Morphological Structure of Grasses and Legumes
Description of a Grass
Identification of grass species at the vegetative stage can be completed based upon few leaf characteristics:
structure of the sheath,
presence and size of the auricles,
presence and importance of ligule,
limb and structure of prefoliation
Vernation describes the structure of future leaf within the sheath where it is formed. It can be:
folded (like perennial ryegrass)
or rolled (like orchard grass).
Auricles are located at the base of the limb, exactly at the transition between the limb and the sheath. There are three possible situations:
auricles are absent (case of orchard grass or bluegrass)
auricles are short (case of perennial ryegrass)
auricles are long and clasping (case of crabgrass)
Ligula is a membrane which is located on top of the sheath. There are four possible configurations:
ligula is absent (case of red fescue)
ligula is short (case of perennial ryegrass or dog’s tail)
ligula is long (case orchard grass, tall fescue and Timothy)
ligula is very long (case of brown bentgrass)
Description of a legume
A legume can also be recognised at the vegetative stage, upon following observations:
leaf structure and the number of leaflets in each sheet
shape of leaflet and the presence of a mucro
shape of stipules
stem structure and the presence of stolons
The number of leaflets varies according to species. They are 3 for many species (including clover), but can reach 12 or 15 for species such as sainfoin or coronilla.
Mucro is a sharp, short point, generally at the tip of a leaf or the tip of the midrib of a leaflet. There are 3 mucros for many species (including clover), but can reach 12 or 15 for species such as sainfoin or coronilla. The presence of mucrons facilitates separation of clovers, which have never any mucron, from lucerne which always carry a mucron.
Stipules are small appendages located at the junction between petiole of the leaves and the stem. They are generally reduced to a small tooth. Species of Lotus genus have very large stipules, a size of a leaflet.
But to distinguish legumes, it is much easier to wait for flowering…
Number of Tillers and Growth Points
Figure alongside shows the evolution through seasons (from winter to summer) of the number of tillers/ m² for a grass groundcover at different heights of defoliation ( from 20 mm (extremely cropped) to 160 mm).
From 140 to 160 mm, the increase in height of defoliation results in a reduction of tillers. This is related to the fact that the increase of height of defoliation (160 mm) leads to a stronger shade of all points of growth which are at the base of the tiller, and thus to the reduction of newly developing tillers.
A defoliation at 20 mm escapes this tendency with always very low number of growing tillers. The reason is that for such heights, we have an extremely high reduction of the leaf area of the plants and their photosynthetic capacity, which reduces their potentiality for growth and vegetative multiplication. Going from the winter into spring and then summer, we can observe an increase in the number of tillers for all heights of defoliations.
This study was conducted in United Kingdom was accomplished on perennial ryegrass (Hernandez-Garay et al, 19991)).
The counterpart of the modification in number of tillers/m² is the mass of each of these tillers. It can be seen from the graph, on the one hand, that mass of tillers increases from winter to summer, but we can also observe on the other hand, much more contrasting situations depending on the height of defoliation. As we could expect, there is a much higher mass per tiller for defoliation at 160mm, the lowest being obtained at 20mm.
The second element influencing the number of tillers or the number of growth points is the interval between two defoliations. In grasses, when the gap between two cuts is increased, the size and the mass of each of the tillers increase, and the number of growing tillers is mechanically reduced.
An identical study was conducted on white clover (see the third graph).
This graph shows two relatively contrasted attitudes between two varieties: Rivendell Small Leaf White Clover, and Aran clover. Quite dramatically in Rivendel, we notice that a lengthening of cutting frequency leads to a very significant reduction amongst the points of growth (Simon et al, 20042)).
Aran responds less to this phenomenon, with an inability to increase the number of points of growth. This is partly explained by the size of its organs, such as the size of its stolons.
This functioning of groundcover and the relationship between mass and number of tillers has practical consequences on animals’ digestion at the time of grazing.
This graph illustrates the relationship between the mass of leaves available by surface (here expressed in Kg/ha) and the bite size taken by animals (here dairy cows). When increasing the mass of leaves per unit area, we find that the bite size increases almost linearly, if the plant cover is vegetative or headed. It is therefore the mass of leaves that presents a limiting factor.
The number of bites taken by an animal being almost constant, it is the mass of available limbs which conditions the amount of dry matter ingested by the animal (Delagarde et al., 20013)). There are many practical consequences of these simple relationships.
In case of high defoliation, an increase in the weight of tiller will induce an increase in the size of the bite and thus an increase in the quantity of grass ingested by grazing animals. In return, there will be a reduction in the number of tillers, which means probable increase in the number of holes in the vegetation, and thus a long-term risk of soiling.
A contrario, in the case of a close-cropped defoliation, a dense cover is set, similar to lawn. In this context, the bite size is reduced, and therefore the amount of grass ingested is greatly limited.
In the case of a very close-cropped defoliation, there is excessive reduction in the amount of tiller, resulting in holes in the vegetation and therefore a decrease in density and degradation of vegetation cover. This situation occurs in the case of overgrazing, especially in the summer.
In general this leads to a law regarding pasture management . It is important to find a compromise between density of tillers and bite size. It is also important to maintain a balance between growth of vegetable cover (biomass accumulation) and sampling (defoliation by the animals).
This graph shows the need for this balance between productivity (less stress) and disturbance (sampling by grazing or mowing) of the vegetable cover.
In the case of deviation from the median (equilibrium zone), two scenarios can be observed.
If the productivity greatly exceeds sampling, there is an increased competition within cover, a reduction of growth points and an absence of openings in the cover.
On the contrary, if the sampling exceeds productivity, the cover opens and presents many holes, supporting the establishment of seedlings but also a high mortality of this cover.