DairyCo

How the grass plant grows

Published 9 February 10

This section looks at:

How the Grass Plant Grows
Understanding the Sunlight and Sugar Relationship
Down to Grass Roots
Grazing at the Three-leaf Stage
How Hard to Graze?
Measuring Grass

How the Grass Plant Grows

To maximise the production from grazed grass it is necessary to ensure the optimum production and intake of the most palatable, highest feed value grass. Grass naturally wants to grow a seed head, but this will reduce sward palatability and digestibility, so grazing systems need to prevent this happening if grass quality and quantity are to be maintained.

Effective systems graze the plant at the point where it is growing fastest with the highest ratio of leaf to stem. The plant grows fastest between the emergence of the second and third leaves, when sunlight capture is maximised. The amount of grass grown is 25-30% more with the third leaf present than with just two. The fastest growing paddock on-farm has three leaves. A clump of ryegrass is made up of tillers. Each tiller has its own independent roots. They reproduce by budding off daughter tillers at the base, which build up new clumps. Buds are your future sward density.

Grass is a living plant in a continuous cycle of growing and dying, so a ryegrass tiller only has three live green leaves at any time. Water and fertiliser will influence the size of those leaves.

Grass has a central leaf pushed up from the growing point at the base of the tiller; a second leaf, collecting sun and producing sugars; and a third leaf doing the same. The oldest leaf will die as the newest leaf takes its place, so the plant will only ever have three living leaves.

A new leaf takes a minimum of six days to appear in spring. But depending on the temperature this will be extended to 30 or 40 days in winter. This can be influenced by soil moisture in extreme cases.

Therefore, in spring when it is taking six days for a new leaf to appear, three leaves will appear in 18 days, so the pasture will be ready to graze 18 days after grazing or cutting. When new leaves appear at a slower rate, the optimum time between grazings will be longer. However, the plant also wants to reproduce, so it aims to grow reproductive tillers which put all their energy into producing a seed head.

This is influenced by:

• Day length.
• Temperature - with a ryegrass typically requiring a temperature of about 100C for a week.
• Grass variety.

A reproductive tiller becomes fibrous and suppresses the daughter tillers at the bottom. When managing a grazing sward we need to understand this and manage grass to resist reproduction and to maximise grazing potential.

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Understanding the Sunlight and Sugar Relationship

The first leaf grown after grazing uses up sugars stored in the bottom 4cm of the tiller to initiate its growth, not from plant roots. This sugar store is built up again as the plant puts out its second and third leaf. This first leaf is the smallest, the second leaf is larger and finally the third leaf is the largest. It is possible to maximise the sunlight capturing ability of grass by preventing back grazing or grazing of silage aftermaths and allowing the third leaf to grow before the plant is grazed. When cows graze off the pale lime green new regrowth of the first leaf the plant is unable to replenish its sugar store thus regrowth slows. Grass with just the first leaf is also an unbalanced feed. It is high in nitrate, low in sugar, low in calcium and magnesium, and high in potassium.

Allowing the tiller to grow its second leaf gives it a chance to build up its store of sugars. Then its rate of growth will increase. When grazed too frequently, parent tillers can't support daughter tillers as there is too little sugar available. 

Down to Grass Roots

There is evidence that rotational grazing increases the efficiency of the roots at capturing moisture and nutrients from the soil and this helps prolong grass growth through the summer.

Roots do not start to grow until at least one leaf has grown, as all the plant energy is diverted into this first leaf. In a dry period, roots need to work hard at picking up any available moisture and when the first leaf is grazed off this is affected.

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Grazing at the Three-Leaf Stage

In Victoria, Australia (from the Target 10 campaign) the average dairy farmer gets cows to graze 8t of DM/ha of grass. In one year of practising the three-leaf principle, this can be increased by 1-2t DM/ha without any extra inputs. Some farmers achieve 15t of DM/ha of grass eaten.

Tiller quality is best at the three-leaf stage in terms of:

• Sugar
• Nitrate
• Minerals
• Starch
• Fibre

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How Hard to Graze?

Grazing below 4cm will diminish plant reserves for plant regrowth. The aim should be to leave a height of 4-5cm between clumps and 8-10cm over clumps, with a maximum of 30% clumps in a paddock. When there is more grass than this post-grazing, consider topping the paddock to 10cm within 24 hours before the lime green regrowth occurs as topping this will slow regrowth.

(Anecdotally a mower is better than a topper as it cuts cleaner and you should get better regrowth.) If you don't top in this situation, the amount of ungrazed grass can increase after each grazing. It should not be necessary to top after every grazing when the amount of grass allocated is correct, but it can be useful to get the grazing back under control.

Measuring Grass

Grass production can be measured in two ways:

1. Using a plate meter and measuring kg of dry matter/ha of grass present - which is the method used by grass+.

2. Counting the leaves - the three leaf method.

There are other ways to measure grass such as the use of cages and cutting and weighing but these methods may not be as practical on farm as a plate meter or the three leaf method.

Both systems rely on grazing grass at the optimum point in terms of growth and quality. Both are rotational grazing systems which allow grazing management to suit the grass habit and to achieve the maximum return.

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