Knowledge of differential responses of grasses toutilisation is limited. Selective grazing, however,cannot be prevented because different animals havedifferent nutritional requiremenis, and efforts to forcenon-selective grazing can affect animal performance.There is, however, a n~ to reduce the levels ofselective grazing if rangeland production is to beimproved and sustained in good condition. The tilleringprocess is a common response to disturbances such asdefoliation (McNaughton 1992) that compensates for theimpact of grazing. Establishment of young plantsrequires titlering. It is essential for the regeneration ofthe sward following the removal of the terminal mer-istem by clipping or grazing during inflorescence devel-opment (Jewiss 1972). Stout et al. (1981) found thatdefoliating binomial species to 5 em stubble heightreduced tillering rate. Rapid tiller formation and growthafter defoliation is essential for the survival of perennialgrasses under grazing (Eriksen & Whitney 1981; Rich-ards et af. 1987; Busso & Richards 1995).

Heavy defoliation in T. triandra can reduce herbageyield by up to 60% (Coughenour et al. 1985). Heavygrazing, therefore, suppresses growth of some grasses(Beaty & Powell 1976; Singh & Mall 1976; Coughenouret al. 1985; Hodgkison et al. 1989; McNaughton 1992).

Soluble carbohydrates are often considered the pri-mary source of carbon for regrowth after defoliation.Storage of carbon is, therefore, essential in forage plantswhere carbon supply from leaves is frequently inter-rupted by grazing or defoliation (Danckwerts 1993).Poor correlation has been reported between labile carbonreserves and regrowth after defoliation. Grazing re-moves carbon/energy-assimilating tissues, thus distort-ing the physiological balance between assimilation andacquisition of mineral nutrients below ground. Dyer etaJ. (1991) suggest that storage of labile carbon reservesin sinks or pools readily available to the plant mightallow rapid mobilisation of these plant resources follow-ing grazing. Defoliation reduces the photosyntheticcapacity of the plants that are selected by animals

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Afiican Journal of Range & Forage Science 17(1,2&3):52-59 53

·····1936'1997 -"*. 1007/1996 Table 1 Annual rainfall received at trial site at MatoposResearch Station before and during the study period

MeasurementsThe initial number of tillers was recorded. Any newtillers emerging were recorded throughout the growingseason at two-week intervals. Tillers counted at eachcensus were those having at least two leaves. Tillerswere marked using coloured rings, with each colourrepresenting a census date. At the end of the seasons(24 April 1996 and 1997), a tiller count was done todetermine the number of live tillers from each treatment.Tiller mortality was calculated as a percentage relativeto total number of tillers that had emerged. Clippedherbage was oven dried at 80°C for 48 hours. The dateof defoliation was recorded. At the end of the experi-ment, after two growing seasons, all plants in the plots

3.0 ha per livestock unit (LU = 500 kg animal). Duringthe study, the study site formed part of a grazing systemstrial (1993 onwards) where it constituted one of the five9-ha paddocks in a rotational grazing treatment.

Field samplingIn the 1995/1996 growing season, thirty 1 m x I m plotswere selected at the study site based on the followingcriteria: the presence of T. triandra, aerial cover in thechosen plots was above 50010and the basal area of thetarget tufts between 28-32 cm2, Tufts were consideredseparate if the distance between them was greater thanor equal to 3 em or if it was apparent that they werediscrete. In each plot, a target T. triandra tuft wasmarked using a white flag for ease of identification.Five treatment combinations were randomly allocated tothe 42 plots with each treatment replicated six times.The treatment design was a 22 factorial, with twodefoliation heights (intensities: 5 cm and 10 cm), twosimulated defoliation regimes, selective defoliation oftarget T. triandra or non-selective defoliation of all thegrass in plot, and a control. The defoliation heights of 5cm and 10 cm simulated heavy and light grazingintensities, respectively. Intensity of defoliation isdefined as the proportion of the plant remaining afterdefoliation. The length of herbage remaining on t.¥plant is the base from which regrowth begins. After thefirst defoliation, subsequent defoliation was done whenthe grasses in the 10 cm defoliation treatment hadgrown to a height of 17 cm. Heady & Child (1994) statethe importance of this method on measuring regrowthpotential in plants and as an alternative to fixeddefoliation frequencies.

Figure 1 Mean monthly rainfall for the 199511996, 199611997and 1997/1998 seasons.

(Davidson & Milthorpe 1965; White 1973; Christiansen& Svejcar 1987).

Effects of selectivity of defoliation and height ofdefoliation (grazing intensity) on tiller numbers, rate ofproduction and mortality, and on herbage production,were investigated on T. triandra (Forsk.) a palatable,preferred (Danckwerts et aJ. 1983; Danckwerts & Nel1989), perennial, tufted grass (Gibbs-Russell et aJ.1991). The study attempted to test the hypotheses thattillering is higher under lightly non-selective defoliationand that herbage yield is low under heavy defoliation ingrasses.

ProcedureStudy areaThe experimental site was located on a section ofMatopos Research Station, Mahiye, (20°23'S, 2S028YE,1 320m a.s.1.)in south-western Zimbabwe. Topographyis gently undulating. The area is underlain by quartzmeta-gabbro parent material giving rise to siallitic,medium textured red clay soils (Dye 1983) of moderatelyhigh fertility. Soils are prone to surface crusting, whichresults in bare patches (Ward et aJ. 1979). Mean annualrainfall is 600 mm (median 570 mm) and rain fallsbetween October and April (Department ofMeteorologi-cal Services 1981). Rainfall before and during the studyperiod is presented in Table 1 and Figure 1. Winters arecold (15°C minimum and 22°C maximum) and oftenthere is ground frost along the watershed where nightground temperature can be as low as -~C. Thedominant grass species in the area are CymbopogonpJurinodis, Themeda triandra, Heteropogon contortusand Hyparrhenia fi/ipendu/a. The study site is de-scribed as an Acacia tree-bush savanna of varyingdensity (Rattray 1%1). The dominant woody species inthe area are Acacia karroo, Acacia ni/otica and Mayte-nus senegaJensis. Prior to the study (1980 to 1990) thestudy site was part of a four-paddock (9 ha each)rotational grazing trial in which stocking rate had been

Season (Jul-Jun)

1990/19911991/19921992/19931994/19951995/19961996/1997

Rainfall (mm)

267.5166.6432.4437.2574.9623.5

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·2Census ~me (Weeks)

African Journal of Range & Forage Science 17(1,2&3):52-59

Results

for tiller numbers, tillering rate and herbage yield weretransformed to meet the assumptions for analysis ofvariance (Dunn & Clark 1987; Fernandez 1992). Thecube-root (xIl3) transformation was used. Where signifi-cant differences occurred amongst treatment combina-tions, further analysis of variance was performed, split-ting the treatment combinations into defoliation regimeand height. Data for the chemical composition were notsubject to statistical analysis since there was no replica-tion; samples were lumped by treatment.

Tiller productionThere were significant differences in tiller production inboth the 1995/19% and 1996/1997 seasons amongsttreatments and census dates (Figure 2). Tiller produc-tion was greatest during the early parts of the season,peaking in weeks six and ten, and declining toward theend of the season. More tillers were produced underconditions of non-selective defoliation, within eachdefoliation regime, in plants subjected to 10 cm defolia-tion height. Production was higher in the 1995/1996growing season compared with the 19%/1997 season.Interaction between season and treatment was signifi--cant (P<O.OOI). Interaction between treatment andcensus dates was not significant (.P>O.05) in the 1995/1996 season but was in the 1996/1997 season (P<O.OOI).There were no significant differences in tiller productionbetween defoliation heights (P>O.05) in the 1995/19%season. Differences occurred in the 1996/1997 season(P<O.OOI),where more tillers were produced under 10cm heights (57 tillers per plant) than at 5 cm height.Tiller production was significantly influenced by defo-liation regime (P<O.OOI) in both seasons (Table 2).More tillers were produced in non-selectively clippedthan in selectively clipped plants.

Rate of tiller productionRate of tiller production significantly differed betweendefoliation treatments (P<O.OOl). A high rate of tillerproduction occurred in plants subjected to non-selectivedefoliation (Table 3). No differences occurred betweendefoliation heights in the 1995/1996 season but in the1996/1997 season tillering rate was higher under lightdefoliation (lO cm stubble height, 5 tillers per plant perweek) compared with heavy defoliation (l tiller perplant per week). Tiller production was significantlygreater in the first than in the second season (P<O.OOI).

Tiller mortalityPercentage tiller mortality in T. triandra plants signifi-cantly differed between defoliation treatments(P<O.OOI). Mortality was highest (65.4%) under heavyselective defoliation and lowest under light non-selectivedefoliation (Table 4). There were no significant differ-ences between seasons (P>O.05). Tiller mortality dif-fered significantly between defoliation regimes (P<O.OI)in the first season with higher mortality in plants

54

a --- Select Tl. 5 em -+- Select Tt. 10 em -.-Ccntre!..•.. Nonselect. 5 em ..•.. Ncnselocl.10em

10

i •&j ..'0~ 2~Z

10 12

-2 "

b --- Select n. 5 em -+- Select Tl1 0 em -.- Centre!..•.. Nonselect, 5 em ..•.. NCIl1IeIoct. 10 em

10

Figure 2 Variation in tiller production of Themec:18 triandraunder different defoliation treatment combinations during twogrowing seasons, (a) 1995/1996 and (b) 1996/1997.

o

were clipped to ground level and the target tufts wereweighed after oven drying. Light intensity reaching thebase of tufts was measured in February, March andApril 1997 using LICOR data logger. On threeconsecutive days, in March, light was measured from09hOOto 14h30 to determine diurnal changes in lightintensity in the control, selective and non-selectivedefoliation treatment. The LICOR data logger consistsof two light intensity sensors. One of the sensors (LI)was placed at the base of the target tuft, while the othersensor (L2) was placed in an open area mounted on atripod stand, a meter above ground. Relative lightintensity reaching the base of the tuft was calculated asLIIL2. Percentage light intensity reduction was thencalculated as:

(1-(L lIL2)]*100.

Laboratory analysesHerbage samples were bulked by treatment and species.ground to pass through a I-mm screen and analysed forsoluble carbohydrates using the Deriaz method (1961)and a glucose standard (glkg of glucose). Crude proteinwas analysed using the A.O.A.C m~thod (1965).

Statistical analysesANOVA was performed on the data using procedures inthe SAS computer programme (SAS/STAT 1987). Data

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African Journal of Range & Forage Science 17(1,2&3):52-59

Table 2 Mean number of tillers produced by Themeda trisndraunder different defoliation regime and height treatment combi-nations (df=4, n=6), and under different defoliation regimes(df=1) and heights (df=1) in two growing seasons (df=1; n=12).Means in the same row or column of the table with differentsuperscripts differ (P<O.OO1)

Treatment Season

1995/1996 19%/1997

Clip T. triandra to S em '147 b(j

Clip T. triandra to 10 em a46 "21Contro~ (no defoliation) lld71 d88

Clip nonselective to 5 em ad77 c20

Clip nonselective to 10 em ad79 d87

SE 12.2 12.2

Defo6ation regime

Nonselective defoliation a81 bS6

Select T triandra b49 c17

S.E 6.6 6.6

DefoHation height

5 em stubble height aM blS10 em stubble height B66 aS7

SE 6.2 6.2

Table 3 Rate of tiller production (number of tillers per plant perweek) of Themeds frisndrs under different defoliation regimesand heights (n=6, df=1, SE=1.2). Means in the same row orcolumn of the table with different superscripts differ (P<O.OO1)

Treatment Season

1995/1996 1996/1997

Defoliation regime

Nonselective defoliation a9 b5

Select T triandra bS ct

Defoliation height

S em stubble height B7 bl

10 em stubble height a7 as

subjected to selective defoliation regime compared tothose subjected to non-selective defoliation (Table 4).There were, however, no differences between regimes inthe second season. Defoliation height did not influencetiller mortality in the 1995/1996 season (P>O.05), but itwas significantly higher in plants subjected to heavydefoliation (5 cm stubble height) in the 1996/1997season (Table 4). Interaction between defoliation heightand defoliation regime was significant (P<0.05). Thissuggests that the two factors are linked in their effect onmortality, with heavy selective defoliation resulting inhigher mortality whilst light non-selective defoliationresulting in lower mortality.

Herbage yieldTotal herbage yield in the 1995/1996 growing seasonwas not affected by defoliation treatment (P>O.05).Generally, more herbage was harvested in plants undernon-selective defoliation (13.6 g plant-i) than in thoseunder selective defoliation (9.1 g plant-I; Table 5.).Herbage yield in the 1996/1997 season was significantly

55

---~._---------------

o9:55 10:<6 10:55 11:<6 11:55 12<6 1255 132 ~55 14:<6

Time of ooy

Figure 3 Diurnal variation in light intensity reaching bases ofThemeda trisndrs tufts subjected to different defoliation re-gimes during the 1996/1997 growing season.

influenced by treatment (P<O.OOI). Within each defo-liation regime, more herbage was obtained in plantsunder light defoliation (10 cm stubble height) thanunder heavy defoliation (5 cm stubble height). Highestyields were obtained from the control plants (26.5 gplanrI) and lowest from the intensely clipped plants (0.5g plant-I; Table 5).

Herbage chemical compositionHigher levels of crude protein were found under non-selective defoliation and control plants, and lower levelsunder selective defoliation (Table 6). Generally,amounts of soluble carbohydrates were higher undernon-selective defoliation and in control plants.

Light intensity reductionThere was a significant difference in light reductionaccording to time of day (P>O.05). Light intensityreduction was, however, higher in the morning and latein the afternoon. By midday (between Ilh55 and13h55), light intensity reduction was lower. Lightintensity reduction significantly differed amongst defo-liation regimes (P<O.OOI). Light intensity reductionwas highest (92.2%) under control and lowest undernon-selective defoliation (21.SOIo, Figure 3).

Light intensity reduction did not differ from onemonth to another (P>O.05). It was, however, lower inFebruary (early part of season) and in April (late part ofseason), and higher in March (mid growing season;Figure 4). Light intensity reduction was affected bydefoliation treatment (P<O.OOl). It was higher underselective defoliation and control (no defoliation), andlower under non-selective defoliation.

Discussion

Tiller dynamicsThe study has shown that defoliation treatments affectstiller production. Under non-selective defoliation condi-tions there is a reduction in competition for resources

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56 African Jownal of Range & Forage Science 17(1,2&3):52-59

Figure 4 Monthly variation in light intensity under differentdefoliation treatment combinations during the 1996/1997 grow-ing season.

Table 4 Percentage tiller mortality of Themeds triendrs underdifferent defoliation regime and height treatment combinations(df--4, n=6), and under different defoliation regimes (df=1) andheights (df=1) in two growing seasons (df=1) (n=12). Means inthe same raw or column of the table with different superscriptsdiffer (P<O.05)

such as light (see Figures 3 and 4). Similar findingswere reported by Jones & Cross (1971). These workersobserved that non-selective defoliation of Eragrostisspp. and Paspalum dilatatum was a more importantfactor than defoliation intensity on tiller production ofthe grasses.

The effects of defoliation height emerged in thesecond season of study (1996/97 growing season). Stoutet al. (1981) observed similar results. These workersfound that defoliation of pinegrass to 5 cm stubbleheight resulted in rapid decline in tiller production asthe number of defoliation years increased and thisdecline could be attributed to a fall in plant vigour as aresult of heavy defoliation. This results in reducedcarbohydrate levels (perry & Chapman 1976). Singh &

3.2 63.6 85.7 185.5 195.6 20

Crude protein Solublecarbohydrate

Treatment Season

1995/96 (dF3) 1996/97 (dN)

Clip T. triandra to 5 em 9.1 ao.5Clip T. triandra to 10 em 11.1 a2.2Control (no defoliation) 1>26.5Clip non-selectively to 5 em 11.2 a2.7Clip non-selectively to 10 em 13.6 el1.0SE 2.23 3.62

DeCoHatiOllregime

Non-selective defoliation 12.4 ~.9Select T. triandra 10.1 b1.3S.E 1.579 0.52

DefolDtion height

5 em stubble height 10.2 a1.610 em stubble height 12.3 b6.6SE 1.57 0.52

Clip T. triandra to 5 emClip T. triandra to 10 emControl, (no defoliation)Clip nonselective to 5 emClip nonselective to 10 em

Treatment

Table 5 Herbage yield (g plant1) of Themeds trisndrs underdifferent defoliation regime and height treatment combinations(df=4, n;6), and under different defoliation regimes (df:=1) andheights (df=1; n=12) in two growing seasons (df:=1). Means inthe same raw or column of the table with different superscriptsdiffer (P<O.OO1)

Table 6 Effects of different defoliation regime and defoliationheight treatment combinations on the chemical composition ofherbage, crude protein (%) and amount of soluble carbohy-drates (g kg-1 glucose) within Themeds trisndrs

Mall (1976) also attributed the decline in tiller produc-tion to reduction in the amount of reserves available fortranslocation to roots resulting in insufficient resourcesfor regrowth or tiller production.

Tiller production declined as the season progressedbut there were sporadic increases in tiller production,probably related to rainfall events as observed byDanckwerts (1988) in semi-arid grassland. Danckwerts(1988) suggested that grass growth in semi-arid grass-lands take place in short sporadic spells after a rainfallevent. Tiller production in the second season was belowhalf of the first season production. This might be anindication of the adverse cumulative effects of defolia-tion. a reduction in plant vigour, alluded to by manyworkers (e.g. Davidson & Milthorpe 1965; Beaty &Powell 1976; Perry & Chapman 1976; Derigibus et al.1982). Rapid tillering ensures fast production ofsufficient leaf area for complete light interception(Jewiss 1972). It is those plants that produce tillersrapidly after a disturbance that will persist longer in anenvironment prone to disturbance like grazing (Eriksen& Whitney 1981; Richards et al. 1987). Management

.t.Vo Feb

131

~~~---~~----

Treatment Season

1995/1996 1996/1997

Clip T. triandra to 5 em "64.40 "66.45Clip T. triandra to 10 em 846.81 bl3.27Control, (no defoliation) 0331.79 0338.37Clip nonselective to 5 em e1>25.37 e1>24.61Clip nonselective to 10 em el>21.98 eb27.98SE 7.494 7.494

DeColiation regime

Nonselective defoliation a23.86 a26.25Select T. triandra b55.14 a33.04S.E 5.679 5.679

DeColiation height

5 em stubble height 842.23 842.1310 em stubble height a32.85 abI9.72SE 5.820 5.820

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African Journal of Range & Fomge Science 17(1,2&3):52-59

should target such grasses so as to improve rangelandcondition. Tillering rate was higher under non-selectivedefoliation where light penetration was higher. Therewas increased shade effect on clipped plants. Lightdefoliation ensures that there is sufficient leaf arearemaining for photosynthesis. Thus, under light defolia-tion intensity (10 cm stubble height) tillering rate washigher. Barnes (1972) made similar observations whileworking on Cenchrus ci/iaris and Panicum maximum.Tiller production was faster in the first season than inthe second season. Hodgkinson et al. (1989) found thatfrequent defoliation reduced the rate of tiller productionin T. triandra. Heavy selective defoliation resulted indecreased plant vigour as shown by the low level ofsoluble carbohydrates in plants subjected to this treat-ment (Table 6). Similar observations were made byAnderson et al. (1989) and Danckwerts (1993) workingon Panicum virgatum and also by Christiansen &Svejcar (1987). The results showed that tiller mortalitywas highest under selective defoliation. Effects ofdefoliation height became significant in the secondseason when mortality of tillers was highest in plantssubjected to heavy defoliation (5 cm stubble height).Danckwerts et al. (1984) also found that the response todefoliation height in became apparent in the secondseason as recorded in this study. Perry & Chapman(1976) reported that low non-structural carbohydratereserves in heavily defoliated grass result in high tillerand plant mortalities. They attributed the high mortali-ties to loss of vigour due to heavy defoliation. Mortalityof tillers in undefoliated plants was higher than undernon-selective defoliation and lower than under selectivedefoliation. This confirms observation by Eriksen &Whitney (1981) which suggests self-thinning in pro-tected plants.

Herbage yieldResults of the present study show that herbage yield inT. triandra, at each harvest, in the 1995/1996 growingseason was influenced by defoliation treatments withmore herbage being realised under light non-selectivedefoliation. Lowest yields were under heavy selectivedefoliation. This supports the observation by Beaty &Powell (1976) who showed that heavy frequent defolia-tion of Panicum virgartum reduced herbage yield andtiller production. Singh & Mall (1976) also showed thatdefoliating Andropogon pumi/us, an annual grass, to 5cm stubble height (heavy defoliation) resulted in amarked reduction in herbage yield than defoliating it to10 cm stubble height (light defoliation). Singh & Mall(1976) attributed the reduction in herbage yield underheavy defoliation to reduced carbohydrate manufacturedue to reduced leaf area. There were peaks in herbageyield within season, a response to rainfall events.Strugnell & Pigott (1978) found a strong correlationbetween herbage yield and rainfall in a Hyperrheniafi/ipendula-T. triandra range in the Rwenzori NationalPark, Uganda. According to McNaughton (1992),

57

canopy closure is a major contributor to the capacity ofgrasses to compensate for defoliation. non-selectivedefoliation reduces canopy closure and ensures thatmaximum light interception occurs. This ensuresmaximal photosynthesis for regrowth, resulting in in-creased herbage yield. Eriksen & Whitney (1981) whofound reduced herbage yield (% dry matter) in plantsgrown under shade recorded similar results. The lowyields were correlated to a decline in carbohydratelevels.

Total herbage yield for the whole 1995/1996 seasonwas not affected by defoliation treatment. Effects oftreatments were masked by the fact that more herbagewas removed, initially, in the 5 cm stubble heighttreatment than in the 10 cm stubble height treatment.Within season yields, at each harvest other than the first,were higher under light non-selective defoliation thanunder heavy selective defoliation. Beaty & Mall (1976)reported similar results. The higher yields were main-tained for the greater part of the season. Defoliationregime and height of defoliation affected herbage yieldin the 1996/1997 season. Total herbage yield in the1996/97 season was highest under no defoliation asobserved by McNaughton (1992); Derigibus et aJ.(1982) and Anderson et al. (1989). MacNaughton(1992) observed highest herbage yields where T. trian-dra was unclipped and unshaded as observed also byEriksen & Whitney (1981) in their work on six foragespecies in Hawaii. Shading (which can be caused byselective defoliation) reduces leaf growth and increasestiller mortality (Christiansen & Svejcar 1987).

Herbage chemical compositionResults from the present study have shown that crudeprotein levels were high under light defoliation. Thissuggests impeded photosynthesis when compared toheavy defoliation (Anderson et al. 1989). Amounts ofsoluble carbohydrates were high Under non-selectivedefoliation. Similar observations were made by Ander-son et al. (1989) working on Panicum virgatum,Derigibus et al. (1982), Singh & Mall (1976) onAndropogon pumi/us, and Davidson & Milthorpe (1965)on Dactylis glomerata. In these plants, light forphotosynthetic activity is not limiting. Danckwerts(1993) and Dyer et al. (1991) suggested that lightdefoliation induces plants to fix large amounts ofcarbon. Intense herbage removal decreases the amountof soluble carbohydrates, especially under conditions ofselective defoliation (Christiansen & Svejcar 1987).White (1973) reported that selective removal of herbagefrom some plant result in the ungrazed plants taking upthe available nutrients and water away from the grazedplants. This reduces the performance of defoliatedplants.

It should be noted that the effect of defoliationtreatments on nutrient levels could not be explicit fromthe single sampling undertaken at the end of the secondgrowing season (after 2 years). The levels of soluble

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carbohydrates in plants fluctuate tremendously withingrowing season. Continuous sampling would havebeen preferable.

There were low levels of soluble carbohydrates underheavy selective defoliation. This suggests that heavyselective defoliation results in a rundown of reservecarbon in plants (Anderson et a/. 1989; Smith 1972).Danckwerts (1993) postulated this hypothesis for T.triandra. Eriksen & Whitney (1981) observed lowlevels of non-structural carbohydrates under low light(shade) environment in temperate grasses. They sug-gested that the same could be true for tropical grasses.This study confirmed this suggestion.

ConclusionDefoliation selectivity was a major factor affecting theperformance of Themeda triandra. It influenced thelight environment, with nonselective defoliation reduc-ing the competition for light between neighbouringplants. Light intensity reduction under nonselectivedefoliation is lower than under selective defoliation.The undefoliated neighbouring plants shade plantsdefoliated selectively. Reduction in unequal competi-tion for resources (which might be caused by selectivedefoliation) can be achieved through nonselective mow-ing or through short duration grazing at high stockdensities. Under short duration grazing, plants are notseverely defoliated and high stock number will ensurethat selection is limited. This might affect the perfor-mance of animals but it strikes a balance with goodsustainable range condition, hence sustainable animalproduction. This will curtail the deterioration ofsemi-arid grazing systems, which cover large areas ofZimbabwe and indeed the world.

AcknowledgementsI am grateful to the following organisations: Institute ofEnvironmental Studies (University of Zimbabwe) forfacilitating, co-ordinating and administering the fundsfor this work; University of Utrecht (The Netherlands)for funding the project and for the valuable assistancefrom W. Dijkman. Many thanks go to P. Frost, 1.Gambiza and B. Campbell.

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