PRE-EMERGENT herbicides are an important front in the war against herbicide resistance in the cropping belts of Queensland and NSW.
But, as they are not sprayed directly onto the weeds like their post-emergent cousins, their performance is affected by how they behave in the soil and how and when they are incorporated.
Help is at hand with the Grains Research and Development Corporation (GRDC) offering two informative video tutorials.
Video one explains the solubility and binding characteristics of pre-emergent herbicides, while the second video investigates incorporation by sowing techniques with different types of sowing implements.
The videos collate information generated through the many research programs supported by GRDC that focus 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.
As the name suggests, pre-emergent herbicides are applied before weeds germinate and typically have more variables that can affect efficacy than post-emergent chemistries.
Performance often varies according to soil conditions and type, as well as herbicide placement and method of incorporation.
The focus on pre-emergent herbicides as a tool to control hard-to-kill weeds has been largely pre-empted by the increasing frequency of failure of post-emergent chemistries across the northern cropping belt, particularly on grasses and fleabane in-crop and on fallow.
Herbicide resistant winter grass weeds such as glyphosate resistant annual ryegrass (ARG) Group A resistant wild oats, as well as summer grass weeds such as feathertop Rhodes grass (FTR) and awnless barnyard grass are posing significant management challenges for northern growers and advisors.
With pre-emergent herbicides becoming an important component of integrated weed management programs in the northern grains region, industry consultant and video co-contributor John Cameron, ICAN, said key considerations for growers when using these chemistries included location of the weed and crop seeds, soil type, stubble load, soil moisture and rainfall (both amount and timing) and where the herbicide is placed.
For herbicides applied at sowing and incorporated by sowing, the type of sowing equipment used and how it interacts with other factors is also important both for weed efficacy and crop safety.
“Behaviour of pre-emergent herbicides in the soil is governed by the solubility of the herbicide, how tightly the herbicide is bound to soil components and the rate of breakdown in the soil,” Mr Cameron said.
“Minimising crop damage requires an understanding of how sensitive the crop is to the herbicide used and where the crop is sensitive as well as how to keep the germinating crop seedling separated from the herbicide.
“Here, knowledge is needed of how to manage crop seeding depth, soil throw at sowing to manage where the herbicide is positioned in the soil after sowing, the impact of 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 or organic matter.
“These topics are covered in the two videos, giving growers an overview of the information they need to ensure that applications of pre-emergent herbicides incorporated at sowing are both effective and the risk of crop injury is managed.”
Growers in the summer cropping regions of NSW and Queensland are increasingly using pre-emergent herbicides for control of grassy weeds in winter cereals and Mr Cameron said the video on incorporation by sowing techniques would have particular relevance.
“With several of the soil applied pre-emergent grass herbicides used in winter crops, crop selectivity/safety in cereal crops is reliant on spatial separation of herbicide in the soil from the crop seedling,” Mr Cameron said.
“To do this, tyned seeding implements are used to strip soil away from the treated row and throw it into the interrow area.
“Managing the soil throw out of the treated row at sowing, balances the mechanics of the sowing implement, sowing depth, row spacing, soil conditions and speed of travel. Too much soil throw will result in herbicide treated soil being thrown over the top of the adjoining crop row, potentially increasing the risk of damage.
“The video includes some excellent graphics depicting where pre-emergent herbicides are in the soil relative to the crop as influenced by different types of soil movement in the seeding operation. It also covers in schematic form an overview of some of the key concepts of how pre-emergent herbicides behave in the soil.”
Growers and advisors requiring more detail on any topics related to pre-emergent herbicide use can CLICK HERE to download a copy of the soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers.
