How best to manage our native pastures?

January 31, 2009
By: Mark Kesby, Projects Officer, LPLM

I had thought that by simply changing from set stocking to rotational grazing we could increase both the diversity of species and productivity of our native pastures. However, research has shown this to be a simplistic view. The closer you look at this issue, the more complicated it gets.

Before we introduced sheep, cattle and rabbits to our native grasslands and woodlands they had a diverse range of grasses, forbs (broadleafs), shrubs and trees and consequently a wide range of birds and animals. Overgrazing (especially sheep and rabbits), drought, fertilising and adding exotic legumes and grasses have greatly reduced the range of natives that these pastures now contain.

There has been considerable research conducted in southern NSW which suggests that the critical factors that determines the range of species in a native pasture are phosphorous levels (amount of fertiliser added over time), grazing management and the drought strategy adopted. While I recognise that this research has been conducted in a different climate to here, it would seem probable that much of the findings will apply to us.

The research team is from the Future Farm Industries CRC. They surveyed 24 properties in southern NSW and northern Victoria in 2006 and 2007 and interviewed the property managers. The properties had a wide range of past and present practices from set stocking and high fertiliser use through to no fertiliser and cell grazing by sheep, cattle or both. They also reviewed all relevant past and present research.

The researchers found that highly fertilised native pastures contained only a small number of native grasses along with introduced annual clovers, grasses and broad leaved weeds and can be highly productive. Many of the native grasses and all the forbs have disappeared.

High rates of fertiliser reduce native forbs like Glycine

These pastures are very susceptible to overgrazing and subsequent domination by annual grasses and broadleaf weeds unless carefully managed in drought.

Native pastures with a long history of prolonged grazing with little or no fertiliser typically have low productivity and low stocking rates. The research found that many of these paddocks still had a high plant diversity. Native pastures that were previously fertilised (or cropped) were very different in that they had very low diversity and usually contained lots of weeds. Sound familiar?

Many native perennial grasses are highly adapted to our low fertility soils (particularly phosphorous). They rely on specialised fungi growing on their roots (Mycorrhiza) to help obtain scarce nutrients from the soil. As soil phosphorous increases with fertiliser use, the associated mycorrhiza declines and the increased soil fertility favours short-lived fast growing plants (typically exotics). Wallaby Grass and Weeping Grass (Microlaena) are examples of natives that can tolerate moderate fertility while Kangaroo Grass cannot.

To me this indicates that if we want to manage a paddock for biodiversity, then it is important to choose a paddock with the right history and a reasonable range of species. To increase biodiversity, don’t fertilise and conservatively stock with decent rest periods at critical times. If you start to fertilise native pastures, then you will start to lose natives (especially forbs) as fertility rises. This may increase productivity but needs to be managed carefully as these pastures are less resilient. In particular, drought management is critical. Overgrazing at such times of stress can result in the loss of perennials which are replaced by annual grasses and broadleaf weeds.

 Download the full paper here ... (pdf)