Information and Advisory Note Number 47 Back to menu
Grazing by large herbivores is a major factor determining the structure and species composition of upland ecosystems. The manipulation of grazing regimes by land managers can thus be a powerful tool in the management of upland habitats. To predict the effects of unmanaged, or managed, grazing at a particular site, an understanding is needed of the foraging behaviour, diet selection and intake of the large herbivore species found in the Scottish uplands.
2 1 Upland sites are not uniform but consist of a mosaic of vegetation types
differing in altitude and aspect Table 1 lists the most common broad categories
of vegetation found in the uplands which are of relevance to the understanding
of foraging behaviour of large herbivores. Each of the vegetation categories
listed in Table 1 represents a food resource for grazing animals and each has a
feeding value which not only differs from that of the other vegetation
categories but also vanes throughout the year.
2.2 Feeding value of a vegetation type has three components the nutritional content of the forage, the ease with which this can be extracted by digestion and the ease with which it can be grazed. The attributes of vegetation which affect feeding value include-
2.3 Digestibility is the proportion of the dry
weight of the plant material which can be converted into energy or animal tissue
Fig 1 shows the seasonal variation in the digestibility of some of the commonest
upland plant species to sheep. Table 2 lists the attributes of the commonest
plant types which affect their feeding value.
2 4 Foraging behaviour is determined not only by the attributes of the
vegetation but also by the attributes of the grazing animals. The large
herbivores most common in the uplands are sheep, red deer, cattle, goats,
hares and rabbits. Horses and ponies are also occasionally found
Table 3 lists the most
important differences in the attributes of these species which affect their
grazing behaviour.
2 5 Range use by large herbivores is determined by a number of factors including
feeding value of different vegetation patches, shelter from wind or ram, social
behaviour and human disturbance or management. Of these, the first is probably
the most important factor. In general, large herbivores spend most time on
vegetation types which have the highest feeding value.
2 6 In practice, the other factors influencing herbivore movements generally
cause them to be distributed over all vegetation types but with a bias towards
those which give the highest nutritional return. The differences in the grazing
attributes of each large herbivore, (Table 3) together with differences in the
other factors determining movement, lead to each using its range in a different
way.
2 7 Of all the upland large herbivore species, most is known about the foraging
behaviour of sheep. Many of the principles concerned with the impact of feeding
value on seasonal foraging preferences are therefore best illustrated with
reference to sheep but apply equally well to other herbivore species.
2.8 Sheep typically group together in flocks of about one hundred animals. The
flock stays largely within its home range which is often bounded by topographic
features such as streams or mountain ridges. Within the flock, there are
sub-groups of ewes frequently made up of related animals. Each of these
sub-groups occupies a smaller home range, or heft, typically where they were born
and reared The heft will normally include a range of vegetation types. The
individuals which occupy a heft will tend to spread out while foraging The
combined hefts will cover most, or all, of the home range of the flock so the
flock will normally be spread out over its whole home range.
2.9 Super-imposed on the behaviours described above is a range of others. During
the day, sheep will tend to graze on favoured vegetation types, often at the
foot of the hill. At night-time they have a tendency to move uphill. This is
thought to be an evolutionary response to the threat of predation In summer,
sheep may move uphill to avoid flies, to cool down or to exploit later growth of
grasses at higher altitudes. In winter they will use walls or woods to shelter
from high winds. Shepherding is rarely earned out these days but, when it is,
sheep are normally herded up the hill daily to encourage the sheep to spread
their grazing pressure more evenly. Sheep congregate around winter feeding sites,
where these are provided.
2 10 The need for sheep to find the most rewarding food supply leads to seasonal
changes in the grazing pressure on different vegetation types. The graphs shown
in Figs 2a, b have been generated by the Macaulay Land Use Research Institute
hill grazing management model. This model gives site specific predictions of
sheep foraging behaviour and seasonal intake from each vegetation type Figs 2a,
b show how the amount of each plant type in the diet of a sheep might vary
seasonally at a hypothetical site composition of a range of different vegetation
types (Table 4). This is just one example which serves to illustrate some
general principles and should not be taken as typical since the area of each
vegetation type available to the sheep, and the potential growth of each
vegetation type, has a major impact on seasonal preferences and therefore on
diet composition.
2 11 Fig. 2a shows how intake is likely to be distributed between the vegetation
types when the stocking rate is low (1 ewe ha-1). In May, both species-rich
bent/fescue and reseeded grassland are highly preferred Although the reseeded
grassland has a higher digestibility, there is less of it at the site and it is
quickly grazed to a short sward. Some species-poor bent/fescue is also eaten
because it has a relatively high digestibility. The main growing season for purple
moor-grass is later in the year so it is not eaten in May because there is still
very little live material. Heather is not eaten because its digestibility is too
low. As summer progresses, the new growth of purple moor-grass becomes abundant
and the amount in the diet increases until August, when the purple moor-grass
starts to die off.
2 12 Over summer/early winter the proportion of reseeded grass in the diet
decreases as the sward becomes progressively shorter. The sheep move first on to
the species-rich bent/fescue and then, as this is also used up, on to the
species-poor bent/fescue By December, the reduced digestibility and standing
biomass of the grasses causes the sheep to begin eating heather The
digestibility of the diet is sufficiently low by this time that total intake
falls significantly (see 4.1). By January, heather is the second largest
component in the diet. In February and March, the proportion of grass in the diet
starts to increase again with the limited new growth at this time and as dead
material is removed by grazing. In April, species-poor bent/fescue briefly
becomes the most preferred vegetation type because, although both it, and the
other evergreen grass types, have started their spring growth, the latter are
still short after heavy grazing the previous year.
2 13 If the stocking rate is doubled to 2 ewes ha-1 (Fig. 2b), a similar pattern
is seen
but the sheep move on to the vegetation types which are less digestible, but
have a higher standing biomass, earlier in the year because the heavier grazing
rates deplete each vegetation type in turn more quickly.
2 14 The percentage of the total annual production of vegetation removed by
grazing from each of the vegetation types (the utilisation rate) at both
stocking rates is shown in Table 5. There is no direct relationship between
stocking rate and utilisation rate. Differences in the attractiveness of each of
the heather types (see 2 20 and 2.21) lead to the different utilisation rates
shown in Table 5. In general, however, all heather types have a very similar
attractiveness compared with other vegetation types. This means that, if there is
no pioneer heather, the intake from
the other heather types will increase to compensate
2 15 If sheep run out of good quality grass and are forced onto poor vegetation
during summer, their body condition will suffer and they will need to be given
supplementary feed to enable them to survive the winter. In the example described
above, total annual intake per ewe of digestible dry matter is predicted by the
model to drop from 276 kg to 241 kg when the stocking rate is doubled.
2.16 If sheep are kept at the maximum density compatible with ensuring adequate
body condition at the end of the winter, without supplementary feed, they will
be more likely to graze heather heavily if the area of heather relative to that
of good grass is small. This is because the density of sheep which can be
supported without supplementary feed is higher if there is a large proportion of
good grassland at the site. When the sheep start eating heather, the small area
of heather present receives very heavy grazing.
2 17 Figs. 3a, b illustrate this point by showing likely utilisation rates
(generated by running the MLURI hill grazing management model) of species-rich
bent/fescue and of heather as sheep stocking rate increases at two sites with
different proportions of grass and heather. The sheep are assumed to weigh 50
kg. The annual intake of digestible material by an average sheep is also shown.
As stocking rate increases, the sheep can, at first, eat as much grass as they
need, but eventually they are forced to increase their intake of heather as the
grass sward becomes very short At this point the annual intake of digestible
material starts to decline. The minimum requirement for a 50 kg sheep to
maintain body condition is about 230 kg digestible dry matter per year The
dotted lines in Figs 3a, b show the stocking and utilisation rates at
which this intake level is achieved. When there is a small proportion of heather
relative to grass (Fig 3a), the utilisation rate on the heather is significantly
higher than when the proportion is high (Fig 3b)
2 18 If small amounts of supplementary feed are given, the digestibility of the
diet will rise. This will increase the sheep's capacity to consume more forage and
it is possible that even more heather will be eaten. However, if sufficient
supplementary feed is given to cover most of the nutritional needs of the sheep,
less heather will be eaten.
219 Heather which grows at the edge of an area of preferred grass will be more
heavily grazed than that which is in the middle of a large patch. This may be
because sheep try out other, easily accessible, vegetation types whilst
concentrating on the most preferred. In this way, areas of heather can decline,
not because of overall heavy grazing pressure but because of a gradual decline
from the edges.
2.20 Sheep show strong preference for newly burnt heather over the older age
classes (listed in Table 1) for three possible reasons; the relative ease of
access to the new shoots of heather, the new growth of grass which comes up
alongside the re-sprouting heather and the higher nitrogen content of the
heather shoots. Pioneer heather is preferred over the older age classes, probably
also because of the relative ease of access. Burning heather in small patches
spread throughout the
heather areas will help to spread grazing pressure more evenly over the hill.
2 21 Heather growing in blanket bog or wet heath tends to be less heavily grazed
than that of the pioneer or building phases of dry heath because sheep do not
like getting their feet wet. However, if sheep have no other option they will
graze both these vegetation types but with a preference for wet heath over
blanket bog. Climatically suppressed heather tends to be avoided partly because
of the exposed locations in which it is found and possibly also because the
current season's shoots are very short and dense and hard to graze in any
quantity.
2 22 Red deer show similar foraging preferences to sheep however they have a
higher propensity to browse dwarf shrub, shrub and tree species. This is partly
because of competition between sheep and red deer when they share the same range
(see 2 31 below) and partly because their larger guts allow them to digest rough
vegetation better.
2 23 Red deer range over much larger areas than sheep and are often segregated
into hind and stag groups (see 2 31 for a possible explanation of this). Stags
tend to wander over a wider area than hinds. Stags taking part in the autumn rut
spend very little time feeding.
2 24 Red deer will often congregate at high density in sheltered wintering areas.
This is exacerbated by the common practice of supplementary feeding stags in
winter. Red deer are woodland animals by preference and will make use of any
available woodland for shelter and forage.
2 25 Cattle cannot graze short grassland efficiently but, because of their
larger gut size, have a greater ability than sheep or red deer to digest poor
quality forage. As a result they are more likely to move onto mat-grass or purple
moor-grass in summer. Most breeds of cattle grazed in the hills currently have to
be taken off in winter. Hardy breeds can be left on all year but will normally
need supplementary feeding in winter. Cattle need to have a water supply and will
often concentrate around it on hot days.
2 26 Cattle tend to move around their range as a herd. Although they are usually
not found on very steep slopes, or above about 600 m, they will move off the
more nutritious vegetation and use their range as fully as do
sheep if they are on the hill for enough of the year to become familiar with all
of it
2 27 Domestic goats are usually confined to lower, enclosed pastures but there
are several local populations of feral goats in the Scottish hills. When wet,
goats lose heat rapidly. They are therefore restricted to areas with rocky
outcrops or caves where they can shelter from rain. Survival of kids is low in
Scotland. This, together with the limited availability of shelter, means that
most populations are not increasing. Goats, like sheep, live in family groups of
females each with its own territory. Males operate singly or in peer groups. The
forage preferences of goats are similar to those of red deer but goats have a
slightly greater ability to eat low quality forage. Due to their greater agility,
they are also able to exploit vegetation on cliff ledges and in boulder fields
to a greater extent than can red deer or sheep.
2 28 Mountain hares feed largely at night and rest during the day uphill in long
heather. They feed mostly in heather-dominated vegetation, preferring stands of
younger, pioneer heather. They select strongly for grasses growing amongst the
heather, resulting in a diet of less than half heather m summer and almost
totally heather in winter when these grasses are largely dead or have been
removed by grazing. They are generally found higher up the hill than brown hares
which feed largely on grassland.
2.29 Rabbits can occur in large numbers anywhere in the uplands (except at high
altitudes) where there is abundant good grassland and either scrub cover or
sandy soils for burrowing in. They have small mouths, can be very selective and
have a distinct preference for short, high quality grasslands. They will eat
heather if there is little grass available. The grazing pressure of rabbits is at
its highest close to warrens and adjacent to areas of scrub used for cover.
2 30 Horses and ponies are found only rarely in the uplands. They tend to graze
in mixed sex groups. Unlike ruminants, which have a four-chambered stomach, they
have only one stomach. This means that food stays in the digestive tract for less
time and digestion is less efficient. Ruminants also ruminate, or chew the cud,
thereby breaking down food into smaller particles which are easier to digest. The
process of rumination makes digestion more effective but reduces the time
available for feeding. The digestive
system of horses and pomes therefore allows them to process food quickly, and in
large quantities, but does not extract as much of the nutrients from their food
as does that of ruminants. However, with their upper as well as lower incisors
(Table 3) they can crop grass swards shorter than can most ruminants. Horses and
ponies can therefore consume large amounts of food quickly and can more
effectively graze very short swards As a result, horses and ponies have a
greater tendency to concentrate on high quality grasslands than would be
expected from their body size If only poor quality forage is available, they
require large quantities.
2 31 Interactions between grazing species
can be positive or negative. All species compete for the available vegetation.
Smaller herbivores, with smaller mouths are able, however, to graze grass swards
to a shorter sward height and still gain the daily intake of digestible material
that they need. The larger mouth sizes of larger herbivores restrict the
efficiency with which they can graze short swards. Large herbivores also have a
higher daily intake requirement which they are unable to satisfy from short
swards. In theory, larger herbivores will move off grasslands before smaller ones
as the grass sward becomes shorter. In that sense, small herbivores can 'outcompete'
larger ones. Thus, in theory, stags move off good grasslands before hinds, deer
before sheep and sheep before rabbits. Large herbivores are, however, better at
digesting rougher vegetation and can survive where vegetation quality is too low
to support smaller herbivores. Cattle can graze mat-grass or purple moor-grass
to a level which allows more nutritious grass species to increase in abundance,
thus increasing the overall livestock productivity of the hill.
2 32 Horses and ponies go against this trend in that, despite their large size,
they can graze short swards effectively because they have both upper and lower
incisors. They are thus able to 'outcompete' cattle, goats, sheep and red deer on
good quality grasslands.
2.33 Sheep are thought to disturb the feeding of mountain hares. They also
compete directly with hares for food and may cause a reduction in hare numbers.
3.1 Just as grazing pressure at the landscape
level is not uniform, so grazing pressure within a vegetation type is also not
uniform. If a vegetation type contains a mix of plant species and a mix of live
and dead material, an animal will select the most nutritious diet permitted by
the size and shape of its mouth in relation to the size and distinctness of the
different components of the vegetation. Selective ability may also be limited by
the amount of time the herbivore is prepared to spend on one bite.
3.2 Most herbivores grazing grass vegetation types containing a high proportion
of dead material are capable of selecting a diet which contains a lower
proportion of dead material than is in the sward. The ability to select live
material depends on the size and shape of the herbivore's mouth (Table 3) and on
the distribution of live and dead material in the sward. As the proportion of
dead material builds up in a sward, an animal's ability to select live material
declines and the digestibility of the diet also declines.
3 3 At low densities, animals grazing on good grass concentrate on a few
patches, thereby creating a mosaic of patches of different sward height. By doing
this, they ensure that they always have access to patches where there is no
build-up of dead material and digestibility remains high.
3.4 Many plant species can have a variety of growth forms depending on
environmental conditions and grazing history. Selection, or avoidance, by a
herbivore, of a particular plant species can depend on the plant's growth form
in a particular patch. Thus sheep generally avoid mat-grass tussocks, but if the
mat-grass is growing as small tufts, rather than as tussocks, as part of a
heavily grazed bent/fescue sward, they cannot avoid it. The mat-grass is then
eaten along with the other components of the sward If, however, grazing pressure
is relaxed and the sward becomes longer, sheep can avoid the mat-grass and
tussocks may form. These will then be avoided thereafter. At high grazing
pressures, sheep will be forced to graze the inter-tussock vegetation so close
to the mat-grass tussocks that they will be forced to eat some of the mat-grass.
3.5 Sheep seen grazing on heather-dominated vegetation will often be eating
grasses which are growing amongst the heather rather than the heather itself,
although some heather will be grazed in passing If, however, the heather is
scattered through a
short grass sward and is the same height as the sward, sheep will not be able to
avoid it and it will be grazed along with the grass.
4 1 The amount that a herbivore can eat when there is unlimited food available
is directly related to the digestibility of the diet and the animal's size. When
the digestibility is low, the gut takes longer to digest the food so less can be
processed. Large animals have larger guts so can process more food. Intake is
linearly related to metabolic live weight (live weight raised to the power of
9,75). The following equation has been derived for sheep.
Intake (kg dry matter per kg of metabolic live weight) = 0.167 x digestibility
(proportion) -0.044
As a crude approximation, this equation can be used for other animal species to
compare intakes Table 6 lists average live weights of different herbivore
species and breeds and predicted intakes for summer and winter.
4 2 If approximate animal numbers are known, these intake figures can be used to estimate the likely relative magnitude of the impact of grazing by a range of different herbivore species on a patch of vegetation
Armstrong, H M (1993) The MLURI hill grazing model using a computer to help set
stocking rates ENACT, 1(4), 7-9
The MLURI hill grazing management model is available from Ann Malcolm, Macaulay
Land Use Research Institute, Aberdeen AB15 8QH It comes with a manual and
tutorial and costs £30
Helen Armstrong
Comments were kindly provided by lain Gordon of the Macaulay Land Use Research Institute, Tony Waterhouse of the Scottish Agricultural College and Nigel Smith and Barbara Hogarth of SNH.
Helen Armstrong/Angus MacDonald
Scottish Natural Heritage
2 Anderson Place
EDINBURGH EH6 5NP
Tel 0131-447 4784