Campbell, Shane D.

McKenzie, John R.

Pattison, Mike J.

Vitelli, Joseph S.

University of Queensland eSpace

886
Fifteenth Australian Weeds Conference
Summary   In recent years a concerted effort has been 
made by research organisations to better understand 
the biology and ecology of the invasive shrub parkinso-
nia (Parkinsonia aculeata L.) and to develop integrated 
control strategies. Some land managers have also been 
testing innovative methods. One such example is the 
use of camels, with a small but increasing number 
of landholders purchasing them for the purpose of 
woody weed control in northern Australia. On two 
properties in the Charters Towers region, a trial was 
established to quantify whether camels have a detri-
mental affect on parkinsonia or are potential dispers-
ers. In browsed and unbrowsed paddocks, the level 
of pod production, size of seedbanks and presence of 
seed in camel dung were directly compared. Under 
heavy continuous browsing, pod production per tree 
averaged less than one in browsed paddocks in com-
parison to 3801 per tree under unbrowsed conditions. 
The soil seedbank present under the canopy of trees 
was similarly reduced, dropping from 207 seeds per 
square meter in unbrowsed paddocks to one seed per 
square meter in browsed areas. In terms of dispersal, 
camels appear to be low risk, with most dung samples 
containing less than two seeds. These results suggest 
that the preferential browsing nature of camels could 
compliment traditional control methods and lead to a 
reduced seedling recruitment from the depleted soil 
seedbank. However, further research is warranted to 
clarify camel diet preferences particularly with regards 
their potential impact on native shrubs.
Keywords    Browse,  parkinsonia,  dispersal,  soil 
seedbanks.
INTRODUCTION
Browsing by livestock, including sheep, goats and 
camels has been used to control weeds (March 2000, 
US Department of Agriculture 2002, Dörges and 
Heucke 2003). In recent times, camels have been 
introduced into northern Australia to control prickly 
acacia (Acacia nilotica L.) and parkinsonia (Parkinso-
nia aculeata) (Deveze 2004). In an integrated parkin-
sonia management program, the primary objective of 
existing biocontrol agents and browsing animals is to 
reduce the plant’s reproductive potential. If effective 
The effect camels have on the ability of parkinsonia 
( Parkinsonia aculeata L.) to set seed
 John R. McKenzie1,  Mike J. Pattison1,  Shane D. Campbell1,2 and  Joseph S. Vitelli1,2 
1 Queensland Department of Natural Resources, Mines and Water, Tropical Weeds Research Centre, 
PO Box 187, Charters Towers, Queensland 4820, Australia
2 CRC for Australian Weed Management
this will result in a reduction in the size of the soil 
seedbank. Consequently, seedling recruitment should 
be less following the implementation of conventional 
control methods. 
Currently the major biocontrol agent Penthobru-
chus germani (Pic), which is a seed-feeding bruchid 
released in Australia in 1995, assists in reducing viable 
parkinsonia seed input into the soil. Seed predation 
by this insect has been measured at between 80–90% 
(Lockett et al. 1999) and up to 99.68% (Donnelly 
1998). The insect is fairly widespread throughout the 
range of parkinsonia infestations in Queensland and 
the Northern Territory (Lockett et al. 1999, Lukitsch 
and Wilson 1999), although an egg parasitoid Uscanna 
sp. can reduce the impact Penthobruchus has in some 
areas (van Klinken 2005).
Information on the impact of camels on parkin-
sonia or other woody weeds is limited. Plant analysis 
studies on parkinsonia have shown it to have a low 
leaf fodder value, due to its moderate to high total 
fibre content levels, and a nitrogen content of 3.68% 
with no condensed or hydrolysable tannins indicating 
high digestibility (Hunter and Steward 1993). Despite 
the low leaf fodder value, parkinsonia is browsed by 
camels and sheep (Dörges and Heucke 2003, Lepape 
1980). Feeding trials by Dörges and Heucke (2003) 
found camels to browse selectively on the new foliage 
of plants. Parkinsonia was cultivated as a browse plant 
for sheep at the Cape Verde Islands (Lepape 1980). 
Studies have also shown camels to be more efficient 
than ruminants in extracting protein and energy from 
poor quality forages (Fowler 1999).
Co-grazing, which is the combination of different 
grazing animal species, allows a wider selection of 
vegetation to be preferentially used therefore improv-
ing grazing efficiency at the same time as maintaining 
or improving animal production (Phillips et al. 2001, 
DBIRD 2001, US Department of Agriculture 2002). 
Studies have shown that cattle preferentially graze 75% 
grass (US Department of Agriculture 2002) and camels 
prefer 95% dicotyledons (Dörges and Heucke 2003).
Traditional weed control options can be expensive 
particularly in terms of labour, chemical and machin-
ery hire. To treat a medium to dense infestations of 
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Fifteenth Australian Weeds Conference
parkinsonia (2200 plants ha-1), using labour-intensive 
control methods such as basal bark spraying cost $420 
per hectare in 2001 (McKenzie et al. 2004), whilst 
mechanical control options such as blade ploughing 
and Ellrott ploughing cost $156 and $126 per hectare 
respectively (McKenzie et al. 2004). These techniques, 
though effective, require follow-up control after 
recruitment from the soil seedbank and need to be 
continued until the seedbank has been exhausted.
This paper considers the effect of browsing by 
camels on parkinsonia pod production and its impact 
on soil seedbanks. The potential of camels to disperse 
parkinsonia seeds was also investigated.
MATERIALS AND METHODS
Two properties in the Charters Towers area were 
selected for the camel browsing experiment. The two 
landholders purchased camels in 1999 and 1998 re-
spectively, as a tool for managing parkinsonia. At the 
commencement of the experiment camels had been 
grazing the experimental sites for one and three-and-
a-half years at Site 1 and 2 respectively.
Wambiana (Site 1) is about 75 km south-west of 
Charters Towers and had two 30 camel herds grazing 
in rotation on a parkinsonia infestation along the Cam-
paspe River. The site consisted of mature parkinsonia 
plants throughout the paddock and a grass cover of 
approximately 1500 kg ha-1. Scartwater (Site 2) is 
approximately 150 km south-east of Charters Towers 
and had 60 camels in a continuous grazing regime on 
a parkinsonia infestation near the Suttor River. The 
parkinsonia on Site 2 was confined mainly to an area 
that is inundated most years and had very limited grass 
at the time of assessment.
Treatments at each site included a control (no 
camel browsing) and a camel-browsed plot. Control 
plots for both sites had no history of camel browsing 
and were located in paddocks directly adjacent to 
the browsed areas. A camel-proof fence separated 
the treatments and they were similar in soil type and 
density of parkinsonia.
All data was collected in December 2002 to allow 
the parkinsonia pods to mature but prior to senescing. 
This enabled accurate pod counts on trees to be un-
dertaken and soil seedbanks to be measured prior to 
replenishment from new season seed production. 
Sampling method   Fifteen isolated, seed-bearing, 
mature trees were selected for each treatment at each 
site. Trees without touching canopies were chosen so 
that seed from neighbouring plants would not con-
found seed collection data from underneath the drip 
zone of the sampled trees (J. McKenzie and A. Grice 
unpublished data).
Plant parameters   Basal diameter at 20 cm above 
the ground, height in cm, canopy dimensions in two 
directions and percentage leaf cover were recorded 
for each plant. Seed pods were collected from each 
tree and counted.
Soil seedbank   Previous studies have found that 
ninety-eight percent of the soil seedbank of parkin-
sonia is located within the drip zone of an adult tree 
(J. McKenzie and A. Grice unpublished data). Conse-
quently, in this study the drip zone of the plant was 
used for determining the parkinsonia soil seedbank. 
The canopy circumferences of each plant was scribed 
on to the ground surface, and divided into quarters 
using the plant’s stem as the centre. Fifteen cores 
were randomly collected from each quarter and then 
bulked. Each core was 5 cm diameter by 5 cm deep. 
The bulked sixty cores were later sieved though a 2 
mm sieve to retrieve parkinsonia seed.
Faecal disposal   Fifteen discrete fresh (green in col-
our, not grey) camel faeces were collected within the 
browsed areas at both sites to determine the presence 
or absence of seeds. Faecal extracted seed underwent 
germination and viability testing. At the time of collec-
tion, camels had access to mostly mature pods.
Seed viability test   Once recovered, seeds were 
counted, and damaged seed recorded and removed. 
The remaining intact seeds were placed in Petri 
dishes containing a Whatman™ No.4 filter paper and 
moistened with a systemic fungicide Fongarid™ at 1 
g L-1 distilled water. The Petri dishes were placed in 
a germination cabinet set at a diurnal temperature of 
25 to 35 ± 1°C and moistened daily with the fungicide 
mix. Seeds were considered germinated when the 
radicle extended at least 1 mm beyond the seed coat. 
Germinated seeds were counted and removed daily for 
seven days. At this time the seed coats of any remaining 
seed were scarified with a scalpel to break dormancy. 
These seeds were then placed back in the germination 
cabinets for a further seven days with any germinated 
seed recorded and removed daily. 
Statistical analysis   Plant cross-sectional area (the 
basal area at 20 cm height), pod production and vi-
able soil seedbank for browsed and control plots were 
analysed using the two-sample t test for each site.
RESULTS
Rainfall for the two previous years (2001 and 2002) for 
this area was 49 and 52 percent of the average. 
Based on cross sectional area plants studied 
were similar in size irrespective of treatment or site, 
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Fifteenth Australian Weeds Conference
averaging 66.1 (±32.5 SEM) cm2. Throughout the 
trial camels were observed feeding on fresh growth 
including flowers, young pods and leaves. This lead 
to parkinsonia foliage being reduced by 66% and 77% 
in browsed plots at Sites 1 and 2 respectively, when 
compared to the non-browsed controls (see Figure 
1). 
Browsed plants at both sites contained signifi-
cantly less seedpods than unbrowsed plants. At Site 
1, browsed plants produced a mean of 48.8 ± (25.3 
SEM) pods per plant in comparison to unbrowsed with 
a mean of 2842.5 ± (623.5 SEM) pods per plant (t = 
−4.476 df = 14, P = 0.001). At Site 2, under continu-
ous browsing, the mean number of pods per plant was 
0.2 ± (0.2 SEM) compared to unbrowsed plants with 
a mean of 3801.7 ± (1277.3 SEM) (t = −2.976, df = 
10, P = 0.014) (see Figure 2).
Site 1 had no significant differences in viable soil 
seedbanks between treatments (t = −1.878, df = 28, 
P = 0.071). However, at Site 2 there were vast differ-
ences, with the seedbank in the unbrowsed plot 46 
times higher than the browsed area. 
Camel dung was found to contain small quantities 
of viable parkinsonia seed. For Site 1, seeds per dung 
averaged 1.8 ± (0.9 SEM), ranging from 0 to 11 seeds 
per dung, whilst Site 2 had on average 0.3 ± (0.1 SEM) 
seeds per dung, ranging from 0 to 1. 
DISCUSSION
This experiment demonstrated that camels have the 
ability to reduce seed production, leading to a net de-
cline in the soil seedbank over time. Browsing of fresh 
growth and flowers by camels could have attributed 
to this significant reduction. Parkinsonia pods did 
not appear to be deliberately eaten by the camels but 
were consumed along with the palatable leaf material 
or when limited preferential food was present such as 
in Site 2. In another experiment where camels were 
fed parkinsonia seed it was found that seed was only 
eaten incidentally with other desirable attractants such 
as molasses, salt and grain (J. McKenzie unpublished 
data).
Reduced time frame could explain why Site 1 
had not achieved a significant reduction in the viable 
soil seedbank compared to Site 2. Camels were only 
introduced to the paddock at Site 1 one year prior to the 
commencement of the experiment, compared to three-
and-a-half years for the paddock studied in Site 2.
Approximately 15% of seeds consumed remain 
viable after digestion by camels (J. McKenzie unpub-
lished data). When confined to an infested area the 
threat of seed spread by camels is inconsequential 
compared to the benefits of both less seeds being 
produced and a low seed viability of consumed seeds 
by camels. However in areas where camels are not 
confined, they could act as vectors for dispersal. 
During the trial impacts from camels were also 
observed on native flora. The impacts of camels to 
off-target species should also be considered when 
contemplating the use of camels as a tool for par-
kinsonia management. Under some grazing regimes 
camels have the potential to contribute to a decline in 
preferred food plants (Dörges and Heucke 2003). At 
Site 2 observations suggested that Prickly pine (Bur-
saria incana Lindl.) and Emu apple (Owenia acidula 
F.Muell. ex Benth.) were heavily grazed while other 
species such as white wood (Atalaya hemiglauca 
F.Muell.) and Bendee (Acacia catenulata C.White) 
were grazed to a lesser level. Dörges and Heucke 
(2003) found that white wood had a palatability rating 
similar to parkinsonia.
Figure 1.   Mean percentage leaf cover of parkinsonia 
plants growing at two sites (1 = Wambiana and 2 = 
Scartwater) following browsing by camels. Vertical 
bars indicate the LSD at P <0.05.
Figure 2.   Mean pods per parkinsonia plant and 
viable soil seedbank under individual parkinsonia 
plants growing at two sites (1 = Wambiana and 2 = 
Scartwater) following browsing by camels. Vertical 
bars indicate the LSD at P <0.05.
non grazedgrazed
1 2
Site
0
4
8
12
Le
a
f c
o
ve
r 
(%
)
Po
ds
1 2
Site
0
1000
2000
3000
4000
5000
6000
1 2
Se
ed
s 
(m
-
2 )
Viable soil seed bankPods per tree 300
200
100
0
non grazed
grazed
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Fifteenth Australian Weeds Conference
CONCLUSION
In conclusion, camels could play a role as part of an 
integrated management program. The ability of camels 
to reduce the quantity of seed entering the soil seed-
bank, should result in landholders having less seedling 
regrowth to treat once they control initial infestations 
using conventional techniques (such as machinery 
and herbicides). However, care is required to manage 
herd numbers and to ensure that off-target damage 
and overgrazing due to dietary overlap of species is 
minimised.
ACKNOWLEDGMENTS
We sincerely thank the Lyons family of ‘Wambiana’ 
and the Pedracini family of ‘Scartwater’ for conducting 
this experiment on their properties.
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The effects camels have on the ability of parkinsonia (Parkinsonia aculeata L.) to set seed

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The effects camels have on the ability of parkinsonia (Parkinsonia aculeata L.) to set seed

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