The impact of overgrazing on forage production

Wednesday, 21 June 2023

We need to ensure post-grazing residuals are available to aid rapid regrowth when conditions improve.

Overgrazing occurs when the plant is grazed before its energy reserves and roots are fully recovered from the previous grazing. The most efficient time to graze is when the full photosynthetic potential has been achieved.

Younger leaves are more photosynthetically efficient, so grazing too low, i.e. below the two-leaf stage, can reduce grass growth by up to 85%.

In summer, the regrowth of that first leaf will start after three days:

  • Leaf 1 produces 10–15% of the grass in a sward
  • Leaf 2 produces 30–40% of the grass
  • Leaf 3 produces 45–55% of the grass

Graze at 8–10 cm, leaving a residual of 4–5 cm – sugars are stored in this stubble and help to grow the next leaf after grazing. During the grazing phase, plants reduce root growth, hence why under continuous grazing management, roots tend to be shallower.

Continual grazing does not allow root reserves to be fully replenished; ensuring they have a long enough rest period between grazing allows energy to be restored.

Getting the balance right

It is about protecting root reserves and trying to maintain plants in a vegetative state. As already mentioned, overgrazing reduces root reserves and prolongs plant recovery and regrowth. When the grazing period is short, and the rest period long, the roots are more active.

Plant cover and healthy roots result in better infiltration and moisture into the soil. Overgrazing can lead to depletion of the humus and glomalin content in the soil, which causes a reduction in the amount of moisture that can be absorbed and retained by the soil.

How to manage your grass after the rainfall

Many farmers are feeding supplements to ensure demand is met while trying to preserve some ground cover. The cover from higher residuals will also shade the ground from the heat and help to reduce further moisture loss.

Even with the recent rainfall, it is important that supplements are maintained to ensure the grass has a chance to recover.

This is likely to take around 21 days to allow the grass plants to rebuild their roots reserves and to push up some leaves before the leaves are removed by a grazing animal.

It could be that you maintain animals on sacrifice fields that may need reseeding or can be allowed to recover later in the year.

Currently, the lack of moisture is affecting the uptake of nitrogen (N), so there is no value in spreading it.

It is likely that after the first rain, N that has been mineralised in the soil or not utilised from previous applications will drive the recovery growth, but additional N will be needed to boost grass production, especially if silage cuts are still required. Remember to only apply N when soils are moist enough for active growth.

Otherwise, N will be lost to the environment. The response rate per kg N may vary from 10–15 kg DM (Dry Matter) on high-fertility soil; it will not be as high as in the spring.

Parasites are likely to be a problem with the recent rainfall, as although the dry weather has reduced the parasites available for ingestion, the larvae would have just moved into the soil and are likely to return once there is moisture.

Regular monitoring of faecal egg count (if possible) or liveweight gains to ensure parasites are not affecting performance will be important.

Further information

GREATsoils - practical information on soil management

Improving pasture for better returns

Forage For Knowledge - interactive grass growth dashboard

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Katie Evans

Senior Knowledge Exchange Manager - National Specialist (Grass Forage & Soil)

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