The increased use of pre-emergent herbicides and zero till cropping in the northern cropping belt has focussed the spotlight on planting management as growers look to maximise herbicide efficacy and minimise costly crop damage.
Pre-emergent herbicides typically have more variables that can affect efficacy than post-emergent chemistries and if not applied strategically, can result in poor crop emergence.
Minimising crop damage requires an understanding of crop seeding depth, where the herbicide is positioned in the soil, soil type including the amount of organic matter and crop residue on the surface, herbicide solubility and its ability to be bound by the soil.
The Grains Research and Development Corporation (GRDC) is supporting a number of research programs focussed on generating a greater understanding of pre-emergent cereal herbicides, their interaction with seeder type and soil type and the resulting implications for weed kill and crop safety.
Over the past 20 years seeding equipment used by growers has changed considerably to enable greater stubble retention and more uniform seed placement while creating minimal soil surface disturbance.
As a result, knife-point and press wheel seeders have been widely adopted and more recently, disc seeders have also been utilised.
Research associate from the University of Adelaide’s School of Agriculture, Food and Wine, Dr Sam Kleemann told advisors and growers attending the recent GRDC Grain Research Updates in Coonabarabran and Warren that the physical separation of crop seed and herbicide was imperative if crop damage was to be avoided.
“The behaviour of pre-emergent herbicides from both a crop safety and weed control perspective can vary considerably depending on the seeding equipment used and the level and type of soil disturbance it creates,” he said.
“Given that most pre-emergent herbicides are non-selective, herbicide safety at sowing is often obtained by creating positional selectivity, that is, the physical separation between crop seed and herbicide.
“Achieving this separation involves the seeding system displacing and throwing herbicide treated soil into the inter-row to create a clean slot from which crop seed can safely germinate.
“This objective is more easily achieved by tined seeding systems fitted with knife-points which can more aggressively engage the soil than low disturbance discs.”
The switch by some growers to the no-till, knife-point press wheel has underpinned a revival in the use of older products like trifluralin and triallate (Avadex), as controlled soil throw under these systems often provides adequate separation between the germinating crop seed and treated soil to provide sufficient crop safety.
Other growers have adopted the disc-based zero till cropping system which enables them to maintain or even narrow their crop row spacings under high stubble loads.
GRDC supported trial work has shown that some herbicides and rates of herbicides are better suited to disc systems than others, highlighting the importance of herbicide selection and following label recommendations.
Dr Kleemann said inadequate soil throw and poor incorporation of trifluralin with single disc systems often caused crop damage and poor weed control and could lead to massive build ups in the weed seed bank.
“Such large build ups in the weed seed bank can have serious effects on the productivity of subsequent crops grown in the rotation,” Dr Kleemann said.
“In general, keeping weed seeds on the soil surface will improve control by pre-emergence herbicides, so minimising soil disturbance is recommended.
“It’s critical that growers choose the right herbicide for the job. Not all pre-emergent herbicides behave the same so it’s extremely important to follow label recommendations closely.
“Growers should also be mindful that volatile herbicides like trifluralin and triallate (Avadex® Xtra) require some incorporation to limit herbicide losses from volatilisation and sunlight degradation.”
Sam Kleemann, University of Adelaide
School of Food & Wine
0418 256 475
Sarah Jeffrey, Senior Consultant Cox Inall Communications
0418 152 859
GRDC Project Code