The forage revolution which took place in Europe during the 1960s favoured forage species in pure crops, primarily grass crops, managed with high levels of nitrogen fertilisation and mainly mown.
This vision of intensive agriculture has now disappeared, and the search for a multifunctional agriculture, economically efficient and preserving the environment, is challenging dogma of pure crops.
This leads to the issue of the potential benefits of crops that mix forage species.
Many research studies have been carried out in recent years on this subject.
Production of Aerial Biomass
The first founding study regarding the potential benefit of increasing the number of crop species was published by Hector et al. in 19991). This study conducted in eight locations demonstrated that, in some studied environments, the increase in number of species is accompanied by an increase in aboveground biomass.
The same study demonstrated that one of the important determinants for increase in produced biomass is the increase in the number of functional groups (Lavorel et Garnier, 20022)) present in groundcovers.
Hector’s groundbreaking study was carried out in various environments, but in general under conditions of average, even reduced, fertility. It was reproduced in many environments and followed by many publications.
Amongst other works, we should emphasised a study by Guo et al. (2006)3)
They demonstrate that, in all cases and throughout the growth of sown grasslands, an increase in the aboveground biomass was observed with the number of species, but also the existence of a plateau, a limit beyond which the biomass does not increase.
Another example of the effect of diversity is provided in the study published by Kirwan et al. in 20074).
The principle of this study is based upon the usage of four species (two grasses and two legumes) mixed in a set of two, three or four, according to an original device.
This study was conducted in twenty-eight European environments belonging to four categories of different fertilities.
In all cases, a benefit with the increase in number of species, compared to monoculture, is observed. This study shows also that benefits are observed mainly when combining one grass with one legume, but that the combination of two legumes or two grasses is also beneficial.
Finally, to conclude this overview of examples showing benefits related to diversity, we should emphasised studies undertaken in Jena, Germany and published by Roscher et al. (2005)5), Fischer et al (2008)6) et Marquard et al.(2009)7)
These studies also show an increase in above-ground biomass with the number of mixed species. Nevertheless, those demonstrate that this benefit exists only for the aboveground biomass and absolutely not for the underground biomass.
The studies of Marquard et al (2007), have shown that the aboveground biomass of the community increases partly with the number of species, in particular in the presence of legumes, while the benefit is nil without them.
This tends to suggest that a certain level of nitrogen fertility is necessary to express this benefit of diversity.
Even if the increase in species diversity does not lead automatically to a benefit in terms of aboveground biomass productvity (Huston et al, 2000)8); Sanderson et al, 2005)9), this overview illustrates a frequent benefit related to the increase of interspecies diversity or functional diversity. A frequently asked question is whether the same benefit can be achieved by increasing the diversity within each species.
An original study was conducted by Surault et al. (2008)10). Forage production of grasslands mixtures and associations with initial varied diversity of species. Seminar AFPF, 26-27 March 2008, Paris.)) mixing within a single species (perennial ryegrass) two, three varieties belonging to different groups of precocity.
The graph above shows two species studied, Hamilton (early variety) and Herbie (intermediate variety), and three in the inferior graph, Hamilton, Herbie and Ohio (late variety).
These graphs show the production for each harvest during four years of study.The cumulated biomass shows no increase, meaning that the mixture produced exactly the average of each of these varieties.
Likewise, the mixture is always located in an intermediate position, implying that mixture production has a greater stability than each of the components, which may be of considerable interest for the requirements of the farmer.
Effects on Biochemical Composition
The effects of diversity of species on the biochemical composition and the food value of collected forage were also studied (Deak et al, 200711); Huyghe et al, 2008). There is no benefit for major components such as protein content or digestibility. The value of variables for forage resulting from multispecies grasslands is determined by the value of each species and its proportion in the harvested forage.
Nevertheless, the interactions between forages were observed by Aufrère et al (2005)12) and Julier et al (2002)13) for the degradability of proteins. In fact, whenever one of the species contains condensed tannins, the solubility of all the forage proteins decreases. Tannins of the rich species will also tan other proteins present in fresh forage. This phenomenon can be observed by cultivating a mixture of white clover (species deprived of tannins) and the Birdsfoot trefoil (species rich in tannins) or lucerne and Sainfoins.
The presence of one species rich in tannins can be sufficient to eliminate the risks of bloating when used in pasture.
Mechanisms Underlying Functioning of Mixtures and Associations
Competition consists of the division between two or several individuals of an available resource in limited quantity in the environment and limiting the growth of individuals.
Competition can lead to the exclusion of some of these individuals. Experimental and theoretical studies by Tilman et al (1997)14) illustrate how competition in homogeneous or heterogeneous situations can lead to an increase in production with an increase of diversity.
The intensity of competition depends largely on environmental conditions and the amount of resources, and begins at the phase of seedling establishment, as demonstrated by Körner et al. in 200815).
Complementarity is the ability of species to acquire and use various resources for their growth, or to use the same resource at different times and places (Gross et al, 200716)).
There are many examples. We can cite the functioning of mixtures of grasses and legumes for the acquisition of nitrogen. Grasses assimilate mineral nitrogen while legumes fix nitrogen from the air.
We can also cite the complementarity between species having different rooting depths, for example a ryegrass and a tall fescue. Ryegrass uses the surface level while tall fescue uses deeper levels.
A third example is that of species with their growth cycles spread out through time. This way we obtain the benefit of certain grasses and legumes, the first growing earlier in the spring, and the latter ensuring production in the summer.
Complementarities are frequently found in situations of intermediate fertility. One of the key points for the phenomena of complementarities is the relative abundance of different species. Complementarity between two species is impossible if their abundance presents a big contrast, if one is very present and another almost absent.
Facilitation between two or more species (Brooker et al, 200817); Bruno et al, 200318)) Facilitation between two or more species allows, on the basis of a fundamental niche, and in order to extend this same niche, one species to facilitate the presence of close species by:
increasing the amount of resources (a legume can bring nitrogen to a grass)
improving the quality of habitat
offering a refuge against predation
increasing the recruitment and arrival of new seedlings.
This configuration of facilitation is regularly found in the case of grass/legume mixing, since a well-settled mix benefits from the transfer of nitrogen from legume to grass, either by rhizodeposition (Hogh-Jensen et Schjoerring, 200119)), or by senescence of organs and in particular nodules.
A more original example of facilitation is provided by the tolerance to pathogens, including fungal pathogens, in a situation of increased diversity (shown in the graph below).
By increasing a number of plant species, the number of present fungal species increases, which is true also when observing an increase in the number of functional groups. In contrast, the level of infection, measured in numbers or by the infection rate, decreases rapidly with an increase in the number of species or functional groups.
This is clearly a case of facilitation, since we can register this effect as a form of refuge against predation.
- Plant Physiology [lien vers Physiology of Groundcover]
- Plant Morphogenesis [lien vers la page]