REVISED DECEMBER 2015 PRE-EMERGENT HERBICIDES FACT SHEET NORTHERN, SOUTHERN AND WESTERN REGIONS UNDERSTANDING PRE-EMERGENT HERBICIDES AND HOW THEY INTERACT WITH THE ENVIRONMENT With the continued evolution of herbicide resistance, growers are being forced to introduce a range of different weed control tactics. A tactic that has rapidly increased in recent seasons is the use of pre-emergent herbicides, especially in the summer crop and fallow. To predict field performance of these herbicides, an understanding is needed of their chemical properties and how they interact with the environment. The value of pre-emergent herbicides When devising a weed control strategy, consider the use of pre-emergent herbicides as an additional tactic available to help drive weed numbers down. Used alone they usually will not achieve the objective of driving down weed The presence of stubble or existing seedbank numbers, but when used vegetation can intercept herbicide amongst a suite of tactics, they can be before it reaches the soil surface. particularly effective. KEY POINTS Pre-emergent herbicides: offer an alternate mode of action to Knowing which weeds are in the paddock and where the weed seeds are located many post-emergent options; (shallow or deep) is important in selecting a herbicide to be applied. can reduce selection pressure on Be aware of whether a herbicide is subject to volatilisation or photodegradation in subsequent post-emergent herbicide order to determine an incorporation strategy that minimises loss to the environment. applications; Solubility influences how much rain is required for herbicide incorporation, how easily remove much of the early season weed a herbicide will be taken up by a germinating weed and crop, and if a herbicide will be competitive pressure on a crop and can subject to moving down the profile, potentially causing crop injury or loss to leaching. protect yield better than post-emergents, Sandy or low organic matter soils will have less binding and allow for greater herbicide especially in weedy paddocks; availability for crop and weed uptake. can save costs, especially in the fallow Herbicides that bind tightly to soil and organic matter generally require higher where multiple knockdown applications application rates, stay close to where they are applied (unless the soil moves), and may be required; persist for longer. can reduce the time pressure on Soil pH affects how long some herbicides persist for and how available they are for spraying operations, especially in plant uptake and soil binding. situations when double knocking is a The persistence of a herbicide and the way in which it breaks down will dictate the requirement; length of residual control and plantback constraints to sensitive crops. have a major role to play in patch Rainfall after application is important for incorporation and availability to the weeds and eradication where a weed blow-out crop. Rainfall and temperature also affect degradation. can be GPS logged and a pre- Choice of application rate will affect length of residual, and possibly crop selectivity. emergent herbicide can be applied to manage the patch; Level 4 | 4 National Circuit, Barton ACT 2600 | PO Box 5367, Kingston ACT 2604 | T +61 2 6166 4500 | F +61 2 6166 4599 | E [email protected] | W www.grdc.com.au Page 2 FIGURE 1 Interactions, loss and breakdown pathways of could result in an increased proportion of soil applied herbicides. herbicide getting to the soil include: winds across the rows; application speed less than 16km/h; use angled (rear facing) nozzles where Volatilisation Photochemical decomposition the angle offsets the travel speed to have droplets moving predominantly Plant & weed downwards through the stubble; uptake Spray large droplets travelling at higher speed. interception Incorporation Narrow fan angles (e.g. 65-80 degree) increase droplet speed; keeping water rates high to increase the Run off Adsorption by soil number of droplets produced; organic matter and clay narrower nozzle spacing (25cm vs Microbial 50cm); and decomposition minimising boom height, but ensuring at Chemical least double overlap. breakdown Leaching/ In situations where pre-emergent herbicides deep percolation are desired to be used in-crop, then directed spray set-up (layby application) can be used to minimise crop canopy can be applied after the last cultivation Availability of a pre-emergent herbicide is interception. to manage the few weed emergences an interaction between the solubility of the that would otherwise be allowed to herbicide, the strength of binding onto soil Photodegradation grow and return seed to the soil after a colloids and organic matter, the prevailing Photodegradation occurs when a cultivation event is undertaken; and climatic conditions, the environment, and herbicide undergoes a chemical reaction the rate of herbicide applied. are useful in crops where there is a in the presence of sunlight, becoming lack of post-emergent options, such Stubble and crop interception broken down and lost to the weed control as grass weed control in sorghum or system. For most pre-emergent herbicides broadleaf weed control in pulses. The presence of stubble or existing used in Australian farming systems, vegetation can intercept herbicide before it Factors influencing the photodegradation is not a major path of reaches the soil surface. This can have two activity of pre-emergent breakdown as standard incorporation negative effects: herbicides practices such as cultivation, planting or adequate rainfall after application are 1. the herbicide tied up on the stubble or To understand how pre-emergent typically adequate to prevent unacceptable in the crop canopy may not be available herbicides will perform, it is important to losses. However, when a herbicide is for soil incorporation and subsequent understand the properties of the molecule sprayed onto the soil surface in summer weed control; and and the soil type and how they interact and with no following rainfall or mechanical are broken down in the environment. 2. the existing standing organic material incorporation, losses from this pathway will may result in an uneven coverage of the be at their highest. soil surface and so may result in areas with insufficient herbicide coverage, Some of the common herbicides that can resulting in weed escapes. undergo some level of photodegradation include: the Group C herbicides atrazine, Herbicide tie-up on stubble and plant fluometuron, simazine, terbuthylazine material depends upon the herbicide and diuron; sulfosulfuron; pendimethalin; solubility and the strength of binding to picloram; and s-metolachlor. organic matter. Depending upon the molecule involved, the herbicide may If these herbicides are applied under warm, be tightly bound and hence lost to the dry conditions with no rainfall expected system in terms of weed control despite in the coming weeks then mechanical subsequent rainfall (for example trifluralin). incorporation should be considered to reduce losses to photodegradation. By comparison, a product that is loosely bound and soluble can be washed off Volatilisation stubble and into the soil by rainfall (for Some of the pre-emergent herbicides example chlorsulfuron). used in the Australian grains industry Where high levels of stubble or plant material are considered volatile. This means they exist, the level of spray droplet interception will transition to a gaseous phase after Well-timed pre-emergent herbicides can can be reduced by alterations to how application if left on the exposed soil provide season long control. herbicide is applied. Some techniques that surface without incorporation. Therefore Page 3 TABLE 1 Vapour pressure for selected pre-emergent herbicides and Solubility summary of incorporation requirements. Many pre-emergent herbicides are taken up o ^ Herbicide (example) Vapour pressure (mPa @ 25 C) by the roots of the germinating weed. For ® tri-allate (Avadex ) 12 root uptake to occur, the herbicide needs to trifluralin (Treflan®) 9.5 be available in the soil moisture. If the soil is ® Actives with a vapour pressure of greater than 1mPa are s-metolachlor (Dual Gold) 3.7 generally considered volatile and are likely to require dry, there is little herbicide in the soil water dimethenamid-P (Outlook®) 2.5 incorporation. Refer to individual product labels for that is available for root uptake. This is specific situations. pendimethalin (Stomp®) 1.94 why many pre-emergent herbicides may clopyralid (Lontrel®) 1.36 fail to provide good weed control under prosulfocarb (Boxer® Goldp) 0.79 dry conditions. ® metribuzin (Sencor ) 0.121 Herbicides with low solubility often require ® terbuthylazine (Terbyne ) 0.12 larger volumes of rainfall to achieve ® propyzamide (Kerb ) 0.03 incorporation and tend not to remain imazapic (Flame®) 0.01 as available in the soil moisture, so they atrazine (Gesaprim®) 0.0039 Actives with a vapour pressure of less than 1mPa are are not easily taken up by plant roots. triasulfuron (Logran®) 0.0021 generally considered non-volatile and do not usually require any specific incorporation recommendations after Typically, they need very good moisture isoxaflutole (Balance®) 1.0 x 10-3 application. conditions after application, and for the diuron (various) 1.15 x 10-3 period of desired weed control, for pyroxasulfone (Sakura®) 2.4 x 10-3 effective performance. ® -3 diflufenican (Brodal ) 4.25 x 10 Conversely, herbicides with high solubility ® -5 picloram (Tordon ) 8.0 x 10 are relatively easy to incorporate