dnnals oj Arid Zone-I6
(3), 349-359, 1977
Impact of overgrazing on the grasslands
K. A.
Central
SHANKARN1\RA
Arid Zone Research
Y AN
Institute,
Jodhpur.
ABSTRACT
Overgrazing in grasslands results in slow but inexorable deleterious
change in botanical composition which however varies with the type of grass
t:over, its palatability, forage productivity and the way in which it is utilised
and the sequence of climatic events'. It triggers off succession and invariably
the degenerated stagl 'Consists of inferior less palatable perennials and annuals
such as Oroperium lhomeaum, Arislida adseensionis and Eragrosris unioloides
accompanied by reduction in the per cent cover of the desirable species.
The physiognomy and growth of tbe desirable species are also affected
from "basket" to "saucer" shape which are strongly influenced by grazing and
soil moisture relations.
The impact of browsing is nowhere more pronounced
than in top feed trees such as Zizyphns nummularia, Salvadora oleoides which
assume "bush form" whereas Prosopis cineraria and Anogeissus pendula
become "pillow cushion" form.
The dynamics of herbage production
is influenced by the severity of
grazing which is attributed
to the destruction of shoot apex resulting in
lowered growth rate as evidenced in Isei/ema laxum, C. eiliaris, C. setiger/ts
and Sehima nervosum grasslands.
Concurrently
the root biomass and root
number also get decreased with overgrazing.
Heavy intensity of grazing on Lasiurus-Aristida rangeland showed lowest
weight gains in Tharparker yearling heifers both in low and medium rainfall
regime
The trampling by hoov~s of stock appeat more damaging leading to
disintegration
of humus and organic matter and altering the mechanical
composition
of the soils. The hydrology is drastically impaired with high
bulk density, lower infiltration and increased runoff. Overgra;zing in grassland gave maximum runoff. (27%) and maximum soil loss (2100 lb/acre).
Likewise in land use having overgrazed fallow the runoff was 22% but soil
loss was as high as 2029 lb/acre.
INTRODUCTION
In India five distinct grass covers have been recognised namely (I) Sehima
iAchanthium cover
(2) Dichanthium-Cenchrus-Lasiurus
cover
(3) Phragmites-
350 : K. A. SHANKARNARA Y AN
Saccharum-Imperato cover (4) Themeda-Arundinella cover and (5) Temperate-Alpine
cover (DabaBghao and Shankarnarayan,
1973). The response to the impact
of overgrazingvaries
with different grass covers.
Normally the various grass covers remain as a stable preclimax in equilibrium with the environment when grazed to carrying oapacity.
During normal
years of rainfall the carrying capacity of different condition class of rangelands
in Rajasthan on year long basis per 100 ha has been worked out by Ahuja
(1975) as follows:
Condition of class of
rangelands
No. of adult cattle units (300 kg
body wt.) per 100 ha
Excellent
Good
Fair
Poor
Very poor
25-30
20
17
13
0-6
In this paper an account is described
Adult Cattle Units
ACU
ACU
ACU
ACU
on the impact of overgrazing on the
grasslands both direct and indirect.
,
Effect of overgrazing on succession :
Grazing tends to damage the most palatable plants and then progressively
less palatable species so reducing their frequency in the community.
~hese are
the decreasers of Dyksterhuis (1946). This is based not only on the palatability
of species but also on the stage of the species. In his studies on the relative
palatability of six desert grasses Dabadghao and Marwaha (1962) reported that
Dichanthium annulalum arid C. ciliaris showed consistently high degree of palatability at all stages of growth.
~owever, the highest palatability shown by C.
ciliaris during preflowering and more particularly during ripe and dead ripe
stage compared to Dichanthium makes it most desirable species amongst grass
species. Viewed in this light, the lower palatability shown by C. setigerus
and L. sindicus upto flowering stage compared to Dichanthium- i~ more than
compensated by the more or less equal palatability during the ripe stage and
much higher value in the dead ripe stage.
Th~se data incidentally throw light on the process of deterioration of
gr-assland by grazing. The main cause of the deterioration of the grassland
is the selective grazing habits of the animals due to which the most preferred
IMPACT
OF OVERGRAZING
ON GRASSLANDS
C
351
species are always hit hard. The highest palatability shown by Dichanthium
in early stages indicates that this grass will be sought out and grazed in early
siage in preference to others. Grazing year after year at this vulnerable stage
of growth would eliminate this species first out of the grassland community
under the impact of grazing.
Similarly in a grassland dominated by mixed
stand of C. ciliaris and L. sindicus, the former would be the first to go out. This
applies to other grass covers.
This process of selective grazing leads to different trends in succession.
Depending upon the intensity of grazing, whether severe, medium or protected,
Gupta and Saxena (1972) described the succession in grasslands of Cenchrus
ciliaris-C. setigerus and Lflsiurus sindicus (Figs. 1 and 2).
CENCHRUS SETIGERUS-C.CILIARIS
J
MG
ELEUSINE
CYNODON
tp
COMPRESSA
DACTYLON
! tp .
MG
OACTYLOCTENIUM
SINDICUM
UROCHLOA PANICOIDES
ERAGROSTIS SPECIES
OIGITARIA ADSCENDENS
! t
MG
P
INDIGOFERA CORDI FOLIA
CENCHRUS PRIEURII
C. BIFLORUS
TRAGUS BIFLORUS
tp
SG!
ARISTIDA FUNICULATA
A. ADSCENSIONIS
SG
4
tp
BARE SOIL
FIG. I. Successional trend in Cenchrus set'igerus-C.
ciliaris grassland on old alluvium with loamy
sand to sandy loam soils.
3~2:
K. A. SHANKARNARAYAN
LASlURUS SlNDlCUS
1 t
MG'
P
CYMBOPOGON
SP.
CENCHRUS -t SP.
MG
IP
ELEUSINE COMPRESSA
OACTYLOCTENIUM SINDlCUM
!
! t
SG
P
CENCHRUS BIFLORUS
LATIPES SENEGALENSIS
!I
SG
P
INDIGOFERA L1N1FOLlA
I. CORDIFOLIA
ERAGROSTIS SP.
ARISTIDA ADSCENSIONIS
! 1
SG
P
ARISTIDA MUTABILIS
A. FUNICULATA
MELANOCENCHRIS JACQUEMONTU
OROPETIUM THOMEAUM
SG
! t
P
BARE SOIL
F lG. 2. Successional trend in Lasiurus grasslands
on hummocky undulating terrain with
loose sandy soils.
Impact of overgrazing on plant cover
In its optimum state Sehima nervo,mm recorded a plant cover of 11.5%
at Palri (Pali district) but was reduced to 0.1 to 0.2% due to the impact of
grazing (Gupta & Saxena, 1972).
Likewise Dichanthium annulatum type in optimum expression in sandy
clay loam soil in Harji, Ahor block recorded a basal cover of 24%. But due to
unrestricted grazing and high palatability,
it was reduced to 5%.
IMPACT OF OVERGRAZING ON GRASSLANDS:
353
Cenehrus eiliaris and C. setigerus type predominate on sandy to sandy loam
soils and maximum expression of this type was recorded by Gupta and Saxena
(1972) in Sojat in an exclosure having a basal cover of 19.8 %. 'This was reduced
to a mere 0.5 % under high intensity of grazing. Not only that, undesirable
grasses such as Aristida funieulata,
A. adseensi(mis, Tragus biflorus, Oropetium
thomaeum, Eragrostis eiliaris, E. pilosa, Digitaria adseendetts, Eleusine eompressa,
Dactyloetenium sindicum and Urocholoflpanicoides recorded increase in total plant
cover.
The combined effect of anthropogenic factors, grazing intensity, rainfall and topography on succession in Madhya Pradesh grasslands are presented
in Figure 3 (Pandeya, 1962) which is self-explanatory.
CLlMJ~TIC CLIMAX FORESTS
Anthropogtnic fectors
(Grosslond$-S~riolronks)
~
GRASSY CLIMAX
Imperato I Arundlnello
Anthropogebic foctors
t
rRA1NFALL
ZONES -----LOW RAJNFALL TYPES
(Dichclllthium BothriochJoa/Cenchrus).
I
--..
MEDIUMRAINFALL,SUBSEQ\.ENT DRY
'i
r SOIL VARAINTS-,
INTENSEl)'
ANNUAL PROTECT1ON
.
GRAZED
(LOWLANDS)
(Coarse and thin)
t
I
f
Melonocenchrusl
MORE LIME
MORE
Microc1lloo/Urochloo
' 1JI0lSTURE
( i) Eremopogon/Bothrioch!oo/Heteropogon
Ischoemum.f
LOWLAND- Cynodon/Bofhriochloal
(ii)Chrysopogon/Sehima
IseUema .
Dichanthium
(m) Cymbopogon mortinii
(iv) Ophiuros/lseilema
Fig.3 Succession in gross covers in MADHYA PRADESH (PANDEYA.1962)
TOPOGR~PHIC TYPES
ANNUAL PROTECTION
(LONLANDS)
UPLAiNDS-Andropogon.IHeteropogon
Tbemedo
quodrivolvis
SlOPI~S - Heteropogon/Bothriochloa
J
iNTEN~iELY GRAZED
Studying the effects of overgrazing on botanical composition of ThemedaPJ:eudo.ntlzistiriaglassland, Bharucha and Shankarnarayan (1958) recorded the
replacement of dominant grasses by Eulalia fimbriata. As the intensity of pasturing
in'creased the latter is succeded by Erogrllstis unioloides.
Simultaneous with the regression the soil is washed off exposing rocky
aDd stony ground fit only for the establishment of useless weeds such as BltllnU
eriantha, Bon1U1yabradziata and &"ena strieta.
In..iicators of overgrwn:
The following species are avoided by cattle and therefore, indicate
cx':essive grazing and browsing when they form the chief species in a stand.
Butea tnIIflIJsjJerma, HDlarTlwu1. an.tidysentrica, Cauia tl!l.TiculaJa. Capparis daidu4.
354 : K. S. SHANKARNARAYAN
Zizyphus
mauritiana, Calotropis procera and Adhatoda
vasica particularly
in Aravallis
between Jaipur and Chomu.
Effect on growth form
Development of vegetation depends upon the intensity of grazing. If the
intensity of grazing is reduced, the dominant grasses grow erect and acquire
'basket' shape covering the ground extensively. But with continued heavy
grazing they grow in "saucer" shape.
Thus Dichanthium allllulatum, D. caricosum, Bothriochloa pertusa and to some
extent /seilema anthephl}.roides which grow in 'basket' form in non-grazed areas
acquire "saucer" form in grazed places and grow prostrate giving out branches
trailing over the ground and rooting at the nodes.
Pandeya (1964) observed that it is the intensity of grazing which is the
decisive factor in shaping the physiognolny and structure of the grass communities. Bor (1941) further explains that grasses have developed in response to the
stimuli of the unfavourable habitat. This fact is only evident in well grazed area
..................
where tall luxuriant grasses of savannah get replaced by grasses
forming short turf which provides poor pasture. Thus the physiognomy and
phenology of dominant species are strongly influenced by grazing and soil
moisture relations.
The impact of overgrazing is more pronounced on the trees and shrubs
browsed upon by animals. Saxena (1975) observed that browsing combined with
lopping and other biota effects on common top feed species e.g. Zizyphus mauritiana, Salvadora oleaides, Capparis decidua, Prosopis cineraria and Anogeissus pendula
at early stage of growth year after year, assume bush form in the case of
Zizyphus nummuLaria and Salvadora oleoides whereas Prosopis cineraria, Anogeissur
penduLa and Acacia senegal assume pillow cushion form. These are discernible in
waste lands around the villages but in the Oran or biTS these can be seen in
their pristine tree form. For example in the Oran of the village Nimbri Kalan
massive sized trees of Prosopis cineraria (diameter at breast height 2.87 m) and
Anogeissus rotundifolia (d.b.h. 3.57 m) have been recorded recently by the author.
Effect on he~bage production in grasslands
Agronomic studies were undertaken in l<eilema grasslands to study the
l'esponse of various cuttings (simulating grazing) intensities on dry matter yield
and consisted of three intensities of cutting namely 5 em height (severe grazing),
10' em height (moderate grazing) and 15 em height (light grazing) (Shankaruarayanet
at., 1976). R.esults in Table 1 showed that dry matter yield
IMPACT
OF OVERGRAZING
ON GRASSLANDS
:
355
generally increased with decrease in grazing intensity and 15 cm cutting
height (light grazing) gave the highest dry matter yield.
This could be
attributed
to the highest growth rate obtained at 10 and 15 cm cutting
height as judged by the efficiency index value of Blackman (1919). This is
bt'cause under 10 and 15 cm cutting height i.e. moderate and light grazing,
ther ~ is a favourable shoot apex development which otherwise gets damaged
at shorter cutting heights (severe grazing). It is also pertinent to mention that
plants subjected to 5 cm cutting height (severe grazing) suffered mortality to
the extent of 68.7% in the experimental plots. Such studies carried out in
Sehima nervosum, C.ciliaris and C.setigerus also showed increase in dry matter
yield with increase in cutting
height or decrease in grazing intensity
(Shankarnarayan ei al., 1977).
In his studies on the effect of biota on the desert, Saxena (1975) described the plant density basal cover and herbage yield in protected and overgrazed site. He has desribed that in protected site the grass community has "high
perennials"
with good cover and high dry matter yield whereas in the open
overgrazed sites the species represent the "low perennials" in most regressive
stage of succession and with relatively low dry matter production.
Ellect on Root biomass
Under 5 cm cutting height (severe grazing) the level of root biomass
production in Iseilema laxum was lowest (415 kgjha) at no nitrogen level which
increased to 715 and 1064 kgjha when 20 and 40 kg Njha were applied respectiv~ly. Under 10 cm cutting height (moderate grazing), application of 20 kg
Njba increased the root biomass to 885 kgjha but increasing the level of nitrogen
to 40 kg Njha had no material advantage.
15 cm cutting height (light grazing)
gave highest root matter (756 kgjha) under no nitrogen. Increasing the nitrogen
level to 20 or 40 kgJha gave little or no effect.
Thus 5 cm cutting height derived maximum benefit by nitrogen
application and this acquires great significance when we consider it to be'severe
grazing intensity.
It implies that Iseilema laxum grassland subject to severe
grazing stress has the resilience to regrowth with nitrogen application
and
prete .tion.
Studies on effect of clipping (grazing) at 15 and 30 days interval on five
gra:;ses, showed that grazing at 15 days (closer) interval has adverse effect on
rool: number and forage yield in all grasses compared with those of 30 days
(Shankarnarayan,
1977).
356:
K. A. SHANKARNAR.A YAN
--
-l1
--
«'l
0\
r--
~
0'1
on
0'1
l"o
~
00
$
-
.".
~
\0
-
..•.
."...• .".....•
0
~
u
..
~
~
..
'>,
8
,-..
<II
'"
V'\
d
\0
N
-H
~
~
.c
co
.....,
.!oil
>0
Q
~
~
CI\
.a
\;S
N
00
<OS
on
- --
'"
-H
~
N
--
-+t
~
....
~
'"
0
E
~
.~
.::::
...,s::
"Cl
00
\0
U
Q\
'>'
$.0
U
.•..
.•..(II
E
-
-
N
N
\0
~
"Cl
c:;
0
....
.d
bO
'4)
.d
bO
.5
....
....:l
....••
till
U
••••0
u
co
<II
Q)
co
....
.c
'"
ii
...
:c
§
&-4
Eo<
(II
'N
<II
~
~
...:
.. ,-..
.. ~.. ..
.....•.. :c
'N
c
.•..U
C
-;;
..
as
Q)
Q)
Q)
III
c
'N
:i
V'\
r.Q
r1.i
Q
"0
0
rI:l
:e....,
.!!l
CII
.g
8
u
8
u
8
u
'5
on
.5!!l
...
.c
U
u
"-
0
#.
<II
CII
0
...•
on
~
u
IMPACT
OF OVERGRAZING
ON GRASSLANDS
357
Effect of grazing on Animal production
Studies on the growth of yearling heifers were conducted by Ahuja
(1969) under different intensities of grazing stress of rangeland viz. (a) light
intensity Tl' (b) heavy intensity T2' (c) medium intensity T3 and (d) heavy
intensity plus concentrate feeding (from 16th December to 30th June) T4 and
(e) medium intensity plus concentrate feeding (16th December to 30th June) T 5'
Results showed that (a) Growth of yearling heifers (Tharparkar) on LasiurusAristida pasture with 240 mm rainfall was 70.2,30.9, 46.6, 94.8 and 121.8 kg
under above treatments respectively; pasture (with annual rainfall 526 mm)
was 74.0, 63.4, 70.4, 117.7 and 127.3 kg under above respective treatments.
Thus amongst the treatments wherein no concentrates were given, continuous
grazing had proved to be better for livestock production.
Growth rate was
higher by 70% under T5 over T1 treatments (Ahuja 1969). Cost perkg increase
in body weight of heifers under T 10 T 2. T 3' T 4' and T 5 in Lasiurus sindicus
dry land ranges worked out to Rs. 1.71,5.05,2.91,3.12
and 2.24 respectively.
Investigations on the improved grazing systems showed that on well
developed grassland, deferred rotational grazing had a distinct advantage in
the carrying capacity over continuous grazing. Thus whereas it was 2925 animal
grazing days in 'the former the latter would provide only 2097 animal days.
However the body weight gains of animals were essentially similar in both.
recording an increase of abou t 191 g{heifer{day (Shankarnarayan,
1974).
Effect oj trampling by overgrazing in grasslands
Trampling by hooves of animals are far more damaging than mouths
especially when stocks are concentrated around watering points.
The first ca'lualty in trampling is, as we have seen earlier, vegetation and
some species which are more sensitive than others, quickly disappear to leave
a sparse cover of hardier and usually less palatable plants. The proportion of
bare ground increase rapidly.
Hooves trample, pound and disaggregate the
soil humus so that it can be flown away or distributed
organic matter and
by the animals them-
selves.
Bharucha and Shankarnarayan
(l958) reported on the permclous
effects of overgrazing on both the physical and chemical factors of the soils of
grasslands in western India.
From their studies it is evident that mechanical
composition of the soil is greatly altered by overgrazing and trampling.
The
clay content, which forms the bulk of the colloidal complex of the soil and
from which nutrients are available to plants, have relatively high values in the
358 : K. A. SHANKARNARA Y AN
"cut and grazed" regions (in which the grasses of the pastures are cut at the
end of the monsoon and the cattle allowed to graze on it for one month at its
beginning and one month after cutting ranging from 50 to 69% whereas in the
overgrazed regions (in which the grasses of the pastures are subjected to intense
grazing throughout
the year) it is considerably lower, ranging from 8 to 36%.
Conversely, the soils of the overgrazed regions are sandy. This is due to the
deletion of vegetation by overgrazing resulting in bare area, which exposed to
rainfall bring about serious erosion of the clay portion.
Simultaneously with change in the mechanical composition of the soil,
the moisture content also shows variation. The moistu':e content which is on the
average 7.15% in the "cut and grazed" region of Telegaon registers a decrease
to an average of 3.30% in the overgrazed region.
The base exchange capacity which ranges from 47.3 to 53.0 m.e % in the
';cut and grazed" soils of Telegaon, show an alarming decrease in the "overgrazed" soils where the values range from 28.7 to 31.0 m.e %. Likewise, the
total exchangeable basis and the exchangeable calcium become consistently
low in the "overgrazed"
soils as compared to ,he "cut and grazed" soils in all
the three places.
Effect on Hydrology
Severe trampling and heavy grazing by damaging soil surface have
drastic impact on water balance. Higher bulk density means less infiltration
which in turn deplete~ soil moisture storage. Less infiltration means increase in
run-off.
The runoff per Cl"ntand soil loss under different systems of land use was
studied by Chatterji and ~Jaiti (1974). Results showed that the runoff per cent
varies not only with the type of grasses but also with the systems ofland-use.
Overgrazing in a grassland management has given maximum runoff (27 per
cent) and maximum soil loss (2,100 Ibjacre).
Similarly in a land use having
overgrazed fallow the runoff was 22 per cent whereas the soil loss amounted to
as high as 2029 lbjacre.
It can be concluded that overgrazing leads to degraded vegetative
cover consisting of less palatable species like Aristida adscensionis and Eragrostis
unioloides accompanied by decreased herbage production and loss of body
weight in animals. In its extreme [.)rm, the bare soil is exposed and trampling by .stock sets in the pro~ess of soil erosion and loss of soil texture and
fertility.
IMPACT
OF OVERGRAZrNG
ON GRASSLANDS
359
REFERENCES
1969. Forage production in Arid zone of India. Indian Dairy 21:
263-269.
Ahuja, L.D. 1975. Range Management and fodder production in arid regions of,
India. Mimeo.
Bharucha, F.R. and Shankarnarayan, K.A. 1958. Effects of overgrazing on the
grasslands of Western Ghats, India. Ecol. 3.9:152-153.
Blackman, V.H. 1919. The compound interest law and plant growth. Ann. Bot.
(L~md.) 33:353-360.
Bor, N.L. 1941. Common grasses of the United Provinces. Indian For. Rec. (N.S.)
2: 152-95.
Chatterjee, B.N. and Maiti, S. 1974. Role of grasses in soil conservation in
eastern India. Soil Cons. Digest 2 (l): 15-23.
Dabadghao, P.M. and Marwaha, S.P. ]962. Relative palatability studies on the
important indigenous grass species ("If western Rajastban. Indian Journal
of Agronbmy VII (I): 86-90.
Dabadghao, P.M. and Shankarnarayan,
K. A. 1973: The Grass Cover of India,
ICAR, New Delhi, pp. 1-713.
Dyksterhuis, E.J. 1946. The vegetation of Fortworth prairie. Ecol. Monog. 16.
Gupta, R.K. and Saxena, SK. 1972. Potential gras~land types and their ecological succession in Rajasthan desert. Ann. Arid Zone I I (3 and 4): 198-218.
Pandeya, S.C. 1962. Status of grasslands of Sagar, Madhya Pradesh. India. Trop.
£,0.3: 101-5.
Pandeya, S.C. 1964. Ecology of grasslands of Sagar, Madhya Pradesh. Part II-B.
Composition of association open to grazing or occupying special habitat.
J. Indian Bot. Soc. 43: 606-39.
Saxena, S. R. 1975. Effect of biota on the vegetation. Paper presented in the
Workshop on problems of desert in India organi~ed by Geological Survey of
India (Desert Circle) Jaipur, September 1975.
Shankarnarayan, K. A. 1974. Comparative study of continuous versus deferred
rotational grazing system. Annual Re.port of the Indian Grassland and
Fodder Research Institute, Jhansi, U.P.
Shankarnarayan,
K.A., Dabadghao, P.M., U padbyay, V.S., Ravi Kumar and
Rai, P. 1976. Effect of defoliation management and manuring on the
dry matter yield and quality in Iseilema laxum Hack. Forage Res. 2:55-62.
Shankarnarayan, K.A., Dabadghao, P.M., Ravi Kumar and Rai; P. 1977. Effect
of defoliation frequency intensity and mauuring on dry matter yield
and quality in three range grasses viz., Sehima nervosum, Cenchrus ciliaris
and C. setigerus. Ann. Arid Zone (in press).
Shankarnarayan,
K.A. 1977. Effect of clipping interval on the root sy;;tem of five
range grasses (Unpublished).
Ahuja,
L.n.