GRASS: How it grows

Research has shown that the UK is capable of growing more than 20 tonnes (t) of grass dry matter (DM) per ha, yet the current average in the UK is 7.9 t DM/ha. An increased focus on growing grass and emphasis on utilising home-grown grass and forage crops will enhance business resilience against volatile market trends. Crucial to this is an extensive knowledge of the most important asset, the ryegrass plant.

Grassland management is key to a profitable system, but awareness of the natural growth cycle of a grass plant and how it can be manipulated needs to be improved. Perennial ryegrass, the UK’s most commonly sown species, only has three live leaves on every individual plant (tiller). As the fourth leaf starts to grow, the oldest leaf dies. When grass growth is at its highest, usually in May, a new leaf is produced every four to five days. At peak growth, all three leaves can be replaced within two to three weeks. But when grass growth is at its slowest in mid-winter, it can take 30 days to produce one new leaf.

Time after grazing graphThe best time for grazing is when the plant is at the 2.5 to three-leaf stage. Allowing the grass plant to continue to grow past this stage (above 8-10cm) will mean a fourth leaf will appear. The bottom leaf will then start to die away. It is these dead leaves that prevent swards from being grazed down tight (sward height of 3.5-4cm). As a result, a butt of white, dead grass accumulates at the bottom of the sward. This not only delays regrowth rates but also reduces the metabolisable energy content of the sward, which is the biggest driver of meat production.

Monitoring grass growth rates is crucial when forming a grazing plan. Sward heights should be assessed weekly as the situation can change very rapidly, especially now temperatures continue to rise, both above and below ground. Tracking growth rates on a piece of paper or as a graph can be very useful as it means forage shortages can be forecast and quality can be maintained within the sward.

The dry weather of late means extra vigilance should be taken during grazing. Low residual sward covers (<1500kg DM/ha) in this dry period could have a detrimental effect on grass growth later in the season. It is advisable to remove stock from paddocks at no less than 1,500kg DM/ha cover, this will impact sward utilisation but it will reduce the level of stress on the improved grass species within the ley. When improved grass plants, such as perennial ryegrass, are forced to utilise energy reserves, poorer, less efficient grass species and ‘tap root’ weeds can out compete the improved species for nutrients and crucially moisture within the soil. This could prove costly as reduced sward nutritive values and volumes will have to be buffered as weed populations increase.

Key points:

  • The perennial ryegrass plant can only support three leaves
  • Aim to graze pastures at the three-leaf stage (8-10 cm) to maximise productivity from the sward
  • Target a post-grazing height of 3.5-4cm. This allows for regrowth between grazing
  • Minimise a build-up of decaying leaves at the base of the sward as it reduces animal performance

For more information, see the BRP manual Planning Grazing Strategies for Better Returns.