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Pasture management - the 3-leaf principle

Ryegrasses (annual and perennial) are New Zealand’s major pasture grasses sown by farmers.

How do they grow?
  • Ryegrass plants are a collection of ‘tillers’ or new shoots that grow from the ‘growth point’ in their base.
  • Each individual tiller produces three fresh live leaves.
  • The oldest leaf (first to emerge) starts to die when the fourth leaf begins to emerge and grow.
  • The leaves are the the plant’s factory – they use photosynthesis to produce energy for growth that farmers measure as ‘Dry Matter’ (DM).  DM is pasture minus the water.
  • Pasture begins to grow slowly again after grazing, as each ryegrass plant has to start putting out new leaves again.
  • Growth rate then increases with each new leaf bigger than the previous one.
  • So during:
    • Week 1 you get 10% of pasture growth
    • Week 2 growth produces 35% of potential
    • Week 3 growth produces 55% of potential.
  • Feeding value (digestibility) is generally similar for leaf 1 and leaf 3.
  • If pasture is allowed to grow beyond the three-leaf stage, leaves die; feed value falls as dead material builds up.
  • These tall pastures should be grazed before the canopy closes over and you can’t see the ground among the individual plants, or the clover will suffer from shading.  It may look as if you have a lot of good green feed, but open up the pasture and see the large areas of bare ground at the base.
When to graze?
  • Pastures should be grazed leaving about 4cm as a ‘residual’ for dairy cattle, and about 3cm for sheep.
  • Don’t graze below 3.5-4cm for dairy cows and 2cm for sheep otherwise pasture regrowth will be delayed, and stock will eat more dead matter, causing ‘ill-thrift’.
  • Leaving pasture behind reduces pasture quality in the long term.
  • So graze when the three leaves have grown.
Where problems can arise
  • In the ‘spring flush’, or when Nitrogen fertiliser has been used, plant leaves can grow very large, and lower-quality seedheads soon appear.
  • It gives the false impression that there is a mass of good feed.  Maybe there is in the first grazing but not after that.
  • When this happens the plants produce stem to lift the new tillers and leaves above the shaded area and this results in ‘canopy closure’.
  • So although growth is rapid the feed quality declines rapidly.
  • Very short rotations (less than 15 days) used to counteract this will instead reduce pasture yield and will do little to maintain quality in the longer term.
  • Weeds grow where bare ground has been exposed, and the soil can dry out and reduce subsequent pasture growth.
  • Cutting, like grazing, will stimulate pasture plants, forcing them to start growing leaves again.
  • The plant will remain in ‘negative nutritional balance’ until 1.5 – 2 leaves have grown.
  • Repeated grazing weakens the plants and could kill them all.
  • Key point:  So prevent pastures growing out of control and reaching this ‘canopy closure’ stage by earlier grazing or taking them out of the rotation for a silage cut.
How long does it take for three leaves to grow?

The time taken for one leaf to fully grow is about:

  • 8-15 days in spring (24-45 days for all three).
  • 11-15 days in summer (33-45 days for all three).
  • 13-30 days in autumn/winter (39-90 days for all three).
Measuring this on your farm
  • Pick ten ryegrass tillers across the paddock that the stock are about to graze.
  • Count the number of new leaves/tiller – ignoring leaves previously grazed (with cut ends).
  • Work out the average number of leaves/tiller.
  • Divide the number of days since the last grazing by the average number of leaves/tiller.
  • This gives the leaf appearance rate, which if multiplied by three, gives the average time for a plant to produce three leaves – and hence should be ready for grazing.
  • Then you can see how long it’s going to take for the pastures to reach an optimal grazing time.
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