Research by CSIRO Livestock Industries is moving closer to identifying plants which can be more successfully grown as feed for livestock in salt-affected areas.

A recent project at Tammin in the Western Australian central wheatbelt has found there are many more factors than first thought influencing how sheep consume saltbush - a widespread, salt-tolerant native plant.

About 10 per cent, or 1.8 million hectares, of farm land is affected by dryland salinity in WA's agricultural area and a further six million hectares is at risk.

Project leader, research scientist Dr Hayley Norman, says that, as a result, the quest to identify suitable plant species to allow for the ecologically sustainable and productive use of salinised land is imperative.

Some of the research results have already challenged several existing theories about sheep grazing responses.

While it is commonly accepted that sheep will selectively graze plants with higher nutritional value and lower natural toxins, this is not necessarily the case at Tammin.

The young Merino sheep in the four-week trial preferred the River species of saltbush to Old Man saltbush, confirming anecdotal information from farmers.

To the researchers' surprise, the scientific theories for the sheep's selected consumption did not clearly illustrate the differences between their preferences.

Dr Norman says the experiment results indicate there are other, more complex variables influencing how sheep graze saltbush.

"Sheep preferred the River saltbush plants - although they were less digestible and had less crude protein than the Old Man saltbushes - where we would have expected the opposite," she says.

"Differences in preference were not clearly associated with oxalates, tannins, sulphur or sodium, all factors that could reduce the palatability of plants."

However, at another research site in the eastern wheatbelt near Yealering, researchers have observed clear differences in palatability within saltbush species.

"Sheep will carefully strip every last leaf from an individual plant, while ignoring the neighbouring plant," Dr Norman says.

"We do not know if these differences in preference are linked to factors that drive animal productivity. It is a bit of a mystery that we are continuing to explore."

Further research is now planned for 2005 to examine differences in productivity from sheep grazing the two saltbush species.