In central Europe grassland occupies about 35–40% of the utilised agricultural area. Grassland has a large potential for the production of plant biomass, partly because the leaf area index is higher than for crops and partly because it has the capability to regrow after cutting or grazing. Grassland is adapted to different climatic and soil conditions due to its large variety of species. Most grassland species of the temperate zone are perennial and will grow whenever the temperature is above 5°C and when there is adequate moisture. Assimilation starts at 0°C, but biomass increases significantly above 5°C until it reaches its maximum at temperatures between 17 and 25°C (see also Table 3-8). The minimum annual precipitation lies at 400 to 500mm, but precipitation up to 1000mm per year increases the yield.
Table 3-8: Requirements of grassland of the temperate zone regarding climate, soil and water supply (Heyland, 1996; Whitehead, 1995)
Developmental Temperature [°C] Special climatic Soil Water supply cycle [days] min opt max requirements
Adapted to local conditions
0 17-25 35 Susceptible to frost, snow and changes of temperature
Humus content 400 – 500mm min. annual precipitation
In contrast to crops, the soil texture is less important for the development of grassland. The soil type, especially a well-developed humus layer, influences both yield and content of nutrients, protein and net energy lactose. Grassland is used as fodder; therefore the quantity of biomass produced is as important as its quality, wherein the latter influences the quality and yield of milk as well as meat.
The species, which compose a grassland canopy, can be divided into three groups with varying botanical characteristic features:
o grasses,
o legumes,
o herbs.
Grasses are monocotyledonous species with a similar structure to crops. In botanical terms, they all belong to one family, i.e. the sweet grasses (Poaceae). In temperate zones, the sweet grasses make up the largest proportion of a grassland canopy. They can be divided into two groups according to their growth height: the tall and the short grasses.
The tall grasses, for example meadow fescue (
Festuca pratensis Huds.
), oatgrass(
Arrhenatherium elatius L
.) or timothy (Phleum pratense L.
), mainly influence thedevelopment of biomass. The short grasses, for example cocksfoot
(Dactylis glomerata L.
),bluegrass (
Poa pratensis L.
) or perennial ryegrass (Lolium perenne L.
), have a greatimportance for the density and endurance of the turf. Thus the former influence the yield while the latter influence the livestock capacity.
Legumes and most herbs are dicotyledonous. Legumes live in a mutualistic symbiosis with nodule bacteria and therefore are able to bind atmospheric nitrogen. This leads to a significantly higher protein content and a heightened mineral content compared to herbs. A major advantage of growing a legume in association with grass is that the nitrogen fixed by a legume benefits not only the legume itself, but also the grass growing with it, and the mixed herbage often provides a better diet for livestock than would the legume alone. Their growth height as well as their strength is low, therefore they loose importance with
increasing density of the canopy. Under these conditions white clover (
Trifolium repens L.
)and red clover (
Trifolium pratense L.
) are the most important leguminous species.The herbs show the most variety regarding rooting, habit and persistence. Most herbaceous plants respond immediately to different environmental conditions. Therefore
the condition of a habitat can be judged by its herbaceous composition. Both protein and mineral content are higher compared to the grasses. But their importance is obliged to their amount in the canopy. Up to a proportion of 5% they have a positive influence on the protein and mineral quality of a pasture. But the yield lessens when the herbaceous plants increase to 20% and more. The high quantity of herbs in a canopy is often due to a excessive supply with organic fertiliser. This supports the occurrence of specific species, so called liquid manure flora, such as cow parsley (
Anthriscus sylvestris L),
hogweed (Heracleum sphondylium L
.) and broadleaf dock (Rumex obtusifolius L.
). Dandelion (Taraxacum officinale Web.
) is a very widespread herb because of its high adaptability, but high nutrient supply nurtures its growth. On thickened ground, mainly at the entrances of pastures, species such as silverweed (Potentilla anserina L.
) and broad- leaved plantain (Plantago major L.
) can be found.Some canopies have a fourth layer, covering the soil surface, which is built up of mosses and thatch. Those four layers that may occur in a grassland canopy are shown in Figure 3-8.
A great variety of different plant communities exist which are adapted to specific habitats. The botanical diversity increases from the straw meadow to the forage meadow that is cut twice a year. Meadows that are used more intensively, i.e. both more cuttings and fertiliser, can reach a higher yield, but their floristic composition becomes poorer. Different types of communities develop, depending on the kind of fertiliser as well the intensity of fertilisation.
The type as well as the intensity of use has a great impact on the morphology and species composition. Tall grass species for example react more sensitively to a more frequent use than small grasses and herbs. Therefore the regrowths are both richer in legumes and herbs than the first growth. Grazing is not as rapid and has a more selective effect, and nutrients are partly recycled directly by the excrements of the livestock. But with
increasing grazing intensity the number of species decreases. Figure 3-9 shows the connection between plant diversity and type of cultivation.
The main effect of nitrogen fertiliser is the acceleration of growth and the increase of leaf size. A deficiency of nitrogen restricts the number of tillers that develop and, more importantly, it also restricts the growth of individual leaves and their photosynthetic capacity.
Within grassland canopies nitrogen fertiliser stimulates the growth of the biomass delivering grasses and herbs of the liquid manure flora. Thereby the illumination conditions for legumes and lower herbs are decreased and as a result their fraction recedes. When the nutrient supply is insufficient, the higher grasses as well as the tall herbs cannot develop adequately, while the short grasses and herbs can now spread under full light. This context is shown in Figure 3-10. Their dominance leads to a reduced yield, which is on the one side due to the absence of grasses biomass. On the other hand the herbs tend to crumble while drying and silaging, which reduces the quality and quantity of the forage.
Figure 3-9: Effect of the type of cultivation on the biological diversity of grassland (Ellenberg, 1996; modified)
Intensely utilised grassland is normally fertilised at the beginning of the growing period and after each cut, except for the last one in a year. The rates of nitrogen fertilisation recommended for intensively farmed grassland in temperate regions are generally within the range of 200 to 240kg per ha, depending on soil type, weather and management. When grassland is mown or lightly grazed, the amount of nitrogen leached is normally less than 20kg per ha and year, even if fertiliser is applied. The main reason why relatively little nitrate is leached from mown grassland is that grass has a large capacity to take up nitrate, and this capacity is maintained for much of the year. However, when grassland is fertilised regularly and grazed intensively, more than 100kg nitrogen per ha and year may be leached as nitrate. In water protection areas restrictions for fertilising exist to avoid the leaching from nitrogen into the ground water.
Figure 3-10: Connection between nutrient supply and occurrence of species in a fresh (oatgrass) meadow (Arrhenatherion elatioris) (Spatz, 1994)