PASTURE REGROWTH AFTER GRAZING
Grazing or harvesting pasture removes leaves and deprives plants of their primary food source - light energy. Generally, the first leaf after grazing is relatively small because the plant has little energy for growing this leaf and, therefore, leaf size is restricted.
But once grown the first leaf adds more energy to the plant, so there is more energy for the next leaf, which will be a bit bigger. This pattern continues until the plant has regained its full energy status.
Leaves have a limited lifespan. Ryegrass is often termed a three-leaf plant because it generally sustains a maximum of three live leaves on a tiller at any point. So once the third new leaf has been produced (termed the three-leaf stage, see figure 1), the first leaf produced immediately after grazing will start to die.
Ultimately the pasture will reach "ceiling yield" (shown by x in figure 2), while plants are still producing new leaves, but the amount produced is cancelled by the rate of leaf death. At this point, the rate of loss through the old leaf dying is on a par with new leaf production.
Therefore, dead material will continue to accumulate at the base but no additional leaf material for grazing is accumulated.
OPTIMUM TIME TO GRAZE
The commonly seen S-shaped regrowth curve is shown in blue in figure 3.
This charts the build-up in total pasture cover from the previous grazing. Cover builds up quite slowly to begin with, accelerates, then levels off towards ceiling yield.
Figure 3 also depicts two growth rate curves calculated from the s-shaped regrowth curve, which can help determine the optimum time to graze to maximise leaf accumulation.
Instantaneous growth rate is the daily rate of net pasture growth that contributes to net pasture accumulation. Instantaneous growth increases initially as new leaves are formed and no leaf death is occurring, and then declines as leaves start to die and cancel out the rate of new leaf production.
Average growth rate is the amount of pasture grown since the last grazing (current yield, less residual at last grazing), divided by the number of days.
When the maximum average growth rate is reached (arrow in figure 3) this indicates the optimum balance between the amount of new leaf produced and the amount of old leaf dying. Beyond this point the efficiency of further increases in pasture cover is declining, therefore it is the optimal point to graze.
So how can farmers identify this point?
The curves in figure 3 are all determined by the sequence of leaf production and leaf death (figure 1). Therefore leaf stage can be used to indicate the optimal grazing point.
Generally, maximum average growth rate occurs at approximately the three-leaf stage after grazing (figure 1), so monitoring leaf stage is a practical way to track what is happening.
GRAZING RESIDUAL IMPACT
The previous analysis of pasture dynamics and identifying the optimum grazing point is based on pasture regrowth following grazing residuals of 3.5cm to 4.5cm compressed height (7 to 9 clicks on rising plate meter).
The residual from which pastures regrow impacts on pasture dynamics, subsequent growth rates and the optimum timing of grazing. Figure 4 demonstrates the impact of different post-grazing residuals (low, medium and high) on average growth rate during the next regrowth cycle. This highlights that:
• Time taken to reach maximum average growth rate (shown by the arrows) is very short with a high residual (a: blue line) and very long with a low residual (c: black line).
• The actual growth rate reached at the maximum average is lower for the high residual (y) and the low residual (z) compared with the medium residual (x).
Hence, the amount of pasture left behind after each grazing impacts on the optimum time to graze again and the maximum average growth rate achieved. Therefore, this affects the amount of pasture available at the next grazing. In other words, residuals play a key role in overall pasture growth.
Ensuring post-grazing residuals are consistently managed within a tight range (3.5cm to 4.5 cm compressed height) will enable plants to capture as much sunlight energy as possible, and convert this into feed for cows by:
• Minimising the amount of old leaf material left behind after grazing. This will lead to the best possible growth rates allowing tiller size and density to adapt to a consistent light regime after each grazing. This minimises the lag in the accumulation of new green leaf. When consistent residuals are achieved, about 25, 35 and 40 per cent of total final yield comes from the first, second and third leaves produced.
• Allowing light to reach deep into the sward to stimulate the production of new tillers. This keeps tiller density high, allowing the pasture canopy to quickly regain full light interception.
FARM SYSTEM CONSIDERATIONS
On the farm, implementation of the pasture growth principles is complicated by the need to balance animal requirements and feed supply throughout the year.
There are times when farmers need to bend the rules of pasture growth to improve overall farm system outcomes. Consideration of deviations leads to improved decisions.
Transfer of autumn and winter-grown pasture to feed the milking herd in early spring
A common objective of grazing management during late autumn/winter is to transfer autumn/winter-grown pasture into early spring, to achieve target pasture covers at calving and meet the pasture requirements of the milking herd.
This is achieved by lengthening the rotation in autumn and winter beyond the time taken to grow three new leaves. Although this will result in some leaf death, feed can be transferred from a period of relatively low feed demand to a period of high feed demand.
LATE SPRING - MANAGING PASTURE SURPLUSES AND QUALITY
During late spring, pasture growth rates often exceed herd demand, resulting in periods of temporary pasture surplus and, if not well managed, will result in reduced pasture quality and subsequently reduced animal performance.
Removal of pasture surpluses (taking paddocks out of the round for silage) can reduce grazing intervals, resulting in grazing some pastures before the two-and-a-half to three-leaf stage. Where harvesting surpluses is not desirable, short grazing intervals may reduce pasture growth.
MANAGING PASTURE COVERS
Pre-grazing yield must be managed to optimise pasture utilisation and animal performance, while allowing target residuals to be achieved. The recommended range is 2600 to 3200kg DM/ha for lactating dairy cows.
During periods of rapid growth and on nitrogen-boosted pastures, grazing may need to occur between the two to two-and-a-half leaf stages in order to meet pre-grazing cover targets.
KEY FINDINGS
• Sunlight provides the basic food for plants, in the form of energy.
• Graze between the 2&1/2-3 leaf stages to maximise energy capture, the efficiency of grass growth and long-term yield.
• Allowing growth to continue beyond this point means pastures will reach a ceiling yield after which no further dry matter will accumulate, and pasture quality will decline.
• Achieve an even and consistent grazing residual - between 3.5-4.5cm on a rising plate meter so pastures quickly re-establish leaf area to capture light energy.
• Principles of pasture growth should be balanced with overall farm system considerations.
FAST FACTS
• Leaving a high residual can lead to fast rotations, which means the extra growth available from the third leaf is lost.
• Ensuring post-grazing residuals are consistently managed within a tight range (3.5cm to 4.5cm) will enable plants to capture as much sunlight energy as possible.
• When consistent residuals are achieved, about 25, 35 and 40 per cent of the total final yield comes from the first, second and third leaves produced.
