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Preference
of goats and sheep for browse species under field conditions
R. S. Pande* P. D. Kemp† J.
Hodgson
*Present address: Chababil, GPO Box 10
245, Kathmandu, Nepal. Email: p.kemp@massey.ac.nz
New Zealand Journal of
Agricultural Research, 2002, Vol. 45: 97–102
0028–8233/02/4502–0097 $7.00/0 © The Royal Society of New Zealand 2002 New Zealand Journal of Agricultural Research www.informaworld.com/smpp/.../title~db=all~content=g919964384
Abstract The preference of goats and sheep for browse species in
mixed browse and pasture conditions in New Zealand was determined using 6 goats
and 6 sheep observed for 2-h periods on 6 occasions between February and
December. The goats and sheep were given free access to established nurseries.
The 2 nurseries contained 6 types of leguminous shrubs, 3 non-leguminous shrub
species, and 2 erect grass species, growing in rows with a grass-dominant
temperate pasture. Preference was determined with an index that took into
account the proportion of browsing observations and the relative abundance of
each browse species. Goats browsed in 45% and sheep in 12% of the observations.
The three most preferred species for goats and sheep were the same, but the
order of preference differed. The overlap coefficient for all browse species
(where 0 = no overlap and 1 = identical diets) ranged from 0.47 in winter to
0.79 in summer. The results are contrasted with those from a cafeteria-style
indoor experiment that used the same browse and animal species.
Keywords grazing ecology; goats; sheep; browsing; preference;
diet selection; browse species
INTRODUCTION
Grazing
animals continuously discriminate between vegetation types and plant parts
(Hodgson 1986). Such activity is usually greater under range conditions and
maximal when animals are browsing (Van Soest 1982). Goats are regarded as more
efficient users of browse than sheep (Van Soest 1982; Devendra & Burns
1983), but scant information is available on the preference of goats and sheep
for the browse species commonly available in New Zealand. Lambert et al.
(1989a) compared the preference of goats and sheep for a range of browse
species in a cafeteria-style indoor experiment.
The multi-species nurseries used to provide the browse material in the
experiment by Lambert et al. (1989a) were used in the field experiment reported
here, which was designed to study the preference for browse species and the
browsing activity of goats and sheep on field sites with multiple browse and
pasture species available.
MATERIALS
AND METHODS
Location
The
experiment was carried out at the Ballantrae Hill Research Station, AgResearch
Grasslands, (175°50E, 40°18S, 150 m a.s.l.), 20 km from Palmerston North, New
Zealand. Observations took place at intervals from February 1989 to December
1989 inclusive.
Plant
species
There
were two established nurseries each containing 10 different browse species,
with two forms of gorse (Table 1). Short spine gorse is an ecotype with shorter
spines and a denser bush than typically found on gorse. The browse species were
3–4 years of age and grown in hedges consisting of 5–8 plants per hedge. The
hedges were usually 5 m long and were between 0.4 and 1.7 m high. At each
nursery, hedges of short spine gorse, black locust, and ceanothus appeared only
once and hedges of †Author for correspondence.
the other eight browse species were replicated three times.
Table
1 Browse species in the nurseries.
Common name
|
Scientific name
|
Legumes
|
|
Tagasaste
|
Chamaecytisus
palmensis (Christ) Bisby et K.Nicholls
|
Broom
|
Cytisus
scoparius (L.) Link.
|
Tree
medic
|
Medicago
arborea L.
|
Black
locust
|
Robinia
pseudoacacia L.
|
Gorse
|
Ulex
europaeus L.
|
Short
spine gorse
|
Ulex
europaeus L.
|
Non-legumes
|
|
Tauhinu
|
Ozothamnus
leptophyllus (G.Forst.) Breitw. et J.M.Ward
|
Ceanothus
|
Ceanothus
griseus (Trel.) McMinn
|
Manuka
|
Leptospermum
scoparium J.R. et G.Forst.
|
Erect
grasses
|
|
Toetoe
|
Cortaderia
fulvida (Buchanan) Zotov
|
Pampas
|
Cortaderia
selloana (Schult. et Schult.f.) Asch. et Graebn.
|
Main
pasture species growing underneath the browse species were browntop (Agrostis
capillaris), crested dogstail (Cynosurus cristatus), cocksfoot (Dactylis
glomerata), perennial ryegrass (Lolium perenne), Yorkshire fog (Holcus
lanatus), and white clover (Trifolium repens).
Animals
Six
dry, angora-cross female goats and 6 dry, pure Romney female sheep were used.
The animals were 17 months of age at the start of the experiment, and the mean
body weight was 18.2 ± 0.4 kg and 43.5 ± 1.1 kg for goats and sheep,
respectively. The animals were kept overnight in an adjacent shearing shed
prior to each observation period, and provided with water but no feed, to
encourage feeding during the observation period. The previous grazing
experiences of the animals were on hill pastures.
Table
2 Animal observation and nursery schedule.
Observation date (1989)
|
Nursery 1
|
Nursery 2
|
Season
|
22
Feb
|
sheep
|
goats
|
summer
|
23
Mar
|
goats
|
sheep
|
autumn
|
27
Apr
|
sheep
|
goats
|
autumn
|
23
May
|
sheep
|
goats
|
winter
|
22
Jun
|
goats
|
sheep
|
winter
|
24
Nov
|
goats
|
sheep
|
summer
|
Observations
The
goats and sheep were allowed free choice of all plant species in their
allocated nursery. Each observation period was 2 h (10.00–12.00 h) starting
from release of the animals into the nurseries.Observation dates were scheduled
to cover summer, autumn, and winter seasons, and were arranged in a sequence of
approximately 4-week intervals between 22 February and 22 June 1989, with a
final observation on 24 November 1989. The animal species were alternated
between nurseries (Table 2).
The
preferences of the goats and sheep for browse species were quantified on the
basis of the animal activities; browsing, grazing, and idling. The interval
sampling technique was used to record animal activities at 2-minute intervals
(Hodgson 1982). A shearing shed about 200 m from the nurseries was used as the
observation post so as to minimise the influence of the observer on animal
activities (Jamieson & Hodgson 1979). Distribution of the browsing
activities in relation to the arrangement of the browse species in each nursery
and the defoliation of plant parts are described in detail in Pande (1990).
The
preference of goats and sheep for the individual browse species was adjusted
for both the difference in the proportion of time spent browsing by goats and
sheep, and the different abundance of the browse species. These adjustments
were made by calculating, for each nursery, the proportion of browsing
observations (PBO) each animal species spent browsing each browse species, and
by calculating the relative abundance value (RAV) for each browse species. The
RAV for each replicate of each browse species was calculated from the length,
width, height, and density score of the plants in each hedge (Whittaker 1975).
Density of the foliage of the browse plants was ranked visually from 1 (lowest)
to 10 (highest).
The
PBO and RAV were used to calculate the browsing preference index (BPI) given
below (see Pande 1990).
BPI
= (PBOi/ RAVi/ åni=1
(PBOi/ RAVi )
An overlap coefficient (Cl)
using the simplified Morisita Index (Horn 1966) was used to measure the overlap
for preferred browse species between sheep and goats. The Cranges from 0 (no
overlap) to 1 (identical diets).
Cl=2åni-1
Xi Yi/åni=1Xi
+åni=1
Yi
where:
Xi
=
PBO for animal species X, for browse species i
Yi
=
PBO for animal species Y, for browse species i.
The
effect of animal species, browse species, nursery, and season and their
interactions on RAV, PBO, and BPI were analysed with the SAS General Linear
Model (SAS 1988).
RESULTS
Goats
and sheep were browsing during 45 and 12% of observations, respectively (Table
3). Conversely, sheep grazed more often than goats (Table 3). Both animal species spent a similar time idling,
but this was in 8% or less of all observations (Table 3). There was no
significant effect of season on the observations of animal activities.
Table
3 The percentage of observations of animal activities in
the categories of browsing, grazing, and idling, expressed as means for the six
observation periods. NS, not significant; *, P < 0.05 for within row mean
comparison
Animal activities
|
Goats
|
Sheep
|
%
|
Browsing
|
44.7
|
11.6*
|
Grazing
|
48.6
|
80.5
|
Idling
|
6.7
|
8.0
NS
|
SEM
= ±1.3
|
|
|
Proportional
browsing observations (PBO)
There
was a significant effect of browse species on PBO, and the animal species by
browse species interaction was also significant (P < 0.001; Pande 1990).
The other main effects and interactions were not significant. Goats were
observed to browse tree medic most often, whereas sheep most often browsed
black locust (Table 4). Tree medic, black locust, tagasaste, and ceanothus were
often browsed by both goats and sheep, whereas tauhinu, toetoe, and pampas were
rarely browsed by either animal species (Table 4). Short spine gorse and gorse
were regularly browsed by goats but more rarely by sheep (Table 4).
Table
4 The proportion of browsing observations (PBO), the
relative abundance value (RAV), and the browsing preference index (BPI) for
each browse species for goats and sheep over 1 year.
Species
|
Goat
|
Sheep
|
PBO
|
RAV
|
BPI
|
PBO
|
RAV
|
BPI
|
Tree
medic
|
0.268
|
0.001
|
0.837
|
0.215
|
0.001
|
0.790
|
Short
spine gorse
|
0.249
|
0.003
|
0.154
|
0.056
|
0.002
|
0.053
|
Black
locust
|
0.178
|
0.007
|
0.066
|
0.398
|
0.013
|
0.149
|
Tagasaste
|
0.151
|
0.076
|
0.007
|
0.121
|
0.072
|
0.007
|
Ceanothus
|
0.092
|
0.042
|
0.006
|
0.125
|
0.044
|
0.007
|
Broom
|
0.048
|
0.059
|
0.003
|
0.093
|
0.058
|
0.014
|
Gorse
|
0.143
|
0.252
|
0.001
|
0.041
|
0257
|
0.002
|
Manuka
|
0.012
|
0.089
|
0.001
|
0.009
|
0.085
|
0.001
|
Tauhinu
|
0.002
|
0.111
|
0.000
|
0.013
|
0.107
|
0.001
|
Toetoe
|
0.006
|
0.176
|
0.000
|
0.018
|
0.175
|
0.001
|
Pampas
|
0.002
|
0.182
|
0.000
|
0.000
|
0.184
|
0.000
|
SEM
|
0.008
|
0.031
|
0.024
|
0.008
|
0.031
|
0.024
|
Relative
abundance value (RAV)
The
RAV of the browse species differed significantly (P < 0.0001). The
RAV was unaffected by season, nursery, or animal species. The only significant
interaction effect was nursery by browse species (P < 0.001),
reflecting a difference in the number and hedge size of some browse species between
nurseries (Pande 1990). The four most abundant browse species in terms of RAV
were gorse, pampas, toetoe, and tauhinu, whereas tree medic, short spine gorse,
and black locust were the least abundant (Table 4).
Browsing
preference index (PBI)
The
main effect of browse species on BPI was significant (P < 0.001), but
the main effects of animal species, season, and nursery were not significant
(Pande 1990). The nursery by browse species and the animal species by nursery
by browse species interactions were
significant (P < 0.001; Pande 1990). For both goats and sheep the
greatest browsing preference, based on BPI, was for tree medic. Goats preferred
short spine gorse and then black locust after tree medic, whereas sheep preferred
black locust and then short spine gorse (Table
4). The two animal species were similar in their preference for the other
browse species (Table 4).
The
BPI gave different results for the preference of goats and sheep for some
browse species than the PBO due to the differences in abundance as measured by
RAV (Table 4). In particular, the PBO ranked gorse as the fifth most preferred
browse species by goats, yet due to its high RAV gorse was ranked seventh using
the BPI (Table 4). Similarly, black locust was the highest ranked browse
species for sheep using the PBO but was ranked third using the BPI (Table 4).
Overlap
coefficient (Cl)
The
overlap coefficients for the preferred browse species by goats and sheep were
0.79 (November) and 0.71 (February) in summer, 0.62 (March) and 0.64 (April) in
autumn, and 0.47 (May) and 0.58 (June) in winter, respectively. Hence, the
greatest overlap between goats and sheep was in summer and the least in winter.
DISCUSSION
The
results supported the view that goats are intermediate browsers and sheep are
intermediate grazers (Van Soest 1982). Nevertheless, when only the browsing
activity of the goats and sheep was considered there was a high degree of
overlap on the preferred browse species. Both animal species showed the
greatest preference for tree medic, short
spine gorse, and black locust according to the BPI. Although the overlap
coefficient (Cl)
for goats and sheep was high, the degree of overlap was seasonally dependent.
The availability of species such as black locust was lower in winter, and sheep
were observed to browse ceanothus more in winter which suggests that the
palatability of at least this species was seasonal.
The
use of the PBI enabled the preference of goats and sheep for the individual
browse species to be ranked independently of the differences in the abundance
of the browse species. Although the most preferred species were the least
abundant this appeared to be a coincidental result. The BPI, compared with
using the proportion of PBO, gave a different preference ranking to some browse
species, but the general trend was similar for both indices. The adjustment for
the abundance of a browse species (RAV) affected the preference ranking of
species like gorse and black locust, but largely demonstrated that tree medic,
despite its low abundance, was the most preferred species. The BPI would appear
to be a useful method for adjusting animal preference observations made on
mixtures of browse species of varying
abundance.
The
browse species could be readily placed into one of three preference groups of
high, moderate, and low preference for both animal species, although the
ranking of individual browse species showed some variation between goats and
sheep. The high preference group was tree medic, short spine gorse, and black
locust, the moderate group comprised tagasaste, ceanothus, and broom, and members
of the poor group were gorse, manuka, tauhinu, toetoe, and pampas. This ranking
relates well to the digestibility of the browse species, with the notable
exception of short spine gorse (Lambert et al. 1989b), but corresponds more
poorly, especially for sheep, with the rankings obtained from a cafeteria-style
indoor experiment (Lambert et al. 1989a).
Lambert
et al. (1989b) reported the in vivo digestibility of tree medic,
tagasaste, and black locust as being in the range 76–81%; ceanothus and broom
71–73%; short spine gorse, tauhinu, and gorse 63–67%; and manuka, toetoe, and
pampas 47–48%. The high preference for short spine gorse by both goats and
sheep in our experiment Pande et al.—Preference of goats and sheep for browse
species 101 suggested that digestibility was not the only factor affecting its
palatability.
Many
factors such as digestibility, nutrient content, smell, taste, and physical
properties (e.g., hairs, thorns) influence the palatability of plant species,
and animal preference is also affected by factors such as access to and the
availability of plant material (Crawley 1983). No attempt was made in this
experiment to directly determine the factors that influenced animal preference
though some observations can be made. The high digestibility of the most preferred browse species would have
provided positive post-ingestive consequences to the sheep and goats, thereby
reinforcing the animals’ preference (Bryant et al. 1991).
Additionally,
tree medic, black locust, and short spine gorse are all legumes with consequent
reasonable levels of protein. At least some of the browse species contained
potentially toxic secondary metabolites, with manuka known to contain alkaloid
and phenolic compounds (Corbett & McDowell 1958), but specific defence
chemicals have not been isolated. The differences in physical structure between
the browse species would have affected access to young leaves, and species such
as gorse are spiny on the mature stems.
The
high preference for gorse observed by Lambert et al. (1989a), as opposed to the
poor preference in our experiment, was probably the result of the chopping
process used by Lambert et al. (1989a) which may have improved the palatability
of gorse through, for example, a decrease in its spiny nature, and, perhaps,
poor access to young shoots of gorse in our experiment despite the relative
abundance of the plants. Thus, the comparison between our outdoor experiment
and the indoor experiment of Lambert et al. (1989a), both based on the same
browse nurseries, highlighted the need to examine as many of the factors involved
in animal preference as possible before conclusions can be drawn on the
relative ranking of plant species.
Lambert
et al. (1989a) concluded that goats had a greater preference for short spine
gorse, manuka, and ceanothus than sheep, and that sheep had a greater
preference than goats for broom and pampas. In our experiment goats also had a
higher preference for short spine gorse than sheep, whereas sheep had a greater
preference for black locust than goats. There were no significant differences
in the preference by goats and sheep for individual browse species outside the
three most highly preferred species, tree medic, short spine gorse, and black locust,
but some of the trends were similar to the results of Lambert et al. (1989a).
For example, sheep tended to show a greater preference for broom than goats,
and both sheep and goats exhibited a low preference for tauhinu, toetoe, and
pampas (cf. Lambert et al. 1989a).
One
explanation for the greater browse preference differences between goats and
sheep reported by Lambert et al. (1989a), compared with those reported here, is
that Lambert et al. (1989a) always
imposed choices between pairs of feeds, whereas in the present experiment all
11 browse species and types were always available. In the outdoor experiment
animals were observed to mainly browse the most preferred species, thereby making
it difficult to collect sufficient data on the least preferred species to detect
possible preference differences between goats and sheep for these browse
species. Lacher et al. (1982) also reported that when preferred browse species
were available, less preferred species were either rejected or rarely consumed
regardless of their abundance. Whether browsed by goats or sheep, a mixture of
the browse species examined would be difficult to maintain if it contained
highly and poorly preferred browse species. Low preference browse species would
seem unlikely to be browsed even if more abundant than the highly preferred
species.
ACKNOWLEDGMENTS
R.
S. Pande thanks Food and Agriculture Organisation and New Zealand Ministry of
External Relations and Trade for financial support. The support of AgResearch in
allowing the use of browse nurseries and experimental animals at Ballantrae
Hill Station is gratefully acknowledged.
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