Investigating the links between trifluralin persistence in Western Australian soils and trifluralin resistance in resident annual ryegrass (Lolium rigidum) populations

The pre-emergence herbicide trifluralin is widely used in the minimum-tillage cropping systems of Australia, with the result that resistance to trifluralin is increasing in the major weed of the region, annual ryegrass (Lolium rigidum). Repeated exposure to low herbicide rates is also known to result in the rapid evolution of resistance in weed populations. As trifluralin is highly volatile, readily photo-decomposed, metabolised by soil microbes and to bind strongly to soil organic matter, there are many factors that could result in weed populations receiving reduced (even sub-lethal) rates of the herbicide. To investigate whether trifluralin dissipation could play a role in the increasing levels of trifluralin resistance in annual ryegrass, resistance levels of populations from 18 Western Australian farms were compared with the dissipation rate of trifluralin applied to soil collected from these farms. Although there was no direct correlation between resistance level and trifluralin half-life, there were links between resistance and soil properties which suggest that higher rates of trifluralin dissipation could make a minor contribution to the development of resistance.