In The Paddock
Seasonal sheep flock management tips August 2002

Principles of pasture growth

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Key points

• Ryegrass/white clover pastures are productive and tough
• Optimum growing temperature ranges are 15–20 deg C for ryegrass and 20–25 deg C for white clover
• Large regional differences in total pasture production are mainly due to climate
• Seasonal growth patterns vary little between regions, with a peak in spring/summer
• Optimum pasture growth occurs at pasture masses of 1000–2250 kg DM/ha

Ryegrass/white clover pastures are the backbone of our pastoral industries. They are highly productive in a range of environments and management situations.

They are easy to establish and persist well under set stocking and rotational grazing. They are tough and can bounce back from under-grazing, over-grazing, drought and pugging, if the maltreatment period is not too prolonged.

Understanding pasture growth

Ryegrass grows from buds that develop inside the leaf sheath, just below ground level. This protects the buds from grazing and trampling damage.

New tillers grow from these buds and produce their own root systems.

Each leaf goes through a cycle of growth, maturity, ageing, death and decay. This occurs whether or not the leaf is grazed.

Optimum growth occurs in a soil temperature range of 15–20 deg C. Some growth occurs at temperatures as low as 5 deg C in high fertility soils and 7–8 deg C in low fertility soils.

Growth is severely restricted in very dry and/or hot situations. In late spring ryegrasses go to seed with a dramatic decline in digestibility.

White clover is a prostrate or semi-prostrate plant. It produces new stolons or stems that grow along the ground from an original crown.

There are usually 3–6 leaves per stolon, and continued leaf production depends on continued stolon development.

Clover starts growing when the soil temperature reaches 10 deg C, but the optimum is 20–25 deg C. This is the main reason why clover peaks later in the season than ryegrass.

Flowering also occurs later in the summer than ryegrass, and continues for a much longer period. Unlike ryegrasses, the feed value of white clover does not decline after flowering.

Seasonal patterns

Total pasture production varies enormously from region to region. Climate (rainfall and temperature) has the biggest influence, but soil type and fertility are also important.

As soil fertility increases, low fertility-tolerant species like browntop and sweet vernal decline in the sward. The proportion of ryegrass and white clover increases.

While the leaf quality of low fertility grasses is similar to ryegrasses, they are much harder to keep leafy and palatable going into summer. Annual production is also much lower.

Seasonal growth patterns are reasonably consistent across the country. Pasture growth peaks in spring/early summer, when 60–80 per cent of the year's growth occurs, and bottoms out during winter. Some areas have a growth flush after the first autumn rains.

Low growth in late summer and autumn is due to drought. The winter trough is due to low temperatures.

Total annual pasture production sets the upper limit to farm carrying capacity, while the seasonal pattern of growth has a big influence on livestock management systems. For example, the onset and duration of spring growth usually dictates lambing and calving dates.

The art of effective feed planning is to synchronise pasture growth with animal demands.

Ideally, the increasing feed requirements of late pregnancy coincide with the onset of spring growth and lactation coincides with the peak pasture growth period.

Conversely, by the time pasture quality and production declines in summer, young slaughter stock and dry breeding animals will have left the farm, reducing pressure on available feed supplies.

Grass grows grass

The amount of grass left behind after each grazing influences the rate of new pasture growth.

If too little pasture remains, there will be insufficient leaf area for photosynthesis and growth will be less than optimum. Over-grazing also draws on plant energy reserves and if prolonged, may lead to the death of improved plants like ryegrasses and clovers.

In contrast, if too much pasture remains after grazing, much of it will die and decay. At very high residual levels, digestibility falls to very low levels and the loss of pasture through decay can exceed new growth.

Ryegrass/white clover pastures have a broad range over which growth is optimised (1,000–2,250 kg DM/ha). By keeping pasture covers within this range, feed digestibility and animal growth will be maximised.

The amount of leaf removed at each grazing affects the rate of regrowth (see diagram at right). Because plant B loses less leaf than plant A, it has more leaf area available for photosynthesis and begins to regrow almost immediately after grazing.

More information

Your local farm consultant or veterinarian will also be able to help. Alternatively, contact your local WoolPro extension specialist: