The effect of cattle grazing on soil phy

Agroforestry Systems 40: 109–124, 1998.
 1998 Kluwer Academic Publishers. Printed in the Netherlands.

The effect of cattle grazing on soil physical and
chemical properties in a silvopastoral system in the
Peruvian Amazon
L. A. AREVALO 1, 2, J. C. ALEGRE1, 2, *, D. E. BANDY1 and L. T. SZOTT2
1
2

International Centre for Research in Agroforestry (ICRAF), P.O. Box 30677, Nairobi, Kenya;
Instituto de Investigación de la Amazonia Peruana (*Author for correspondence)

Key words: acid soils, cover crop, humid tropics, peach palm, Peru, silvopastoral system
Abstract. In a six-year-old peach palm (Bactris gasipaes) plantation, centrosema (Centrosema
macrocarpum), a leguminous forage plant, was established as a cover crop which was eventually grazed. This experiment was designed to monitor probable changes in soil physical and
chemical properties and measure peach palm fruit production and live-weight gain of cattle
grazing this silvopastoral system. The experiment was installed on land that was previously
cleared by a D7 bulldozer having a straight blade that mixed the thin layer of topsoil with the
acid subsoil (20–40 cm) and severely compacted the soil. The results demonstrated that the
centrosema cover crop reduced soil bulk density, increased water infiltration rates and reduced

mechanical resistance. In general soil physical properties were improved with the use of
Centrosema as forage and cover crop. Soil acidity and aluminum saturation decreased considerably, while potassium concentrations increased. Calcium and magnesium concentrations
decreased over time as these minerals were stored in the pasture biomass, translocated to fresh
peach palm fruits and/or exported to animals. A strong competition for nutrients was observed
between the peach palm plants and Centrosema. The low production of peach palm fruits was
in response to mechanized land clearing during initial establishment of the plantation, and also
probably due to deficits of N, P, K, Ca and Mg in the soil instead of being a consequence to
the presence of cattle. The average increase in live-weight gains of the cattle was at a rate of
445 g/animal/day during the four-years of the study. Such an increase is substantially greater
than those registered in the area under traditional grazing systems used in the region.

Introduction
In more than 80% of the Peruvian lower Amazon basin, cattle farming depends
on native pasture ‘torourco’ which comprises basically a mixture of Axonopus
compressus S. W. Beauv and Paspalum virgatum L. The low productivity of
native grasses combined with the high stocking rate employed, results in rapid
degradation of soil and thus of the physical, chemical and biological status
of the pasture.
In order to improve these soil conditions and increase meat and milk production, silvopastoral systems, associating trees, pastures and animals can be
a viable alternative. Incorporation of trees in the pasture lands gives the

opportunity to increase land productivity by providing additional products
such as fruits, wood and charcoal, in addition to forage for cattle.
Depending on the density of trees in silvopastoral systems, the selected

110
pasture species should have the following characteristics; grow at low light
intensities, support trampling, withstand grazing pressure, and fix nitrogen.
Previous experimental results have shown that centrosema (Centrosema
macrocarpum) in association with peach palm (Bactris gasipaes HBK) plantations have great potential. This system is of great importance as it preserves
the soil and supplies N to the peach palm plant (Perez et al., 1993). Moreover,
the system minimizes hand labour for plantation maintenance.
Some leguminous forage plants used as cover crops tend to climb trees;
therefore, they need to be frequently cut back. The manpower applied in this
labour can be eliminated if the cover crop is grazed. However cattle grazing
can increase soil compaction which in turn adversely affects tree growth,
thereby diminishing total productivity of the system.
Among different leguminous forage plants evaluated for cover crops under
different light intensities, Centrosema was the best (Perez et al., 1993), and
in addition demonstrated greater tolerance to frequent grazing whether alone
or in association with grasses in grazing systems without trees (Lara et al.,

1991).
The role of the animal component the silvopastoral system of the Peruvian
Amazon has not been adequately researched. Hence, an experiment was
conducted in the peach palm/centrosema system with the objectives of (a)
determining the effect of incorporating the animals on physical and chemical
properties of soil, nutrient recycling and fruit production of beach palm, (b)
regeneration of productivity of centrosema and (c) qualifying live-weight gain
of cattle.

Materials and methods
Site description
The trial was started in October 1988, at the Experimental Station of San
Ramón, near Yurimaguas, Peru. The Station is located at 5°56′ south latitude,
76°5′ west longitude, at 184 m above sea level in a moist humid tropical
ecosystem with a mean annual temperature of 26 °C and annual precipitation
rate of 2,100 mm. Monthly rainfall exceeds 200 mm during nine months of
the year but it is usually around 100 mm during July, August, and the monthly
rainfall rate is over 200 mm except during July, August and September.
In August 1980 one hectare of upland soils (Typic Paleudult) was cleared
with a D7 bulldozer which a straight blade that mixed the thin layer of topsoil

with the acidic subsoil (20–40 cm) and compacted the soil (Alegre et al.,
1988). Thereafter only one season of rice was sown. The results obtained were
very poor and the field site was abandoned until August 1982. On November
1982, peach palm was planted in a limiting nutrient trial, applying different
rates of N, P, and K, for three years (Perez et al., 1993). During 1986 and
part of 1987 the peach palm plantation was managed in the traditional fashion;

111
palm trees with a natural grass understorey. The area had a density of 1,200
peach palm plants/ha, three times greater than the optimum density (400
plants/ha). In May 1987, 50% of plants were eliminated in order to enhance
fruit production. Before initiation of the silvopastoral trial, soil analysis results
showed no residual effects of the applied nutrients (Table 1).
Establishment and management of cover crop
In October 1988, when peach palm was 6 years old, inoculated seed of
centrosema was sown as an understorey cover with a spacing of 1 m between
rows and 0.50 m between plants. About 20% of the area needed gap filling
which was done a month after initial sowing.
Due to low content of nutrients in the soil (Table 1), 20 kg of P ha –1 as
Bayovar rock phosphate and 20 kg K ha–1 as potassium chloride were spread

over the entire area in May 1989. These are the recommended rates of fertilizer for establishing new pastures, which would also be adequate for recouperation of degraded pastures (Ayarza, 1988).
The two main treatments for comparison were: (1) without grazing (trees
and pastures); and (2) with grazing (trees, pastures and animals). The treatments were applied three times across a soil compaction gradient in the
experimental area. The ‘no grazing’ plots were 600 m2 in all blocks, whereas
the plots with grazing were 2,446, 2,253 and 1,509 m2.
Eighteen months after the establishment of centrosema, grazing commenced
with two young bulls of 180 and 171 kg each, at a stocking rate of 3.3
animals/ha. A rotational grazing system was used, with 9 to 14 days grazing
periods and 28 to 30 days rest according to the size of the plots. Subsequently
the stocking rate was reduced to 2.3 animals/ha because of the decreased
availability of centrosema biomass during the dry season and to allow for
recouperation of Centrosema. The cattle received an additional salt/mineral
supplement.

Table 1. Chemical properties of the 0–20 cm soil layer at the start of the establishment of the
experiment in October, 1988, Yurimaguas, Peru.
Treatment

Pa
(µg/ml)


Acidityb
Ca
Mg
Ka
—————— cmol/l ——————

Al Sat
(%)

Without grazing
With grazing

4.8
5.2

5.24
5.07

87

84

a
b

0.53
0.71

0.15
0.13

P and exchangeable K were extracted by modified Olsen method.
Acidity, Ca and Mg measured in 1 N KCl extract.

0.10
0.11

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Measurements
Biannual physical and chemical properties of the soil were monitored: at the

end of the rainy season (April) and end of the dry season (October). Each
plot was sampled according to a pre-determined sampling grid in order to
draw samples from the same location over time. Nine samples were collected
from the no grazing treatment, but 17 samples were collected in the grazing
treatment.
The monitored soil physical properties were: bulk density (BD) at 10 cm
soil depth using cylinders of size 345 cm3 (Uhland, 1950); mechanical resistance (MR) with a cone penetrometer to a depth of 60 cm; gravimetric soil
moisture (Davison, 1965); and infiltration rates (Sorptivity) with cylinders of
10 cm diameter and 30 cm height.
Soil chemical properties were measured at the same time as soil physical
properties in the 0 to 20 cm soil layer. The soil samples were dried and sieved
with a 2 mm mesh. Exchangeable cations (Ca and Mg) were measured in
1 N KCl extract by atomic absorption spectrophotometry (Hunter, 1979). pH
of the soil was measure 1:2.4, soil:water. Both available P and exchangeable
K were extracted by modified Olsen method. P was measured by colorimetry
using molybdate as colour fixer (Olsen and Sommers, 1982) and K by atomic
absorption.
For peach palm tissues, the most representative leaf of 10 trees per treatment and replication were sampled, by obtaining the folioles from the third
part of the leaf at both sides of rachis (Grau, 1986). Samples were dried in
an air circulating oven at 70 °C. Nutrients were extracted with H2SO4 and

H2O2. K, Ca and Mg were analyzed by atomic absorption, N by micro-Kjeldal
and P by colorimetric methods.
At the beginning of each grazing period the forage biomass was estimated
by sampling at 0.5 m2 frame in the plots under grazing, while in plots without
grazing a 0.25 m2 frame was used due to greater quantities of biomass. The
samples were dried at 70 °C, for determining dry weight in an air circulating
oven. Live-weight gain of the young bulls was determined by weighing them
at the beginning and at the end of each grazing period during the trial. The
total gained was divided by the number of grazing days to obtain the daily
increase of live-weight of each animal.

Results and discussion
Centrosema biomass
Centrosema germination was good and although the young plants were
attacked by leaf-cutting ants (Atta sp.) and crickets, 100% ground cover was
obtained within eight months after sowing. Centrosema was allowed to set
seed before being grazed, to ensure its persistence after being grazed. Animals

113
were introduced into the system in April 1990, 18 months after sowing, at a

stocking rate of 3.3 animals per hectare. In 1992, the dry season rainfall was
below the normal mean (Figure 1), therefore, to avoid pasture degradation and
facilitate Centrosema recouperation the animal stocking rate was reduced to
2.3 animals per hectare and maintained at this rate until the end of the study
(Table 2).
Regardless of the season, the non-grazed plot always produced twice the
quantity of available forage and litter compared to the grazed plot. The
quantity of available forage each year was similar over the years.
Changes of soil physical properties
Bulk density
The initial values of bulk density (BD) averaged 1.52 to 1.72 mg m–3 for the
plots before the start of grazing (Figure 2). Bulk density was significantly

Figure 1. Monthly rainfall distribution during the silvopastoral experiment from 1990 to 1994;
Yurimaguas, Perú.

114
Table 2. Forage and litter production affected by grazing vs not grazing in different years at
Yurimaguas, Peru.
Treatment


Years
90

91

92

93

94

RSa

DSb

RS

DS

RS

DS

RS

Available forage (Mg ha–1)
With grazing
6.8
Without grazing
8.7

5.16
9.00

4.0
8.4

05.9
11.1

4.3
9.1

5.1
9.6

4.22
8.35

4.35
9.07

Litter (Mg ha–1)
With grazing
Without grazing

0.94
2.08

0.6
1.9

01.0
02.1

0.7
1.9

0.9
2.5

0.5
1.7

0.5
2.0

a
b

1.0
1.9

RS = Rainy season.
DS = Dry season.

Figure 2. Effect of grazing and not grazing at different times on the topsoil (0–10 cm) bulk
density in a silvopastoral system; Yurimaguas, Peru. (Means with the same letter between
columns are not significantly different with the LSD0.05 test.)

115
reduced by the Centrosema cover crop but there was no significant difference between the two grazing treatments. Bulk density was significantly higher
in grazing than no grazing treatments at 30 months after the start of grazing
and this difference continued until 42 months. However, in subsequent months
there was no difference between the two grazing treatments. The decrease in
BD following Centrosema cover crop could be attributed to an increase in soil
organic matter and reduced soil compaction by the root activity. Several
studies have shown the influence of organic matter and root activity of plants
in lowering soil bulk density (Lal and Kang, 1982; Alegre et al., 1986 and
Szott, 1987). In addition, the abundant and vigorous growth of Centrosema
(Table 2) reduced the potential soil compacting effect of cattle (Alegre and
Lara, 1991).
Mechanical resistance (MR)
Figure 2 shows the changes in mechanical resistance of the soil measured with
a cone penetrometer in relation to the grazing treatments both at the initial

Figure 3. Soil mechanical resistance at different depths and times with and without grazing in
a silvopastoral system; Yurimaguas, Peru.

116
phase and final phase of the trial. Soil MR was significantly greater at the
10 cm depth in 1990 compared to 1994, without considering the presence of
animals in the treatments. With increasing depth, MR differences became
smaller. At lower depths the MR values were similar between treatments over
time.
By the end of the trial in 1994 greater MR values were noted in the grazing
treatment, particularly in the surface 20 cm layer. Apparently the increase in
the soil organic matter content due to Centrosema root decomposition, litterfall decomposition and to cattle excrement and urine was not sufficient to
offset compaction (higher MR values) in the grazing treatment.
The soil moisture content during the trial was similar for both treatments,
therefore did not show any influence on MR. Differences were due to a change
in soil structure, as a result of being compacted by cattle (adult cattle have a
static pressure of approximately 1.7 kg cm–2 in the hoof area). This value is
equivalent to the pressure of wheeled traction (Lull, 1959), probably affecting
the BD up to depths of over 1 m or more (Rhoades et al., 1964). Absolute
values of MR + BD declined from 1990 to 1994, which suggests an improvement in soil physical properties.
Infiltration
Sorptivity (S) is an initial measurement of the non-saturated water infiltration of the soil, offering an indication of its susceptibility to compaction. The
S method is an easier measurement to obtain because it uses a smaller cylinder
(30 cm height by 10 cm diameter) and has a lower soil disturbance effect
compared to the more common two cylinder infiltrometer method (Alegre and
Cassel, 1994).
The grazing treatment showed lower S values than the no grazing treatment (Table 3) except in the measurements carried out at 30 and 66 months.
Only at 30 months the grazing treatment showed statistically higher S values,
than the non-grazing treatment. The lack of significant differences during some
periods has its origin in the high coefficient of variation between S measurements and changes in soil moisture content during the different evaluation periods.
Changes in soil chemical properties
In situ livestock grazing plays an important role in soil nutrient dynamics as
it affects quantity of litter production and its nutrient composition, and nutrient
cycling as a result of the return of nutrients through urine and excretion.
Soil acidity decreased by 33–40% independent of the effect of grazing
(Figure 4). Al saturation also decreased over time like acidity, with a significant grazing effect shown at 54 months. This decrease of soil exchangeable
acidity could be related to Al complexing with organic matter (Davelouis,
1990), and is also possibly due to the recycling of Ca and Mg to the soil
surface through livestock grazing and excretion.

117
Table 3. Changes in sorptivity and soil moisture affected by grazing at different times in a
silvopastoral system; Yurimaguas, Peru.
Treatment

Months
18

30

42

54

66

Sorptivity (cm/√seg)
Without grazing
With grazing

00.020a*
00.075a

00.327a
00.125b

00.229a
00.116a

00.126a
00.097a

00.049a
00.070a

Soil moisture (%)
Without grazing
With grazing

29.7a
29.0a

27.6a
25.2a

20.5a
06.8b

28.3a
28.7a

30.8a
31.0a

Figure 4. Dynamics of soil exchangeable acidity and aluminium saturation in the top layer
(0–20 cm), with and without grazing in the silvopastoral system; Yurimaguas, Peru. (Means with
the same letter are not significantly different with the LSD0.05 test.)

118
Figure 5 shows variations of exchangeable cation concentrations in the soil.
Potassium concentration remained constant until 42 months after the trial
began but increased with the higher K recorded (0.20 cmol(+)/l) by the end
of the trial in the grazing treatment. The increase of K concentration in the
soil could be related to the recycling of nutrients in the above-ground biomass,
root biomass (dry material estimated to be 7.4 t ha–1, or through the recycling of cattle urine and excretion (Ayarza, 1988).
The concentration of exchangeable calcium (Ca) and magnesium (Mg) in
the soil increased during the first 30 months, and then decreased to original
values. Exchangeable Mg was higher in no grazing system than the grazed

Figure 5. Dynamics of potassium, calcium and magnesium in the top layer (0–20 cm), with
and without grazing in the silvopastoral system; Yurimaguas, Peru. (Means with the same letter
are not significantly different with the LSD0.05 test.)

119
system whereas exchangeable Ca was higher in the grazing system. The
observed reduction for Ca and Mg could be attributed to the accumulation of
these in Centrosema (Table 4), and fresh peach palm fruits and to their export
by animals. The quantity of nutrients held in the available forage was always
greater in the plots without grazing, due to higher quantity biomass growth.
Available phosphorus levels showed sharp changes over the years and these
are difficult to explain (Figure 5). The cyclic trend in the increase and decrease
of its concentration is probably related to organic/inorganic transformation
and C content. Organic C increased under both the grazing treatments which
is probably due to the addition of organic residues through litter and root
biomass of centrosema in no grazing treatment and animal dung and root
biomass in the grazing treatment.
Nutrient concentration in peach palm leaves
Tissue concentrations of nutrients decreased over time in both treatments
(Figure 7). These results suggest a strong competition between the cover crop
and peach palm for nutrients caused by the storage of the cover crop (Table
4), uptake by animals and the translocation of nutrients to peach palm fruit.
Nutrient concentrations of N, P, K, Ca and Mg in fruits were 1.07, 0.11, 1.12,
0.09 and 0.05, respectively. By the end of the trial N, P and K concentrations
in peach palm leaves were below critical values (Grau, 1986) while Ca and
Mg were near borderline levels. These results suggest that the insufficient
concentration of N, P and K in peach palm plants, was probably related to
initial trial history. In order to increase fresh fruit production, it is necessary
to add N, P, and K through fertilizers for higher productivity of peach palm.

Table 4. Effect of grazing vs no grazing on the nutrient content biomass at Yurimaguas, Peru.
Treatment

Grazing

Year

N

P

K

Ca

Mg

90
91
92
93
94

0204
0137
0156
0139
0131
0767

13.6
08.2
09.7
08.4
08.3
48.2

150
070
092
107
140
559

103
078
072
061
069
383

11.0
12.4
11.2
09.3
11.3
55.2

90
91
92
93
94

0261
0261
0281
0242
0254
1299

17.4
13.9
17.2
14.4
14.5
77.4

191
104
167
193
200
855

132
115
143
097
129
616

13.9
19.1
19.2
15.3
19.0
86.5

Total
No grazing

Total

kg/ha

120

Figure 6. Dynamics of phosphorus and organic carbon in the top layer (0–20 cm), with and
without grazing in the silvopastoral system; Yurimaguas, Peru. (Means with the same letter are
not significantly different with the LSD0.05 test.)

Peach palm fruit production
Table 5 shows the effect of animal grazing on peach palm fruit production.
No differences between the two treatments were noted.
There were no differences between grazing and no grazing treatments in
respect of fruit production (Table 5). In general, fruit production was lower
in comparison with other peach palm plantations in the research station which
gave yields of 10 to 20 Mg ha–1 (unfertilized with cover crop) (Arévalo et
al., 1993). The low yields were probably related to poor soil conditions at
the establishment of the peach palm. The plantation was established on a
completely infertile subsoil as the area was cleared by bulldozer that mixed

121

Figure 7. Nutrient concentrations in peach palm (Bactris gasipaes) tissues with and without
grazing ata silvopastoral system; Yurimaguas, Peru. (Means with the same letter are not
significantly different with the LSD0.05 test.)

20–40 cm of the soil surface and seriously compacted the soil. These results
suggest that while the introduction of cattle grazing on the Centrosema cover
crop had no effect on production of peach palm fruits, initial soil management
preparation does.
Live-weight gain of animals
The increase of animal live-weights fluctuated from 426 to 457 g animal –1
day–1 over the four years (Table 6). These results are very promising considering the physical and chemical status of the soil under the experiment. The
increase in cattle live-weights on centrosema cover crops in a perennial

122
Table 5. Effect of grazing vs no grazing on peach palm fruit production at Yurimaguas, Peru.
Fruit production (Mg ha–1)

Date

March
March
March
March
March

90
91
92
93
94

No grazing

With grazing

4.2a*
4.0a
3.4a
4.2a
4.5a

3.8a
4.0a
3.2a
3.8a
4.3a

* Means of the values followed by the same letters within a column are not significantly
different with the LSD0.05 test.
Table 6. Animal live-weight gain for different grazing periods in the silvopastoral system at
Yurimaguas, Peru.
Period of grazing

Years
1991
1992
1993
1994
————————— g/animal/day ——————————

1
2
3
4
5
6
7
8

320
220
550
600
382
440
447
450

350
726
292
650
500
457
263
868

576
622
332
294
454
371
326
542

603
688
371
642
293
250
330
464

Mean

426

457

440

455

plantation is greatly superior to the traditional grazing system used in the
region consisting of native pasture ‘torourco’ with animal stocking rates of
less than one head per hectare.

Conclusions and recommendations
Centrosema grown as an understorey cover crop in a peach palm plantation
increased soil organic carbon. Consequently, there was a significant decrease
in soil bulk density in the first 10 cm of soil depth, increased water infiltration and reduced soil mechanical resistance. These results suggest that
Centrosema cover crop can improve the soil physical properties, even when
animals are allowed to graze on the biomass.
Initial soil nutrient reserves combined with an accumulation of nutrients
in the above ground biomass of centrosema caused on N, P and K deficiency

123
in peach palm tissues, which demonstrated a strong nutrient competition
between peach palm and centrosema.
Peach palm fruit production was low, because of poor soil conditions.
Therefore it was not possible to demonstrate the influence of cattle grazing
on peach palm fruit production. The results suggest that for an increase in
peach palm fruit production additional amounts of fertilizers must be added
to the soil.
The increase in cattle live-weight was greater in this silvopastoral system
compared with the conventional system in the region. Averaged over four
years, live-weight gains of 445 g animal–1 day–1 were obtained.
It is possible to recouperate degraded pastures through the establishment
of a silvopastoral system such as Centrosema, peach palm, animals, but it will
be necessary to add nutrients to the soil via fertilizer in order to maintain
sustainable peach palm fruit production.
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