growth, water relations and biomass production of the savanna grasseschloris roxburghianaandcenchrus...

Growth, water relations and biomass production ofthe savanna grasses Chloris roxburghiana and

Cenchrus ciliaris in Kenya

George A. Keya

National Arid Lands Research Centre, P.O. Box 147, Marsabit, Kenya

(Received 11 July 1997, accepted 20 October 1997)

A study was carried out to determine and compare the ecophysiology of theperennial savanna graminoids Chloris roxburghiana and Cenchrus ciliaris in asemi-arid pediment region of northern Kenya. Growth, phenology, waterrelations and biomass production potential were investigated under clippedand non-clipped conditions. Chloris roxburghiana recorded faster growth ratesand greater dry matter fixation than C. ciliaris. This study demonstratedpossibilities of fodder production of up to 11 t ha–1 and 6·6 t ha–1 for C.roxburghiana and C. ciliaris, respectively. Similarly, C. roxburghiana was moreefficient at controlling water loss than C. ciliaris, indicating a better adaptationto drought. On average, growth and leaf area development on clipped plotswas faster than the controls. However, defoliation delayed and reducedreproduction. Although defoliation improved the water status of C. roxburghi-ana, it had no apparent effect on that of C. ciliaris. Growth and green-upresponses of C. roxburghiana occurred 2–4 days following rains. Theproductivity and ecophysiology of these species render them suitable forincreasing land productivity in semi-arid areas.

©1998 Academic Press Limited

Keywords: grass; Cenchrus ciliaris; Chloris roxburghiana; phenology; biomass;transpiration; savanna; northern Kenya

Introduction

Graminoids support large herds of herbivore populations in the savannas of EastAfrica. In this way the social-economic condition and food security of the local humanpopulations is assured. In much of the arid and semi-arid savanna regions however,there has been increased grazing pressure on this basic resource, resulting indegradation and lowered productivity. Thus the ability of the grasslands to support anincreasing livestock and human population has been brought into question (Keya,1997). This is more so in the semi-arid pediment region of the Ndoto Mountains innorthern Kenya, where serious overgrazing led to ecological deterioration in the last 50years (Pratt, 1966; Lusigi and Glaser, 1984; Hornetz, 1994; Keya, unpublished). Thisovergrazing has converted Chloris roxburghiana and Cenchrus ciliaris L. perennial grass

Present address: c/o Dr B. Hornetz, Department of Geography and Geosciences, University of Trier, P.O.Box 3825, D-54286 Trier, Germany.

Journal of Arid Environments (1998) 38: 205–219

0140–1963/98/020205 + 15 $25.00/0/ae970336 © 1998 Academic Press Limited

Figures, tables, equations and references are all linked.

savanna to that dominated by tree/shrub Acacia spp. and the relatively unpalatabledwarf shrub Duosperma eremophilum (Milne-Redh.) Brummitt, thus reducing thegrazing potential of the land.

The tendency towards settled nomadism has also resulted in engagement in non-pastoral activities such as small-scale farming without any prior agricultural advicefrom relevant authorities in order to supplement their livelihood (Hornetz, 1993). Thisagropastoralism, a new development in this area, poses new challenges andopportunities of increasing land and animal productivity using methods such as fodderestablishment, bulking and conservation (Keya, unpublished). Likewise, new opportu-nities for erosion control methods present themselves. The need for reversingdegradation through reseeding is also urgent. This called for studies to evaluatesuitable native grass species to revive and sustain land productivity in this zone. Thenative grass species chosen in this study were Chloris roxburghiana and Cenchrusciliaris.

Chloris roxburghiana (Horsetail grass) is a C4 tufted, ascending perennial grassreaching upto 1·5 m in height. It has wide distribution in east and southern Africa,Sudan, Zaire as well as southern parts of India. It grows in open or wooded and bushedgrassland from 30–1500 m a.s.l. where annual rainfall does not fall below 500 mm inareas of bimodal distribution. The grass is quite leafy and offers good grazing value tolivestock. There are no reported studies on its growth and productivity characteristics.Except for the limited work of Ali (1984) and Hornetz et al. (1992), no studies haveaddressed the soil–plant water, biomass and drought adaptation of this species.

Cenchrus ciliaris (African foxtail or buffel grass) is a C4 tufted perennial grass widelydistributed through the hotter and drier parts of the tropical world. It has also beenintroduced in Central America, Puerto Rico and Australia, and has recently becomewidespread in Texas and along the Rio Grande Valley (Butt et al., 1992). It grows fromsea level to elevations of 2000 m a.s.l. (Heady & Heady, 1982) and can reach up to 1 min height. Cenchrus ciliaris is apomitic (Sands et al., 1970; Pratt & Gwynne, 1977).Polymorphism has been observed with regard to stature, seed colour, seed morphologyand seed production (Chakravarty & Das, 1965; Chakravarty & Kalkan, 1966). InKenya, this species is more common in areas receiving 630–760 mm annual rainfall(Sands et al., 1970). The forage value of this species lies in its persistence underdrought conditions, good nutritive value (crude protein upto 10·9% in growing period)and relatively high production (Mutz & Drawe, 1993). Studies on this species haveconcentrated on establishment characteristics with no reported work on its ecophysiol-ogy in Kenya.

The main objective of this study was to determine and compare the growth,phenology, transpiration, soil–plant water relations and production potential of bothspecies under defoliated and normal conditions. It was hypothesized that limiteddefoliation would be beneficial in terms of growth rates, and plant water relations(McNaughton, 1983; Jones, 1985; Hornetz et al., 1992). Due to its larger and longerleaves, C. roxburghiana was expected to exhibit higher transpiration rates andconsequently a higher above-ground standing crop biomass production than C. ciliaris.This investigation was done in the framework of an overall study to determine thesuitability of these pasture species to restore, improve and sustain land productivity inthe study area in particular, and other similar ecosystems in general.

Study site

This study was done at the Kenya Agricultural Research Institute (KARI)-Ngurunitfield station in Marsabit district, northern Kenya (1°44' N , 37°20' E), located on thefootslopes of the Ndoto Mountains ( ~ 900 m a.s.l.). The area has a bimodal rainfallregime (peaks in April/May (long rains) and October/November (short rains)); mean

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annual rainfall is ~ 650 mm. Soils are Eutric and Calcic Cambisols/Luvisols (Hornetzet al., 1992). Vegetation can be described as the moist phase of thorn savanna (Hornetzet al., 1992), which was originally a tall grass savanna consisting of Chloris roxburghianaand Cenchrus ciliaris, but is now dominated by thorn Acacia spp. and the relativelyunpalatable dwarf shrub Duosperma eremophilum in the understory. This changeresulted from settlement and overgrazing during the last five decades (Pratt, 1966;Lusigi & Glaser, 1984; Mackel & Walther, 1993; Hornetz 1994). It is inhabited by alargely Samburu and Rendille nomadic population.

Materials and methods

The study was conducted on mature grass stands which were established artifically byseeding in 1990. Plots (1–6) measuring 1 3 1 m2 on these stands were identified andmarked for experiments on growth, phenology and ecophysiology of C. roxburghianaand C. ciliaris plants. The experiments were carried out during the 1994 short rainyseason (21 October to 9 December 1994). Two plots (1 and 2) on the C. roxburghianastands were left unclipped (controls) while another two plots (3 and 4) were clipped.Due to a lack of enough suitable C. ciliaris stands, replication was not possible. Henceonly one clipped plot (plot 6) and control (plot 5) could be sampled. Clipping wasdone only once, at the beginning of the experiment (on 22 October 1994), following10 days of rainfall totalling 71·0 mm (soil moisture regimes are shown in Figs 3 and 6).Plants were clipped to 10 cm above-ground. Soil moisture was determined fromgypsum blocks (Eijkelkamp Agrisearch Equipment, Netherlands) positioned in plots at5, 15 and 40 cm soil depth in the centre of each plot. Based on the manufacturer’srecommendations, the blocks were completely soaked in water before burying (on 22October 1994) to improve contact and equilibration with the soil environment. Soilmoisture measurements were taken using the LF90 (WTW) electrical conductivityinstrument. Electrical conductivity measurements (mS) were then converted topercent of field capacity (%FC) based on the calibration curve of Hornetz et al. (1992)developed for the Cambic arenosol of the study site. Measurements were taken dailybetween 1600h and 1700h.

For growth measurements, two individual plants within the plots were marked withwooden pegs for subsequent identification. Two to three individual shoots on eachsample plant were then identified and tagged with coloured strings for subsequentgrowth monitoring. Growth was determined by clasping the shoot upright in the handand measuring its length from the base to the furthest point. Growth at a given intervalwas calculated as the difference between the current shoot length and its originallength. Measurements were initially recorded every 2 days (22 October to 20November 1994) but later (24 November to 9 December) at defined times dependingon whether marked differences in growth were obvious. Phenological observations,namely green-up, vigour and reproduction, were also made on a daily basis.

Leaf area index was measured using the LAI-2000 Plant Canopy Analyzer(LI-COR, Inc. 1992) once or twice within a 7–8-day period. The theory of theinstrument is based on the fact that the amount of vegetative canopy can be deducedfrom measurement of how quickly radiation is attenuated as it passes through thecanopy. The LAI-2000 measures the attenuation of diffuse sky radiation at five zenithangles simultaneously. From transmittances at all five zenith angles, the LAI-2000calculates automatically the foliage amount (LAI).

The measurement procedure (LI-COR, Inc. 1992) involved one above-canopyreading (A reading) followed by six below-canopy readings (B readings). Allmeasurements were made with the 45° view cap facing upwards. All the below-canopymeasurements were done at predetermined points as determined by the vegetationcover patterns within the respective plots. Only one sensor head was used for both A

TWO GRASSES IN KENYA 207

and B readings. Measurements were done in the late evening (sunset) to avoid effectsof direct sunlight. Cloudy intervals were similarly avoided. Other precautionarymeasures included taking measurements with the reader’s back to the sun and the viewcap blocking the sensor’s view of the reader and the sun. All B readings were taken asquickly as possible to minimize the time lag between them and the A readings. The leafarea index values were computed from the A and B readings and stored automaticallyby the LAI-2000 for later retrieval and analysis.

Diurnal course of transpiration was monitored at define times on selected plotsusing the Steady State Porometer (LI-COR). One actively growing plant wasrandomly selected from each plot for transpiration measurements. A single shoot fromeach of the plants was identified for sampling. Three actively growing leaves, one eachfrom the upper, middle and lower layers of each shoot, were then tagged forsubsequent monitoring of transpiration activity which was then taken as the meanvalue from the three sampled leaves. Measurements were taken at 0630–0700h,0900h, 1200h, 1500h and 1800h. The meteorological station (Hornetz et al., 1992)consisted of a wind anenometer for measurement of wind speed at 2 m height(measured three times daily at 0700, 1400 and 1900h), a sunshine recorder (afterCAMPELL-STOKES) for recording of daily duration of sunshine, a thermo-hygrograph for continous recording of relative humidity and temperature (measured at0700, 1400 and 2100h) and a standard raingauge for rainfall measurements. Potentialevapo-transpiration (ETo) was calculated based on McCulloch (1965). At the end ofthe study, plants were clipped to ground level for dry matter determination (air-dryweight) and plant water status.

Results

Meteorological parameters

The relationship between measured meteorological parameters are displayed in Fig. 1.The short rains were timely, adequate and well distributed. Total rainfall received was318 mm compared to 207·8 mm potential evapo-transpiration (ETo), leaving amoisture surplus of 110·4 mm. Mean daily rainfall, ETo and wind speed, respectively,were 6 ± 12·1 mm, 3·92 ± 0·9 mm and 36·7 ± 0·9 km. Similarly, mean daily

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temperature and relative humidity were 23·7 ± 1·7°C and 74·2 ± 10·7%,respectively.

Growth and phenology

Chloris roxburghiana

Regeneration and greening of clipped C. roxburghiana grasses occurred 2–4 days afterrains started. New sprouts arose from both the basal and nodal tillers. Growth ofclipped and non-clipped plants in relation to soil moisture patterns in the respectiveplots is illustrated in Figs 2 and 3. Contrary to expectations, soil moisture remainedgenerally higher at the 40 cm than at the 5 and 15 cm soil depth in these plots, whichwould indicate a rapid water infiltration rate in relation to uptake. Mean maximumshoot growth was, respectively, 84·5–116·7 cm and 77·4–79·1 cm for clipped and non-clipped grasses (Table 1). Calculated over a 49-day observation period, thisrepresented a daily growth rate of 1·724–2·382 cm and 1·579–1·614 cm, respectively.Thus, it is clear that clipped grasses grew faster than the non-clipped controls.Generally, basal tillers grew faster than internodal tillers.

The rapid growth of clipped compared to the non-clipped grasses is furtherillustrated by the results of the leaf area index (LAI) measurements (Fig. 4). Althoughnon-clipped plants maintained a higher LAI, the difference narrowed from 92·1% atthe beginning to 9·3% at the end of the study. This narrowing was achieved by therapid increase in LAI on clipped plots (0·158) compared to the non-clipped controls(0·0985). On average, rate of LAI development on clipped plots was 37·7% faster thanthe controls. The slight drop in LAI during a 5-day spell (25–11 November 1994)perhaps reflects parahelionasty (leaf drooping) in response to moisture stress in thisperiod.

Defoliated plants produced ears and flowers 16–26 and 20–29 days post-clipping,respectively, while the controls achieved this 6–10 and 10–12 days after the beginningof rains. By the end of the study period 58–63% and ~ 100% of clipped and control

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Figure 2. Soil moisture at 5 cm ( + ) and 40 cm (h) depth, growth of shoot 1 ( ) and shoot 2(h) and the phenophases of unclipped Chloris roxburghiana during the 1994 short rains atNgurunit, N Kenya.


plants, respectively, were in various stages of reproductive maturity. Clipped plantsproduced 28% less reproductive elements than the controls. Thus, clipping delayedand reduced reproduction of C. roxburghiana.

Cenchrus ciliaris

Growth and phenology of C. ciliaris on plots 5 and 6 is shown in Figs 5 and 6.Measured maximum shoot growth was 37 cm and 50 cm for clipped and non-clippedgrasses, respectively (Table 1), representing a daily rate of 0·5122 cm and 0·476 cm.LAI development on these plots (Fig. 7) showed a narrowing of the difference from60·8% at the beginning to 10·9% at the end of the study period. Mean daily increasein LAI was 0·098 and 0·082 on clipped and control plots, respectively. LAIdevelopment on clipped plots was therefore 16% faster than on controls. This is inconformity with the 7% faster growth rate achieved by clipped plants over thecontrols.

Transpiration activity

The transpiration pattern of C. roxburghiana is shown in Figs 8 and 9. Mean maximumtranspiration rate was, respectively, 5·184–6·733 µg cm–2 s–1 and 3·073–6·964 µg cm–2

s–1 on non-clipped and clipped plants. Mean maximum transpiration rate occurred inthe period of high soil moisture content (69·1%FC) in plot 1 (Fig. 1; Table 2).However, when the moisture content dropped to 57·3%FC (ETo, 4·34 mm),transpiration rate declined. The situation on plot 3 was such that maximum rate wasachieved when the ETo was highest despite a moisture content of 46·9%FC and50·3%FC at 5 cm soil depth. The effect of low moisture content in the upper soilhorizons was perhaps mitigated by a favourable moisture content (61·5–71·5%FC) atthe lower depth, so that a high transpiration activity was still maintained. Mean dailytranspiration rate was 2·617 µg cm–2 s–1 and 2·540 µg cm–2 s–1 on plots 1 and 3,respectively, indicating that clipped plants transpired less than those that were notclipped.

The diurnal course of transpiration for Cenchrus ciliaris in relation to soil moistureand ETo is shown in Figs 10 and 11, respectively. Maximum transpiration rates wereachieved between 1200h and 1500h, while minimum rates occurred during earlymorning and late afternoon. Mean maximum transpiration rate was in the range5·929–11·540 µg cm–2 s–1 and 6·106–9·582 µg cm–2 s–1 on plot 5 and 6, respectively.

Table 1. Growth parameters of mature Chloris roxburghiana and Cenchrusciliaris during the 1994 short rains at Ngurunit, northern Kenya

Shoot growth (cm)

Grass species Plot Maximum Minimum Mean ± SE Daily mean ± SE

C. roxburghiana 1 (N = 4) 127·9 22 77·4 ± 21·8 1·579 ± 0·4152 (N = 4) 105·5 70·3 79·4 ± 11·4 1·614 ± 0·2323 (N = 6) 152·4 72·0 116·7 ± 13·4 2·382 ± 0·2744 (N = 5) 123·9 53·8 84·5 ± 11·5 1·724 ± 0·234

C. ciliaris 5 (N = 5) 50·0 12·5 33·1 ± 7·2 0·676 ± 0·1466 (N = 2) 37·0 13·0 25·1 ± 12·0 0·512 ± 0·244

N = sample size.Measurements taken over a 49-day period.

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Mean daily transpiration rate was 4·166 and 4·011 µg cm–2 s–1, respectively (Table 2).Thus, clipped plants tended to transpire less than those that were not clipped.Although daily transpiration activity increased with increasing ETo and decreasing soilmoisture content on plot 6, a similar trend on plot 5 was not seen. Leaf temperaturewas always 0·1–0·2°C below ambient temperature except on 1, 3 and 31 November1994.

Dry matter production (above-ground standing crop) and plant moisture status

Table 3 shows the dry matter yield (above-ground standing crop) and plant moisturecontent of the respective forages at the end of the study period. Clipping reducedresidual above-ground standing crop on mature stands of C. roxburghiana and C.ciliaris by about 58% and 62%, respectively. Residual standing crop mass per plant waslikewise reduced by 73% and 52·5%, respectively, for both species. The former speciesout-yielded the latter by 38·1% and 44·4% on control and clipped plots, respectively.Similarly, individual C. roxburghiana plants out-yielded those of C. ciliaris by 28·9 and23·0%. Chloris roxburghiana was therefore, by far the more productive grass species.Although defoliation improved the water status of Chloris roxburghiana, it had noapparent effect on water status of Cenchrus ciliaris.

Discussion

Growth pattern of mature grasses showed that C. roxburghiana was superior to C.ciliaris with regard to growth rate, LAI and absolute growth achieved. The explanationfor this could be a higher water use efficiency (extraction and spending) and a moreefficient energy capture mechanism (large leaves and high LAI, etc.) by C.roxburghiana. In a different study, however, (Keya, unpublished), C. ciliaris wassuperior to C. roxburghiana during the seedling establishment stage with regard togrowth rate, leaf production and absolute growth. This indicates that advantagesgained in the early seedling stage are not necessarily carried over to the maturestage.

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Figure 4. Rainfall (j) and leaf area index (LAI) development in plot 1 (unclipped; + ), plot 2(unclipped, *), plot 3 (clipped, h) and plot 4 (clipped, 3 ) on Chloris roxburghiana stands duringthe 1994 short rains at Ngurunit, N Kenya. Each point on the LAI curve represents the meanof three values.

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With regard to transpiration activity of C. roxburghiana, results of this study tend toagree with Hornetz et al. (1992), who in a limited field study measured maximumtranspiration rates of 2·5–4·0 µg cm–2 s–1 at 1500h for the same species. On the otherhand, maximum transpiration rates of 11·69–18.48 µg cm–2 s–1 (at 1300–1500h) for C.roxburghiana were reported by Ali (1984). Higher transpiration rates (20–25 µg cm–2

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(a) ETo = 3.64 mm (c) ETo = 4.34 mm(b) ETo = 3.83 mm

Plot 1

Figure 8. Transpiration (symbols) and diffusive resistance (bars) of non-clipped Chlorisroxburghiana during the 1994 short rains season at Ngurunit, N Kenya; soil moisture at5/15/40 cm soil depth. ETo = potential evapo-transpiration. Transpiration and diffusiveresistance are the mean of three values.


s–1) were reported for other savanna C4 graminoids in SE Kenya (Maranga et al.,1985). Differences in transpiration rates between species may reflect environmentaladaptations, with the most conservative being those that are adapted to more water-limited environments. In the present study, C. roxburghiana displayed the mostconservative transpiration rates compared to Cenchrus ciliaris indicating perhaps betterresistance to drought during the mature phase. During the seedling phase however(Keya, unpublished), C. ciliaris showed a greater adaptation to drought. Underoptimum soil hydrature, the main driving force of transpirative activity was theatmospheric evaporative demand (ETo). Increases in transpiration rates generallycorresponded to decreases in diffusive resistance and vice versa.

Clipping reduced the mean diurnal transpiration rate of Chloris roxburghiana andCenchrus ciliaris by 3·7% and 6·5%, respectively. Although a reduction of transpirationactivity was reflected in a higher plant water content of clipped C. roxburghiana, thesame was not the case with C. ciliaris. This indicates that plant water status is not only

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Plot 37

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Figure 9. Transpiration (symbols) and diffusive resistance (bars) of clipped Chloris roxburghianaduring the 1994 short rains season at Ngurunit, N Kenya; soil moisture at 5/15/40 cm soil depth.ETo = potential evapo-transpiration. Transpiration and diffusive resistance are the mean ofthree values.

Table 2. Transpiration rates of C. ciliaris, and C. roxburghiana, under given conditions of soil hydrature (at 5 cm soil depth) and potential evapo-

transpiration (ETo)

Soil Transpiration (µg cm–2s–1) ETo (mm day–1)moisture

Species Plot (%FC) Range Mean Range Mean

C. roxburghiana 1 57·3–69·1 2·530–2·929 2·617 3·64–4·34 3·943 46·9–63·7 2·006–3·080 2·540 3·64–4·34 3·95

C. ciliaris 5 52·0–54·0 2·430–6·225 4·166 3·64–4·48 4·116 38·9–62·0 3·468–4·096 4·011 3·64–4·48 4·11

G. A. KEYA216

42.5/37.7/39.6%FC 41.7/34.4/40.3%FC

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52.0/61.9/64.0%FC 54.0/68.3/48.1%FC


Plot 5(d)

Figure 10. Transpiration (symbols) and diffusive resistance (bars) of non-clipped Cenchrusciliaris during the 1994 short rains season at Ngurunit, N Kenya; soil moisture at 5/15/40 cm soildepth. ETo = potential evapo-transpiration. Transpiration and diffusive resistance are the meanof three values.

46.8/40.1/45.4%FC 38.9/39.6/45.7%FC

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62.0/49.4/59.7%FC 58.5/40.8/51.1%FC


Plot 6

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>100

Figure 11. Transpiration (symbols) and diffusive resistance (bars) of non-clipped Cenchrusciliaris during the 1994 short rains season at Ngurunit, N Kenya; soil moisture at 5/15/40 cm soildepth. ETo = potential evapo-transpiration. Transpiration and diffusive resistance are the meanof three values.


a function of transpiration activity and water use efficiency (McNaughton, 1979) butmay also depend on soil water extraction capacity. Soil water extraction capacity inturn depends on root density. I submit that the denser, more fibrous and deeper rootsystem (pers. obs.) of C. roxburghiana compared to that of C. ciliaris, coupled with lowtranspiration rates, resulted in higher water content of clipped Chloris roxburghianaplants. Since enhanced plant water status can prolong the growing period into the dryseason (McNaughton, 1983), then it appears that some degree of defoliation isbeneficial to C. roxburghiana at least in the short-term. However, since defoliation didimpact negatively on the reproductive capacity of this species, long-term detrimentaleffects could result from heavy and frequent grazing.

The differences in above-ground standing crop yield displayed by mature grassstands was perhaps due a higher photosynthetic capacity of Chloris roxburghianacompared to Cenchrus ciliaris. The former species achieved a higher LAI than the latter,a factor which may have enabled it to fix greater quantities of dry matter. The yield ofmature grasses in this study was in the range of 2·86–12 t ha–1 reported for C. ciliariswithout fertilization (Peake et al., 1990) in Queensland, Australia (562–921 mmannual rainfall). The above-ground standing crop production potential achieved in thisstudy should be of great interest to those interested in implementing agropastoral landuse systems and supplementary feeding for livestock, especially during the dry season.There are also implications for erosion control measures. Further on-farm evaluationof these pasture species is however still needed.

This study was carried out within the framework of doctoral studies at the Department ofGeography and Geosciences, University of Trier, Germany. Financial support was provided bythe German Academic Exchange Service (DAAD). Logistical field support was provided by theKenya Agricultural Research Institute (KARI). Yusuf Aila greatly assisted in data collection.

References

Ali, A.R. (1984). Water relations strategies of two grass and shrub species as influenced byprescribed burning in a semiarid ecosystem in Kenya. MSc. thesis, College Station, Texas A& M University. 122 pp.

Butt, M.N., Donart, G.B., Southward, M.G., Pieper, R. & Mohamma, D.N. (1992). Effects ofdefoliation on plant growth of buffel grass (Cenchrus ciliaris). Annals of Arid Zones, 31:19–24.

Chakravarty, A.K. & Das, R.B. (1965). Polymorphism in Cenchrus ciliaris. Annals of Arid Zone,4: 10–16.

Chakravarty, A. & Kalkan, L. (1966). Study on variation in seed yielding components ofCenchrus ciliaris. Annals of Arid Zone, 5: 63–71.

Table 3. Dry matter yield and plant moisture status of C. roxburghiana andC. ciliaris after Ngurunit, 1994 short rains

Dry matter yield

MoisturePlot Species kg DM ha–1 kg DM plant–1 content (%)

1 (unclipped) Chloris (N = 16) 10,118 63·2 ´ 10–3 69·42 (unclipped) Chloris (N = 20) 11,176 60·9 ´ 10–3 60·13 (clipped) Chloris (N = 17) 5444 32·0 ´ 10–3 73·64 (clipped) Chloris (N = 16) 3566 23·3 ´ 10–3 73·05 (unclipped) Cenchrus (N = 15) 6594 44·0 ´ 10–3 77·76 (clipped) Cenchrus (N = 12) 2506 20·9 ´ 10–3 76·1

N = sample size, i.e. number of plants per plot.

G. A. KEYA218

Heady, H.F. & Heady, E.B. (1982). Range and Wildlife Management in the Tropics. London andNew York: Longmans. 140 pp.

Hornetz, B. (1993). On the development and acceptance of agropastoral (agrosilvipastoral)systems in the semi-arid areas of northern Kenya. In: Baum, E., Wolff, P. & Zobisch, M.A.(Eds), Topics in Applied Resource Management in the Tropics, Vol 3: Acceptance of soil and waterconservation, pp. 413–453. Witzenhausen: Cuvillier Verlag. 453 pp.

Hornetz, B. (1994). Agro (Silvi)pastoralesystem in den Semiariden Tropen-Moglichkeiten zurRessourcenschonenden Stabilisierung der Nahrungsmittelproduction an der AgronomischenTrockengrenze (mit Beispielen aus N- und SE-Kenya). Geomethodica, 19: 83–119.

Hornetz, B., Jatzold, R., Litschko, T. & Opp, D. (1992). Beziehungen zwischen Klima,Weideverhaltnissen und Anbaumoglichkeeiten in Marginalen Semiariden und Ariden Tropen mitBeispilen aus Nord- und Ost-Kenya. Materialien zur Ostafrika Forschung. Heft 9. Trier:Geographischen Gesesellschaft Trier. 257 pp.

Jones, C.A. (1985). C4 Grasses and Cereals: growth, development and stress response. New York:John Wiley & Sons. 419 pp.

Keya, G.A. (1997). Effects of herbivory on the production ecology of the perennial grassLeptothrium senegalense in the arid lands of northern Kenya. Agriculture, Ecosystems &Environment, 66: 101–111.

Lusigi, W.J. & Glaser, G. (1984). Desertification and nomadism: a pilot approach in easternAfrica. Nature and Resources, 20: 21–31.

Mackel, R. & Walther, D. (1993). Naturpotential und Landdegradierung in den TrockengebietenKenias. Stuttgart: Franz Steiner Verlag. 309 pp.

Maranga, E.K., Trilca, M.J. & Smeins, F. (1985). Water relations of Panicum maximum andDigitaria macroblephara on a semi-arid rangeland in Kenya. East African Agricultural andForestry Journal, 48: 74–80.

McCulloch, J.S.G. (1965). Tables for the rapid computation of the Penman estimate ofevaporation. East African Agricultural and Forestry Journal, 30: 286–295.

McNaughton, S.J. (1979). Grazing as an optimization process: grass–ungulate relationships inthe Serengeti. American Naturalist, 113: 691–703.

McNaughton, S.J. (1983). Compensatory plant growth as a response to herbivory. Oikos, 40:329–336.

Mutz, J.L. & Drawe, L.D. (1983). Clipping frequency and fertilization influence herbage yieldsand crude protein content of 4 grasses in South Texas. Journal of Range Management, 36:582–585.

Peake, D.C., Myers, R.J. & Henzell, E.F. (1990). Sown pasture production in relation tofertlizer and rainfall in southern Queensland Australia. Tropical Grassland, 24: 291–298.

Pratt, D. (1966). Bush control studies in drier areas of Kenya. III. Control of Disperma in semi-arid desert dwarfshrub grassland. Journal of Applied Ecology, 3: 277–291.

Pratt, D.J. & Gwynne, M.D. (1977). Rangeland Management and Ecology in East Africa. London:Hodder & Stoughton. 310 pp.

Sands, E.B., Thomas, D.B., Knight, J. & Pratt, D.J. (1970). Preliminary selection of pastureplants for the semi-arid areas of Kenya. East Africa Agricultural and Forestry Journal, 36:49–57.


growth, water relations and biomass production of the savanna grasseschloris roxburghianaandcenchrus...

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