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The Grazing Behaviour of Large Herbivores in the Uplands 1 Introduction

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The Grazing Behaviour of Large Herbivores in the Uplands 1 Introduction Information and Advisory Note Number 47 The grazing behaviour of large herbivores in the uplands 1 Introduction 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 Foraging behaviour 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- the proportion of live and dead material available to the animal in a bite, the digestibility of live and dead plant material, the presence of chemicals which are hard to break down in the gut, the content of mineral nutrients, the size of bite possible, the rate at which biting can occur, the presence of spines or thorns, and the presence of silicates in the vegetation which make it harder to bite through 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.
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