During its growth, and over its lifetime, plant cover accumulates biomass, (aboveground biomass) forage. Two elements take part in this production of biomass: on the one hand, the sum of temperatures which determine the speed of development of the leaf area, and on the other hand, the quantity of incidental solar radiation intercepted by the plants. This radiation varies over the year, reaching its maximum around summer solstice in June. The level of interception depends on the established leaf area. This same leaf area is modified at the time of each defoliation, whether by grazing or mowing.
Forage biomass is composed of different organs; leaves (or limbs), sheaths and stems, each having different chemical compositions.
During the increase in biomass, the proportion of different organs changes. Therefore, the biochemical composition of the entire harvest varies.
The figure alongside represents the evolution of biomass of grasses over a period of time by positioning three important phenological stages (ear at 10 cm, beginning of spikes and flowering) (Gillet, 19811)).
During this development, from the beginning of seedlings, there is an accumulation of total biomass, which is based in part on the development of leaves, the mass of leaves quickly reaching its maximum, then the accumulation in stems, especially from the moment the ear develops (and exceeds a stage of 10 cm).
This graph also shows the evolution of the number of plant cover tillers. This number also increases from the seedlings stage to reach a maximum at the phenological stage of “10 cm ear”, before the number of growth points diminishes, this event being generally called a “crisis of tillering.”
Biomass and Nitrogen Content
The nitrogen content decreases automatically with the accumulation of biomass, which is referred to as “the law of dilution.” This law depends essentially on the fact that nitrogen is located mostly in leaves, and that during the accumulation of biomass the proportion of leaves decreases whilst the proportion of stems increases, the later containing less nitrogen.
In the event of nitrogen stress, the accumulation of biomass and its nitrogen content are both reduced (see graph below).
Compared to a sufficient level of nitrogen fertilisation (N3), a reduction in this contribution on a nitrogenised cover (N2 and N1) results on the one hand, in reduction of biomass produced at each stage of harvest, and on the other hand, in reduction of nitrogen content of this same biomass (Gastal et Durand, 20002)).
When there is a slight reduction in the level of nitrogen fertilisation, the nitrogen content of the produced biomass is the first to be affected, whilst the produced level of biomass is affected only in the event of high fertilisation.
In case of over-fertilisation, for example in case of the first wave of harvest (passing from level N3 to N4), we notice an accumulation of nitrogen content without an increase of the aboveground biomass. This means that the photosynthetic capacity of leaves did not increase beyond a certain level of nitrogen content called the “critical level”.
More detailed analysis of the consequences of the modification of plants’ nitrogen nutrition shows that, for plants in hydroponic conditions, a reduction of nitrogen nutrition results in an extremely fast reduction of the elongation of grass leaves.
Biomass accumulated in a grassland groundcover varies greatly throughout the year as a result of different factors: existing leaf mass, non-active incident photosynthetic radiation, and water and nitrogen resources. Therefore there is a significant effect of pedoclimatic conditions. Differences between species, in particular between Grasses and Legumes were also observed.
The figure alongside shows differences in the growth of plant cover (expressed in kg of dry matter/ha/day) for a given region, the temperate humid area, between three types of soil: dry, intermediate and humid.
The difference is low in the spring in May-June when the water requirements of plants are ensured in a similar manner in all three environments.
However, the difference is more remarkable in the summer when growth quickly diminishes in dry areas.
There are marked differences between species, with significant differences between grasses and legumes (Straebler et Le Gall, 19983)).
Grasses have a much greater peak growth early in the spring which corresponds to their earing stage. After the spring peak, their growth rests exclusively on the production of leaves with a significant drop of production in the summer and slight recovery in autumn.
In contrast, legumes, with a more constant morphological composition, for example lucerne display a systematic production of stems and leaves, not exhibiting a significant seasonal effect. Their growths being higher in summer, legumes, and in particular lucerne, are more suitable for summer production. This adaptation is reflected in several phenomena: better uptake of water in the soil due to deep rooting, and a response to the quality of light.
Differences Between Species
Various species in grassland exhibit noticeable morphological and physiological differences affecting their major functional traits.
The graph alongside shows the range of variation for a major functional trait: the dry matter content in limbs. We can observe in this functional trait values ranging, depending on species, from 19% to 27% (for the red fescue).
As this graph illustrates, changes in dry matter content are related to variations of cell wall content, species with highest DM also being those that have a higher content of cell walls.
This variation of cell wall content results in an impact on the digestibility of DM.
Digestibility is measured here by the degradation of DM generated by a bath of cellulase. It is much higher for species with low levels of DM in limbs.
Another major trait with important consequences on agronomic value is earliness of heading, here expressed as a sum of T° required to reach a flowering (T° measured from March 1st onward).
This graph shows that species with later heading present higher seasonal averages of cellulase digestibility. These species demonstrate equally the longest leafy periods.
This graph resulting from the work of De Pontès in 20074) presents values for a single population. This work should not obscure the fact that, regarding the amounts of T° at flowering (earing dates), there may be extremely significant differences between varieties or between populations within a single species.
The most significant difference is undoubtedly perennial ryegrass which presents differences of more than a month and a half between the earliest and the latest varieties/populations.
On the basis of some major functional traits such as the DM content in leaves, phenological earliness or cell wall content, it is possible to combine species with relatively similar characteristics.
Data from this table (Ansquer, 2006)5), enables us to identify four groups that can be described as functional groups. Passing from Group A to Group D, we can see a steady increase in the dry matter content of leaves, an increase in fibre content, as well as species with later and later flowering.
Groups A and B correspond to functional groups having so called “capturing” strategies. Such species are capable of very efficiently acquiring environmental resources, such as water and nitrogen, or luminous resources.
A contrario, species from groups C and D dispose of so-called “conservation” strategies, promoting easy adaptation to poor environment with few resources and ensuring the conservation of these resources.
Grasslands with a large number of species are considered to belong to the functional group of the dominant species.
Recent studies by Ansquer et al. (2009)6) about permanent grasslands combining grasses and legumes demonstrate that functional traits of grasses have their equivalent in functional traits of other families of species in that grassland, especially at the level of functional traits of legumes.
Based upon studies by Cruz, it is possible to use this knowledge of functional traits to understand the behaviour of grasslands according to the availability of nutrients and usage rates, thus on the level of sampling.
In environments with high nutrient availability, therefore more fertile areas, mainly in the case of natural grasslands, species of A and B groups – competing species from the capturing group – are predominant.
In relatively high usage environments like pastures, species from group A will be represented more often, while species from group B will be more common in meadows, with lower usage, mostly mown.
Mowing implies an increase in canopy height at maturity.
In the poorest areas, with low nutrient availability, species tolerant to stress, those with “conservation” strategies, from groups C and D, are favoured.
Here too, in the case of a high usage rate, species from group C will be favoured.
The same reasoning can be applied to sown grasslands (with high nutrient availability) and species from group A, like perennial ryegrass would be favoured in pastures, and species from group B would be favoured in meadows.