New South Wales Department of Primary Industries (NSWDPI) weeds agronomist Andrew Storrie admits that double knockdown weed control will cost graingrowers more than glyphosate.

But, Mr Storrie says, the extra cost of applying two, different mode of action herbicides a week or so apart will be insignificant compared to the management required if glyphosate resistance develops.

The NSW and Queensland DPIs are collaborating on a Grains Research and Development Corporation (GRDC) supported search for integrated weed management strategies that will prevent herbicide resistance developing on northern region grain farms.

The NSWDPI research team tried several double knockdown approaches on glyphosate resistant ryegrass on the Liverpool Plains, with good results, but Mr Storrie says the technology should be useful in any situation where the development of glyphosate resistance is of concern.

"The double knockdown concept is not new, because the name was coined by ICI in the late 1960s with the development of direct drilling," Mr Storrie says.

"Then it referred to the application of Spray.Seed followed by full-disturbance sowing of the crop, giving better weed control than herbicide alone.

"The release of glyphosate in the late 1970s didn¹t change this concept to begin with, because the new product was relatively expensive, leading to its use at low rates that required some cultivation to give full control of many weeds.

"As the price of glyphosate dropped, true no-till was economically possible, but the development of glyphosate resistance has meant that reliance on glyphosate alone is not sustainable."

Mr Storrie says that, while the Liverpool Plains trial has shown that double knockdown works against glyphosate resistant ryegrass, it also shows timing of application and herbicide selection is critical.

Preliminary results from the trial show glyphosate CT alone ­ at 1.2 litres a hectare ­ will check resistant ryegrass, but that only about 23 per cent control can be expected.

Glyphosate followed by paraquat eight days afterwards gave 90 per cent control, and glyphosate followed by Spray.Seed 78 per cent. Delaying the second application to 14 days improved control to 97 per cent for paraquat and 90 per cent for Spray.Seed.

Two successive sprays of Spray.Seed, at 94 per cent control, and two of paraquat (96 per cent) were equally effective while applying Spray.Seed or paraquat before glyphosate reduced control by 10 per cent.

"All applications of Spray.Seed and paraquat in the trial were at 1.5 litres a hectare, because we were very mindful of the costs involved, and we didn¹t want to go beyond what we thought we could reasonably expect growers to spend," Mr Storrie said.