FARMERS are being forced to use poisonous chemicals and revert to outdated tilling methods to cope with a growing breed of herbicide-resistant "super weeds".
The problem, triggered by overuse of the popular weedkiller Roundup, poses health and environmental risks, including soil pollution and toxicity to humans, and is substantially driving up farm costs.
Since the 1980s, Roundup, otherwise known as glyphosate, has been heralded as a farming panacea - cheap, easy to use and relatively safe. But in several countries, including Australia, an over-reliance means weeds have evolved to withstand it.
The first glyphosate-resistant weed in Australia, annual ryegrass, emerged in 1996. Another five have since been added to the list.
In cities, authorities are also battling glyphosate-resistant weeds on roadsides and railway lines.
Monsanto, the owner of the Roundup brand, is the world's leading glyphosate producer, however the chemical is also sold under other brands.
A leading Adelaide-based weed expert, Chris Preston, said the chemical had become "a victim of its own success".
"Glyphosate is one of the biggest [weed resistance] problems because it's such an important herbicide. It's got huge value to the farming system," he said.
A spokesman for the Department of Agriculture, Fisheries and Forestry said as glyphosate use continues, ''the number of resistant plants will continue to multiply''.
Alternative chemicals and tillage, which can also lead to soil erosion, "may result in health [and] environmental risks from using herbicides that are more toxic to mammals or remain in the soils,'' he said.
Glyphosate-resistant weeds can also stifle crop yields and lead to higher farm costs, including fuel for machinery and more expensive herbicides.
Most cases of glyphosate resistance in Australia occur in northern NSW, where year-round rainfall, and resulting weeds, have led to heavy use of the herbicide.
Grain and pulse farmer Graeme Constance, of Bellata, near Narrabri, has reverted to selective tilling and older herbicides known to cause nosebleeds and enter groundwater systems.
The world-first glyphosate-resistant species of awnless barnyard grass was found on his farm in 2007 and has since been documented on 50 properties across Australia.
"Even though we've reduced [the weeds], it's not quickly eradicated," Mr Constance said.
"We're trying very hard to look after our land and do things in an environmentally friendly way … but we have to [grow] food."
A Monsanto Australia spokeswoman said herbicide resistance was one of the greatest modern challenges facing farmers, and ''the threat of resistance to glyphosate-based herbicides is real''.