Permanent grasslands, whether they are natural or have been sown for more than five years, can contain from 10 to up to 50 plant species, unlike temporary grasslands that are rarely composed of more than 2 or 3 species. Species-rich grasslands are of value in several ways. Firstly, this richness enables a diversity of agricultural use values, to feed livestock at pasture or in the form of hay; that was the objective of the research reported here. Secondly, these grasslands fulfil important environmental functions: they constitute a major reservoir of plant and animal diversity, they have a much-appreciated landscape effect and they play a not inconsiderable role in the storage of CO2, the most important of the greenhouse gases.
Until now, the diversity of permanent grasslands has mainly been described in terms of the number of species present, based on taxonomic criteria that reflect a reality that is often of a local nature. Furthermore, their agricultural use value has long been calculated from records of species abundance, each with a specific index "frozen in time", to estimate a global value for a status deemed to be optimum. However, these indicators "speak little" to livestock farmers and have contributed to communicating a productivist image of grasslands through the use of inappropriate terms such as "good" grasses as opposed to grasses with no fodder value.
Apart from yield and quality, two criteria are essential to livestock farmers. The temporality of production defines the time of peak plant growth; it is linked to species phenology. This peak may be observed at each regrowth, and harvesting too long afterwards may result in poor exploitation of the grass available. Flexibility of use enables the delaying or advancing of cutting with reference to the peak production date, but without reducing the quantity and quality of the harvest.
Establishing a link between farming practices and the functioning of grassland plant communities
© INRA / JP. Theau
Type C grassland
By applying the concepts of functional ecology, researchers in the References, Tools and Models for Managing Grassland Farming Systems team (Orphée) in the Joint Research Unit for Agrosystems and Territorial Development (AGIR) at INRA in Toulouse, have proposed a method for the typing of species-rich grasslands that qualifies both their agricultural use value and environmental value (species diversity). Plant species and families (mainly grasses and dicotyledons) are no longer listed but classified in terms of their behavioural similarities. The biological traits of plants, and their agricultural characteristics such as yield and quality (e.g. digestibility), as well as the temporality of production and their flexibility of use, are thus connected.
Four functional plant types (A, B, C and D) have been defined on the basis of their leaf dry matter content (1). This indicator expresses a physiological compromise between the acquisition and storage of nutrients or, in other words, between organ growth and longevity. Thus a species may grow with difficulty in a rich environment, but at the same time growth will be quick because nutrients are taken up rapidly and then stored for a long period because of a lengthy leaf lifespan. These four functional plant types thus differ markedly in terms of the lifespan of leaves, phenology, organ digestibility and, to a lesser extent, growth rate.
Field observations can determine the proportion of each of the four functional plant types. Thus "dominant type A" vegetation corresponds to species such as perennial ryegrass or white clover. This type of grassland is fertilized. Used regularly, it achieves a high yield with early peaks in production and quality (low cellulose content), but this quality declines rapidly if cutting is delayed too much. "Dominant type C" vegetation comprises species such as red fescue, Agrostis or burnet, etc. Production peaks are later. The fodder is less digestible than that from a type A grassland, but digestibility declines more slowly if the harvest is delayed. Type B is intermediate between types A and C, while type D has more marked characteristics than type C: thus both yield and quality are poorer.
The greater the variety of functional types within a grassland (e.g. A and C compared with A alone), the greater will be its flexibility of use. Applied at the scale of a farm or region, this method can thus identify complementarities between types of vegetation (those with early and late peaks) and aid with the design of fodder systems that best exploit the diversity of vegetation.
This study was carried out in the context of the European Vista project (Vulnerability of Ecosystem Services to Land Use Change in Traditional Agricultural Landscapes) (2). It will form the basis for a national permanent grassland typing project based on more generic foundations than in the past.
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© INRA / JP. Theau
Collection of grass samples.
(1) The dry matter content of leaves saturated with water, or DMC expressed in mg/g, is measured according to a fixed protocol and under the same growing conditions.
(2) VISTA : http://www.toulouse.inra.fr/centre/vista-wp5
Reference: Ansquer P, Duru M, Theau JP, Cruz P 2008 Functional Traits as Indicators of Fodder Provision over a Short Time Scale in Species-rich Grasslands. Annals of Botany (in press)
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