Trial looks at herbicide residue yield penalties

It is one of many trials being run by the Liebe Group on Matthew Hyde's farm at Dalwallinu which has had its wettest year since 1999 with 384 millimetres to date, including 110mm received over January and February, followed by 25mm from ex-Tropical Cyclone Seroja on April 12, without any damage.

Herbicides that have long carry-over residues can often limit cropping options, while labels are often not entirely clear and it can be difficult to know ahead of time if rainfall has been sufficient to limit potential crop damage when planting back into a residue.

Liebe Group research and development (R&D) co-ordinator Judy Storer said the topic had been brought up several times by the Liebe R&D committee in previous years.

"We initially decided to do the trial as there have been a lot of growers who were unsure what the consequences of planting into herbicide residues are," Ms Storer said.

"They were also unsure as to if or what sort of yield loss they are getting by planting into that residue and how that might have been different if they used an imidazolinone (IMI) tolerant variety."

IMI tolerant crops refer to varieties that have been bred, normally using genetic modification, to not be affected by a certain type of herbicide.

They are mainly used for weed control in situations where standard herbicides aren't working for a variety of reasons and give growers another tool to use.

"Growers are able to spray a herbicide that the crop is resistant to, but the weeds hopefully aren't, so it'll kill the weeds without affecting the crop," Ms Storer said.

"However, because those IMI varieties have been modified to have genes that make them resistant to herbicides, that often also affects the way they perform as a crop and usually the resistant varieties don't yield as well as standard varieties."

As part of the trial, 10 residue treatments were addressed, each of which was applied in the middle of last year in a fallow paddock.

On May 19 this year, seven different crops were sown over the herbicide residues, including both standard and IMI tolerant varieties of wheat (Scepter and Hammer), barley (Buff and Maximus) and canola (Bonito and Hyola 540XC), as well as a standard lupin.

The results from the trial have been surprising as there has been very minimal noticeable difference between the treatments and only minor differences in the performance due to those herbicide residues.

"Some of the herbicides we sprayed have plant back periods that the label state would kill the crop if planted within 36 months of spraying," Ms Storer said.

"However we had nearly no crop effect, no differences in crop numbers and only minimal differences in how healthy the crop looks despite planting within eight months instead of the recommended 36 months.

"No sign of damage has been noted in the wheat or barley plots, while some signs of stunting were evident in the canola plot, however this did not correlate with any significant reduction in plant numbers."

However, the caveat is that the trial was completed on a heavy clay soil type with pH above seven, which affects the breakdown rate of the herbicides.

There was also a lot of rainfall over summer - 380mm between spraying and seeding - which increased the speed of the breakdown of the herbicides.

"This year is not really a fair representation of how these herbicides would normally behave and the results may differ significantly at sites with lighter, lower pH soils and in seasons with different rainfall patterns," Ms Storer said.

"It does show that if there are certain conditions, including really heavy summer rain, that growers can potentially get off with less penalties than they might think.

"However it is difficult to know that for certain before they put the crop in as there is no testing to see how much residue is left in the soil."

The trial is set to be repeated next year with different seasonal conditions and a lighter soil type which should be a more representative result.