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Understanding how the perennial ryegrass plant grows

20 June 2019
Type Media Article


At present there is a huge push by Teagasc to get farmers to maximise grass growth and utilisation on their farms. We have the Grass 10 campaign to promote sustainable grassland excellence which is achieved by the launch of the Grass 10 farmer training courses, grass walks held throughout the country and the grassland farmer of the year competition. There is a lot to know and understand to maximise grass growth potential and utilisation. Soil testing to establish if ground conditions support good grass growth. It is important to set up good grazing infrastructure, have the correct stocking rate and know the right type and rate of fertiliser to apply at different times of the year. The majority of intensive livestock farmers would prefer to grow Perennial Ryegrass and include clover and grow Italian Ryegrass for silage, as they are high yielding, nutritious and deliver high output per hectare. The Teagasc Pasture Profit Index (PPI) 2019, details varieties such as Abergain, Aberclyde, Nifty etc of which farmers can select a variety to suit their farming system.

The one key thing to understand for successful grassland management is how the perennial ryegrass plant actually grows. Perennial ryegrass can only support growth of three leaves at any one time. When a fourth leaf starts to emerge out of the top of the plant the leaf at the bottom starts to die away. The rate at which the plant produces a new leaf will obviously depend on the time of year and growth rates. For example in early spring a new leaf is produced every 30 – 40 days and therefore you need a long grazing rotation. However during peak growing periods in May and June a new leaf is produced every 7 – 8 days, requiring a grazing rotation in the region of 18 – 21 days. The variation in the rate at which the plant produces a new leaf is the reason why you need to carry out a grass budget on a weekly basis. The aim of the grass budget is to simply match your rotation length to the time taken for the plant to grow three leaves. If the ryegrass plant grows above 10-12cm, a fourth leaf will appear out of the top and the bottom leaf will start to die away. It is these dead leaves that prevent swards from being grazed down tight (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 grass quality.

When the grass plant is grazed out, the first thing it has to do is get enough energy to grow the first leaf. This happens quickly where the butt of the grazed out sward is still green and free from dead leaves. The green leaf quickly traps the energy from the sun and along with energy reserves stored in the crown. This is the reason why silage swards that contain no green leaves after harvest are slow to green up. When the seed head is eaten this will encourage the crown of the plant to produce more tillers and therefore increase sward density. Control pre-grazing heights and ensure paddocks are grazed down tight to maximise growth and utilisation. If you understand how the perennial ryegrass plant grows, avoiding white butt or seed head by grazing before the fourth leave appears you are increasing grass  production and farming in a environmentally sustainable manner.