How to best manage grazing of pastures

However, debate continues about how to best manage grazing of dairy pastures.

Considering the underlying principles of pasture growth and the effects of grazing on growth and pasture accumulation can be useful.

The starting point for understanding plant growth and pasture yield is light capture. Light is used by leaves for photosynthesis, providing energy for plant growth. In this sense, light is the basic foodstuff of plants.

Pasture regrowth after grazing

Grazing or harvesting pasture removes leaves and deprives plants of their primary food source – light.

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.

However, once grown, the first leaf adds more energy to the plant, so there is more energy for the next leaf (so it 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 three live leaves (maximum) on a tiller at any point. So, once the third new leaf has been produced (the 'three leaf stage'), the first leaf 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 out by the rate of leaf death.

Dead material will accumulate at the base but no additional leaf material for grazing is accumulated.

Optimum time to graze

The common 's-shaped' regrowth curve is shown in figure 3 (in blue).

This charts the build-up in total pasture cover from the previous grazing. Cover builds up quite slowly initially, but then accelerates before levelling off towards ceiling yield. Figure 3 also shows two other growth curves which help determine the best time to graze to maximise leaf accumulation.

'Instantaneous growth rate' is the daily rate of pasture growth that contributes to pasture accumulation. Instantaneous growth increases as new leaves are formed, then declines as leaves start to die, which cancels 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 (see figure 3), the optimum amount of new leaf is produced and old leaf is dying. This is the best point to graze because the efficiency of conversion of light into additional pasture cover starts to decline thereafter.

Leaf stage can indicate the optimal grazing point – generally, maximum average growth rate occurs at approximately the three leaf stage after grazing (figure 1).

Grazing residual impact

Pasture residuals impact on pasture dynamics, growth rates and the best timing for grazing.

Ensuring post-grazing residuals are managed within a tight range (3.5-4.5cm compressed height) will enable plants to capture as much sunlight as possible and convert this into feed. This will be achieved by:

minimising the amount of old leaf material left behind after grazing. This leads to the best possible growth rates, allowing tiller size and density to adapt to a consistent light regime after each grazing.

allowing light to reach deep into the sward to stimulate new tillers. This keeps tiller density high, allowing the pasture to regain full light interception after grazing.

Farm system considerations

Implementing the pasture growth principles is complicated by balancing 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.

Transfer autumn/winter pasture to spring feed

A common objective is to transfer autumn/winter pasture into early spring pasture, to achieve target covers at calving and meet the milking herd's requirements.

This is achieved by lengthening the rotation in autumn and winter, beyond the time taken to grow three new leaves. Although this means some leaf death, it allows relatively cheap pasture feed to be transferred to a period of high demand, reducing or avoiding the purchase of more expensive supplements.

Late spring – managing surpluses and quality

During late spring, pasture growth often exceeds herd demand, resulting in periods of temporary pasture surplus which, if not well-managed, mean reduced pasture quality and animal performance.

Removing pasture surpluses (taking paddocks out of the round for silage) can reduce grazing intervals, resulting in grazing some pastures before the 2½-3 leaf stage. Where harvesting surpluses is not desirable, short grazing intervals may reduce pasture growth. However, the benefits of maintaining high pasture quality can outweigh the loss of yield at this time of year.

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 2600-3200kg DM/ha for lactating dairy cows.

During periods of rapid growth, for example when pastures respond strongly to nitrogen fertiliser, grazing may need to occur between the 2-2½ leaf stages in order to meet pre-grazing cover targets. ω

Key points

◦ Sunlight provides the basic food for plants, in the form of energy.

◦ Graze between the 2 3 leaf stages to maximise energy capture, grass growth and long-term yield.

◦ Allowing growth to continue beyond 2 3 leaves means pastures will reach a ceiling yield where no further dry matter will accumulate and pasture quality will decline.

◦ An even and consistent grazing residual – between 3.5-4.5cm on a rising plate meter – means pastures quickly re-establish leaf area to capture light energy.

◦ Principles of pasture growth should be balanced with overall farm system considerations.