Daily intake and the selection of feeding sites by horses in heterogeneous wet grasslands
Daily intake and the selection of feeding sites by horses in heterogeneous wet grasslands
FLEURANCE 0 2
k DUNCAN 2
Bruno MALLEVAUD 1
0 Present address: Les Haras Nationaux, Direction du Développement, Station Expérimentale des Haras , 19370 Chamberet , France
1 Parc Interrégional du Marais Poitevin, place de l'Église , 79510 Coulon , France
2 Centre d'Études Biologiques de Chizé , CNRS-UPR 1934, 79360 Beauvoir-sur-Niort , France
- In spite of the importance of grazed forage in horse nutrition, little information is available on their daily intake at pasture. We determined the intake of 4 non-breeding mares of a heavy breed (average body weight = 674 kg), grazing during the summer in heterogeneous natural grasslands of the Marais Poitevin (France), an internationally important wetland where grazing is an essential process which maintains biodiversity. The mares ate large quantities of forage (21.9 ± 2.4 kg of organic matter per day, i.e. 166.2 ± 20.8 g of organic matter per kg LW0.75 per day) in comparison with previous published values and with the estimated requirements of these horses. The use of the vegetation was very selective, the mares spent about 70% of their feeding time on short grass lawns (sward surface £ 4 cm, biomass < 100 g.m-2), that represented only 10% of the area. This behaviour maintained the plants at young growing stages which are of better quality than ungrazed plants. These results are discussed in relation to the dynamics of the plant communities.
maintenait les plantes dans de jeunes stades de croissance, de meilleure qualité que celle d’une
végétation non pâturée. Les résultats sont discutés en rapport aux dynamiques des groupements
végétaux présents sur le site.
cheval / ingestion / prairie / hétérogénéité / zone humide
Intake rates by horses of green or dried
forages distributed ad libitum are well
2, 4, 5, 18
]. In spite of the
importance of grazed forages in horse nutrition
(these can represent more than 70% of the
annual food, ), little information is
available on intake at pasture , principally
because in the field it is difficult to estimate
faecal output and diet digestibility
accurately. Rittenhouse et al. , using neutral
detergent fiber (NDF) as an external marker
for non productive and lactating mares in a
natural grassland, measured intakes of 113
and 138 g DM.LW–0.75.d–1 respectively.
Nonetheless, the reliability of this method,
and of others based on external markers such
as chromic oxid (e.g. in yearling horses,
80 g DM.LW–0.75.d–1) [
] is debatable [
]. Mesochina et al. , in a recent
study, demonstrated that fecal crude protein
is a good predictor of the digestibility of
forages by horses (dry matter digestibility:
rsd: 0.038; r2 = 0.65; organic matter
digestibility: rsd: 0.036, r2 = 0.74). Duncan  used
this approach to determine the intake of
lactating mares of the Camargue breed (intake:
170 g DM.LW–0.75.d–1).
This study was part of a long term
programme of research on the effects of different
herbivores on the dynamics of the vegetation
in a wetland of international importance,
the Marais Poitevin. The site was in natural
wet grasslands of the Communal des
Magnils-Reigniers, which is of importance for
the conservation of wetland plants (e.g.
Ranunculus ophioglossus), birds (e.g.
breeding Tringa totanus) and mammals (e.g.
otters, Lutra lutra), and has traditionally
been grazed by cattle. The experimental
set-up consisted of 10 plots of 1–2 ha which
were grazed by cattle, horses or both species.
Under cattle at densities comprised between
341 and 1246 kg.ha–1, the plant community
dynamics depended on the biomass density
of the animals : with high densities there
are many rosette plants (hemicryptophytes),
while at low animal densities these plants
are replaced by competitive species (sensu
Grime , mainly perennial graminoids).
Horse grazing lead heterogeneous plant
communities, as the two dynamics were
observed whatever the density of the
animals (345 to 900 kg.ha–1). Tall grasses
appear in patches of lightly grazed plants,
where the animals deposit faeces. This leads
to a loss of plant species diversity in the tall
grass patches, but an increase in structural
diversity. Elsewhere, in heavily grazed
lawns, halophytes and hemicryptophytes
The aim of this work was first to provide
data on the rates of daily food intake in the
horses on these pastures and then to
contribute to this programme by describing the
use of these grasslands by horses. We
measured the time-budgets of the horses and
their selective use of structural types (lawns
and patches of tall grasses) and of the plant
2. MATERIALS AND METHODS
2.1. Study site
The Magnils-Reigniers common is part of
an area of brackish grasslands where the
topography leads to a specific pattern of
plant communities . The shallow natural
drainage channels, which are flooded from
late autumn to early spring, are
characterized by hygrophile vegetation with
subaquatic species (Tab. I). The top of the
catena is occupied by graminoids, forming
the mesophile community. The slopes,
whose soils are saline, are favourable for
sub-halophile species, forming the
We defined two structural vegetation
types, which were represented in all
communities, lawns (sward surface £ 4 cm) and
taller grasses (mostly at the reproductive
stage during the study) where the horses
deposited all their faeces.
2.2. The animals
We studied 4 non-reproductive mares
(Mulassier Poitevin breed), 2 to 7 years old.
They were distributed in 2 sets,
homogeneous for weight, each grazing a 2 ha pasture
(E2D or E2) from April to October 1997.
The mean weight of the animals was 674 kg
during July and August.
2.3.1. Daily food intake
The daily food intake was measured as
OMI = F/(1-OMD) where OMI is the organic
matter intake, F is the weight of faeces
produced over 24 h (g) and OMD is the organic
matter digestibility estimated by the crude
protein (CP) content of the faeces . The
results are expressed in organic matter (OM)
in order to avoid inaccuracies due to
contamination by soil. Faecal output was
measured by collecting the total amount of faeces
produced 3 times a day over 4 successive
days (29.VII.97 to 01.VIII.97) [
dry and organic matter content were
measured in one sample per plot per day, dried at
80 °C to constant weight and by
mineralization at 550 °C. Nitrogen was determined by
the Kjeldahl method. The results were
corrected for the mean weight of the animals,
expressed as live or metabolic weight.
2.3.2. Spatio-temporal organization
of the feeding behaviour
The mares were observed for a total of
48 h, between 21.VII-08.VIII.97.
Individual behaviours were recorded using scan
sampling during periods of 3 h by day and
by night in order to cover the 24 h in 4 days.
The 4 mares, previously accustomed to the
presence of the observer in the pasture and
to the use of a lamp, were checked
successively by the same person at 5 minute
An animal was recorded in feeding
activity if one of these actions was observed:
searching (sorting or gripping food with the
upper-lip), cutting, chewing or swallowing.
The structural type and the community
grazed were noted. In each plot, 100
measures of the sward surface were made (50 on
each diagonal, with a measure every 4 m,
on 21.VII.97, 28.VII.97, 06.VIII.97) using a
50 · 50 cm sward stick. At each measure,
the structural type and the community were
recorded. The mean of the 3 measurements
enabled us to determine the area covered by
each community and structural type in each
plot, and to calculate for each plot a selection
index S for the 2 mares of the pasture (the
ratio of the percentage of the feeding time
passed on the particular structural type or
community, and the percentage of the area
covered by that structural type or
community). The null hypothesis was that the
animals used the vegetation types evenly, i.e.
S = 1.0.
2.3.3. Above-ground plant biomass,
and forage quality
In each plot, samples (Tab. I) were clipped
with shears in a rectangle (50 · 25 cm)
placed at random in each structual type of
each community (mesophile,
meso-hygrophile, hygrophile). The samples were dried
at 60 °C to constant weight to calculate the
plant biomass (dry weight/area clipped).
Some samples were analyzed for nitrogen
(Kjeldahl method), cellulose (Weende
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method) and cell walls: Neutral Detergent
Fiber (NDF) .
The significance of selection for
vegetation types was determined by chi-square
tests, by comparing the numbers of feeding
observations with the values expected if the
animals used the vegetation types evenly.
Kruskal-Wallis and Mann-Whitney tests
were used to compare plant biomass and
grass quality respectively, between plant
communities (across structural types) and
between structural types (across plant
Statistical tests were bilateral, for p < 0.05
using Statistica .
3.1. Daily food intake at pasture
The mares in both pastures selected diet of
good quality (estimated organic matter
digestibility: 61.8 ± 0.8%) with little
variability. Intake was on average 21.9 kg OM.
kg LW–0.75.d–1, i.e. 3.3% of liveweight, i.e.
166.2 ± 20.8 g OM.kg LW–0.75.d–1. The
mean value in terms of dry matter was 172.5
± 23.1g DM.kg LW–0.75.d–1. The digestible
organic matter intake (nutrient assimilation)
was 102.8 ± 13.8 g DOM.kg LW–0.75.d–1.
3.2. Spatio-temporal organization of the feeding behaviour
The mares fed on average 60 ± 1% of the
24 h. This activity occupied about 3/4 of the
night (74 ± 4%) and half of the daylight
(54 ± 1%). The use of vegetation
communities was not homogeneous (chi-square test,
p < 0.01). The mares selected the
mesohygrophile community, using this 2.5 times
as much as expected from the hypothesis of
even use and they avoided the mesophile
community (Tab. II).
At the time of the study, the structural
heterogeneity of the cover was not very
pronounced in the pasture E2, with small
patches of lawns beginning to appear. The
use of the structural types by the mares was
measured only in the E2D pasture. Here the
mares fed on both the structural types, but
the use of the lawns was 7 times as much
as expected (chi-square test, p < 0.01), their
selectivity index being 20 times that on the
taller grass. The preference for the lawns is
verified in each community (M, MH, H),
where the percentage of the area covered
by the lawns were respectively: 9.4%,
37.5%, 10.7% and the selection index were
6.24, 2.58, 7.15 for the lawns and 0.36, 0.05,
0.36 for the tall grasses (chi-square test,
p < 0.01).
3.3. Aerial biomass and vegetation quality
The meso-hygrophile community, which
was selected by the mares, had the least
biomass (p < 0.05) (Tab. I), but the
difference was not significant within pastures
because the variances are high. The mean
biomass of the patches of tall grasses was
5 times higher than the biomass of the lawns
(p < 0.01, Tab. I).
There were no significant difference
between the plant communities for crude
protein, cellulose and NDF contents (Tab. I).
Nonetheless, the lawns which were strongly
selected by the mares were characterized
by a crude protein content 38% higher than
the tall grasses. The cellulose and NDF
contents tended to be lower for lawns (p < 0.06)
4.1. Daily intake
The daily food intake found here (172.5 ±
23.1 g DM.kg LW–0.75.d–1 or 166.2 ± 20.8 g
OM.kg LW–0.75.d–1) is much higher than
the few values available for non-productive
horses at pasture (e.g. 113 g DM.kg LW–0.75.d–1
from Rittenhouse et al. ) and higher than
those of Mesochina  and Ellis et
Lawrence  for growing saddle horses
(92 g OM.kg LW–0.75.d–1 and 80 g DM.
kg LW–0.75.d–1). The values obtained in the
Marais Poitevin are comparable to the
values for breeding and growing wild equid
2, 11, 21
] and for lactating mares
(e.g. in the Camargue, France ).
Nonetheless, even if the needs for production seem
to be the main determinant of food intake
], Meyer  has shown that horses can
have intakes higher than their needs, and
maintain these for a long time. These intakes
are also much higher than those of
comparable beef cattle at pasture (3.3% of
liveweight in horses compared with 2–3% in
cattle (Dulphy et al. ).
In view of the fact that there are
considerable inter-individual variations in intake in
some herbivores [6, 23], and the small
number of animals used in this study, the
measures should be repeated. Nonetheless the
slight variations between days on food intake
(coefficient of variation: 12%) show that
data from four days allow estimation of daily
intake for horses on these pastures with
reasonable accuracy. An obvious improvement
would be to individualise the faeces with
colored plastic marks.
4.2. Duration of feeding activity
The time spent feeding, 60 ± 1% (or
14 h 30) is consistent with previous
published values (14 h–17 h, ) and the
animals in the experimental plots appeared to
have normal feeding behaviour. Nocturnal
feeding comprised 38% of the total time
spent feeding, which is typical for horses
(20 to 50%, ), which is why it was
necessary to conduct 24 h observations in this
study of their feeding strategies.
4.3. Selection of vegetation types
The horses showed more marked
selection for lawns than for any of the
vegetation communities, as has been observed
previously in dry grasslands [
3, 9, 22
mares maintained lawns where the quality of
the grass was high in comparison with the
remainder of the pasture (38% higher crude
protein, lower fibre). In this pasture, the
horses choose their food plants principally
on the basis of their structure (growth stage)
rather than their botanical species.
In spite of the short period covered by
this study, the feeding strategies of the mares
described here clarify causes of the
differences in the vegetation dynamics under
cattle or horse grazing described by Amiaud
. The intense use of the meso-hygrophile
community by horses explains the
rejuvenation of sub-halophyte communities and
the development of hemicryptophytes on
the slopes. In the same way, the opening of
the hygrophile community leads to the
emergence of annual species (Galium debile,
Ranunculus sardous) . The development
of highly competitive species at the top of
the catena seems to be the result of low
grazing pressure and perhaps favourable
conditions of mineral and nitrogen availability
brought by the faeces.
Finally, the high intake levels compared
with cattle and strong selection for short
grass lawns explain why the horses create
and maintain structural diversity in such
grasslands, whatever their stocking level.
The current hypothesis to explain the
heterogeneous use of grasslands by horses
proposes that the animals minimize their
feeding time on patches of tall grasses where
the faeces are deposited in order to
minimize the intake of gastro-intestinal parasites
. The results of this study suggest
another hypothesis: if the feeding strategy of
these horses is to maximise daily nutrient
assimilation, the maintenance of lawns of
good quality could be a nutritional as well as
an antiparasite tactic.
This work was funded by the Parc
Interrégional du Marais Poitevin. We wish to thank
D. Naudon for his support of the project and for
his help in the collection of the data. We are also
grateful to J.B. Bouzillé (CNRS-University of
Rennes I UMR 5558) for including us in this
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