Pasture growth periods
Pasture growth drives productivity in grazing enterprises, so an understanding of the different pasture growth periods throughout a year is essential for producers wanting to maximise pasture utilisation and ensure pastures don’t deteriorate.
In mediterranean-type climates, the three pasture growth periods for annual-based pastures are: establishment, vegetative and reproductive. A fourth period, senescence, is a period where pasture dies or 'hays off’ with a rapid decrease in the amount and quality of the FOO.
Pasture establishment occurs after the break of the season in autumn/early winter. It describes the period from germination and seedling emergence, to the point where the seedling has developed a root and leaf system that can sustain being grazed.
Successful pasture establishment is vital to ensure the persistence of annual pastures. Grazing a pasture too early after seedling emergence can dramatically affect pasture establishment due to:
- uprooting of small seedlings resulting in reduced plant density which is the single biggest determinant of early growth rate
- removal of leaf area (defoliation) resulting in reduced growth of seedlings, poor root development and poor drought tolerance.
Decreasing the grazing pressure, applying fertiliser (particularly nitrogen on grassy pastures), or sowing pasture seed can improve plant density and effective pasture establishment.
Grazing pressure during autumn/early winter can be reduced by supplementary feeding which reduces the amount of daily pasture intake as animals fill up on supplement. Deferred grazing, where animals are removed from the pasture, allows seedlings to establish better compared to grazing through the break which can lead to substantial loss of seedlings. Grazing establishing pastures also keeps FOO low, thereby reducing leaf area index resulting in poor root development and poor drought tolerance.
Vegetative pasture growth describes the winter period where plants have become established and are mature enough to withstand being grazed and defoliated by animals.
During this stage, pastures are actively growing and stocking rates can be maximised. However, over-grazing by grazing animals can result in insufficient leaf area for maximum pasture growth. Small plants may have poorer root development and are more susceptible to cold stress in winter, further reducing growth.
General rule: higher FOO = higher leaf area = faster pasture growth.
A minimum FOO of 1000kg DM/ha is desirable going into winter but optimum pasture growth occurs at around 1400kg DM/ha.
Grazing during the vegetative pasture phase is important to:
- encourage tillering in grasses and branching in clovers
- assist in weed control
- maintain palatability of the pasture by promoting young shoot growth.
Clover seedlings with four to six leaves are generally able to tolerate a moderate grazing pressure. Clover leaves, flowers, burrs and seed all originate in the axils (branches) of leaves along the main stem. Since grazing stimulates branching until flower initiation, the aim up to the point of flowering is to graze clover pastures to maximise the number of branches per plant and, in turn, maximise the potential seed production.
Early growth of grasses can be increased with the application of nitrogen fertilisers to grass-dominant pastures, although the response tends to be site and season dependent. This boost to pasture growth may allow an overall increase in stocking capacity.
Grazing management through the vegetative period should aim to meet animal production objectives, while being sensitive to the productivity and sustainability of the pasture. This can be achieved through various grazing tactics, including strip grazing, grazing to target FOO and grazing to a paddock target.
The reproductive period occurs in spring and is characterised by flowering and seed-set.
Grazing during this period can have an influence on seed production. Overgrazing during flowering will reduce seed production due to removal of flowering parts. This may affect annual grasses more than prostrate species such as sub-clover which buries its seed. Grazing management during spring can influence pasture composition in both the current year and following season.
Senescence describes the period where pasture dies or ‘hays off’ in summer. Plant leaf material breaks down quickly after senescence which means that FOO decreases rapidly. Importantly, this occurs whether the pasture is grazed or not — hence the saying 'use it or lose it'. Research has shown that up to 80% of the peak spring pasture can be ‘lost’ during summer and not ingested or turned into animal product. The rate of decline is influenced by weather. Both quality and quantity declines faster after rain.
The quality (digestibility) of pasture becomes limiting to animal growth when it reaches about 50%. Animals grazing such pastures will lose weight/condition if not fed supplements.