Forage quality
Different plant species have different nutritional qualities. However, forage quality within species varies through the year, generally being highest in late spring /early summer, and then declining. Where there is a lot of dead material within a sward, quality is reduced. Grazing in one year alters forage quality in the next since:
'..heavy grazing of grass and herb swards increases the proportion of young leaves and decreases the proportion of old leaves and dead leaf material. This increases the average digestibility of the sward since dead plant material generally is less than half as digestible as live, green material..'
From: A Guide to Upland Habitats: Surveying Land Management Impacts (MacDonald et al, 1998).
The productivity of woodland and open ground habitats
Forage quantity, or productivity, is a function of the speed with which plants grow. This varies widely according to habitat. Tables 1and 2 below show a range of productivities for woodland and open ground habitats.
Productivity in the tables is measured in terms of 'Dry matter production'. This is a measure of the dry weight of material produced, in this case, over a hectare in the course of a year.
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Table 1: Woodland habitats (for closed to intermediate canopy cover) |
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Habitat Action Plan habitat |
Woodland type |
Estimated dry matter production (tonnes ha-1 year -1) |
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Native pine woodlands |
Acidic dry (with Scots pine) |
0.1 - 0.3 |
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Upland oakwood |
Acidic dry (with no or very occasional Scots pine) |
0.1 - 0.3 |
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Upland oakwood |
Neutral dry |
0.3 - 0.5 |
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Upland birchwoods |
Acidic dry (with no or very occasional Scots pine) |
0.3 |
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Upland birchwoods |
Neutral dry |
0.3 - 0.5 |
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Upland birchwoods |
Acidic wet |
0.2 |
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Lowland and mixed deciduous woodland |
Acidic dry |
0.3 |
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Lowland and mixed deciduous woodland |
Lowland dry |
0.3 - 0.5 |
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Upland mixed ashwoods |
Base-rich dry |
0.5 - 1.0 |
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Wet woodland |
Acidic wet |
0.1 - 0.3 |
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Wet woodland |
Neutral to base-rich wet |
0.5 - 1.0 |
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Lowland wood pasture and parkland |
Refer to relevant open ground habitat estimate (Table 1) |
n/a |
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Lowland wood pasture and parkland |
Regenerating woodland (scrub) |
0.5 - 1.0 |
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Table 2: Open ground habitats (adapted from Waterhouse, Graham and Blaikie 1999) |
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Habitat |
Estimated average dry matter production |
Normal range in dry matter production (tonnes ha-1 year -1) |
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Neutral base rich and semi-improved grassland |
5.0 |
3.0 - 6.0 |
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Acid grassland |
4.0 |
3.0 - 5.0 |
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Molinia grassland |
2.0 |
2.0 - 4.0 |
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Rush pasture |
2.0 |
1.0 - 3.0 |
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Heath |
2.0 |
1.0 - 3.0 |
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Mire |
0.4 |
0.1- 3.01 |
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Bracken (ignoring bracken production) |
0.3 |
0.1 - 1.02 |
1The productivity of a mire will depend on the relative abundance of the various plant species that make up the mire. For example, if it is a mire with an abundance of Molinia or heath species, productivity will be higher than if it is dominated by mosses.
2The productivity of bracken will depend on whether the understorey is dominated by bracken litter, by grasses and spring-flowering herbs or by heath species and Molinia.
More on the range of values of annual dry matter production of ground vegetation.
In woodland habitats, growth is constrained by the shade cast by canopy trees, for example, woodland with rich soils and light shading, such as open hazel woodland, will have far greater quantities of forage available than those with poor soils and heavy shading, such as closed-canopy, young birch woodland on poor, acidic soils (see table 3 below).
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Table 3: The effect of canopy shading on forage availability |
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Canopy shading |
Woodland type |
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Acidic dry |
Neutral dry |
Base-rich dry |
Wet acidic |
Wet neutral to base-rich |
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Heavy |
Very low to nearly none |
Low |
Low |
Very Low |
Low |
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Intermediate |
Very low |
Low to medium |
Low to medium |
Medium |
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|
Light |
Medium |
High |
High to very high |
Low |
High to very high |
The productivity of habitat mosaics will reflect the productivity of the habitats making up the mosaics. For example, a woodland with an unbroken and well-developed canopy will have lower productivity than one with a broken canopy and grassy glades. Equally, open ground habitat which is a mosaic of semi-improved grassland and bracken will have a significantly lower productivity than one which is just grassland.
References
Grant, S.A. & Campbell, D. R. (1978) Seasonal variations in in-vitro digestibility and structural carbohydrate content of some commonly grazed plants of blanket bog. Journal of British Grassland Society 33, 167-173.
Grime, J.P., Hodgson, J. G. & Hunt, R. (1990) The Abridged Comparative Plant Ecology. Chapman & Hall, London.
MacDonald, A.M., Stevens, P., Armstrong, H.M, Immirzi, P. & Reynolds, P. (1998). A Guide to Upland Habitats. Surveying Land Management Impacts. Scottish Natural Heritage, Battleby.
Odeyinka, S.M. & Ørskov, E. R. (2006) Nutritive evaluation of some trees and browse species from Scotland. European Journal of Scientific Research 14, 311-318.
Pollock, M.L., Legg, C. J., Holland, J. P. & Theobald, C. M. (2007) Assessment of expert opinion: Seasonal sheep preference and plant response to grazing. Rangeland Ecology & Management 60, 125-135.
Torvell,L., Common, T. G. & Grant, S. A. (1988) Ecological change in the uplands. (eds M.B.Usher & D.B.A.Thompson), pp. 219-222. British Ecological Society.
Waterhouse,A., Graham, R. & Baikie, G. (1999) Woodland Grazing in Speyside. Forest of Spey Grazing Recommendations.
Welch,D. (1998) Response of billbery Vaccinium myrtillus L. stands in the Derbyshire peak district to sheep grazing, and implications for moorland conservation. Biological Conservation 83, 155-164.