How does grass grow and why is 1,400kg/ha the perfect pre-grazing height?

So given that the majority of grass sown on farm today is perennial rye grass (PRG), what do we know about how it grows?

Well, the first thing to note is that a PRG sward consists of a number of individual plants. Each plant will be made up of a number of tillers and each tiller only maintains three live leaves at any stage.

Only one leaf will grow at a time and, in the summer, a new leaf grows every seven to eight days.

So after 21 days, the grass plant will be at the three-leaf stage and here after, quality will reduce as a fourth leaf will appear, causing the first one to die. This now leaves you with dead material in the sward.

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The plant's stem will also elongate and thicken out at this stage in preparation to support a seed head. The stem is more fibrous and has less feed value for the animals.

So once we go past the three-leaf stage, grass quality is subsequently reduced, which will hit production and utilisation of the sward.

Stick to the rotation

This highlights the importance of sticking to a 21-day rotation during the summer, as with an average growth of 66kg/day over the 21 days, you will be grazing paddocks at an ideal cover of 1,400kg/ha.

At the other end of the scale, what happens when we graze grass at a lower grass height or at the two-leaf stage?

Yes, grass quality will be higher, but you will lose out on yield and slow down regrowths. The loss in yield is easily identified as you are grazing a plant with only two leaves rather than three.

Regrowths

Where the regrowths are hit is in the plant's ability to grow a new leaf after being grazed. Grass absorbs light through the leaf and creates energy via photosynthesis for the plant to grow.

The more leaves and surface area the plant has, the greater energy reserve it can build up in the roots and when grazed has a greater ability to produce a new leaf quickly.

So grazing a plant at the one- or two-leaf stage, the plant energy reserve is a lot lower and, in turn, slows down the plant's ability to produce a new leaf post-grazing.