How do selective herbicides work? Most chemical users at some time or another, have Proherbicides wondered how selective herbicides actually work, and how they ‘select’ the weeds and leave the useful plants alone. A number of the grass selective herbicide products are not herbicidal when stored in their original container. This is Herbicide selectivity because they are manufactured as ‘proherbicides’ that change into the active chemical form once inside the weed. The common phenoxy herbicides, such as 2,4-D and MCPA, are selective for broadleaf weeds and work by An example of proherbicides are grass selective herbicides disrupting the fine balance of biochemical processes that containing diclofop-methyl. Diclofop-methyl is a occur in these weeds. proherbicide, not a herbicide. Once it is applied and absorbed by the leaves of the weed, an enzyme in the The phenoxy herbicides mimic a naturally occurring plant plant cells removes the methyl group from the diclofop, and chemical called Indole Acetic Acid (IAA). They cause the diclofop then kills the weed by inhibiting the production unregulated IAA production in the plant, which leads to of an enzyme that assists fatty acid synthesis. uncontrolled growth, twisting, thickening, elongation, and eventually death as the weed literally grows itself to death. Fatty acids are an integral part of cell membranes, waxes and the leaf cuticle. When fatty acid production stops, the Other selective herbicides target photosynthesis, the weed dies because it cannot continue to carry on the basic process where plants produce energy from sunlight. biological processes essential for life. Normally, blocking photosynthesis would result in a very slow death for weeds as they starve to death however So why don’t selective herbicides kill the useful plants we these herbicides seem to work too quickly for this to be the are trying to grow? The main reason is because the useful case. plants are able to metabolise (break down) the herbicide more rapidly than the weeds, allowing them to survive. In Scientists think that blocking photosynthesis is only part of other cases such as 2,4-DB, some useful plants are unable the story. The other thing that may happen when to break it down into the active form of 2,4-D, thus they photosynthesis is blocked is that highly toxic oxygen and survive while weeds that can break down to 2,4-D do not. hydroxyl compounds form within the weed. These compounds rapidly destroy cell membranes, causing the Herbicide resistance weed to die. The development of herbicide resistant weeds has been a Many selective herbicides target enzymes within plant real issue impacting sustainable weed control for over 20 cells. Enzymes are proteins that work as catalysts in many years. processes within cells, allowing complex chemical reactions to occur under conditions where they probably Herbicide resistance occurs when a plant that would otherwise wouldn’t occur. normally be controlled by a herbicide, survives the application of a usually lethal rate, and the change that Enzyme activity in plants normally occurs in a controlled caused the resistance is heritable. If the change is state of equilibrium. When a herbicide targets an enzyme heritable, it can be passed on to succeeding generations of site, it disrupts the sequence of complex chemical the weed by seeds. reactions, and in many cases, produces highly toxic compounds in the plant, which kills the weed. For a number of years, herbicides have been grouped by their mode of action to allow herbicide resistance to be managed. The table overleaf shows the current Mode of Action groups for herbicides in Australia (as of April 2013). How do selective herbicides work? Mode of Action groups for herbicides in Australia (as of April 2013) Resistance Risk Mode of Action Group Typical Actives High A clethodim, diclofop, fluazifop, haloxyfop, propaquizafop, sethoxydim B chlorsulfuron, flumetsulam, halosulfuron, imazapyr, metsulfuron, triasulfuron Moderate C atrazine, bromoxynil, diuron, linuron D chlorthal, oryzalin, pendimethalin E carbetamide, chlorpropham F diflufenican, norflurazon, picolinafen G acifluorfen, butafenacil, carfentrazone, flumioxazin, oxyfluorfen, pyraflufen H benzofenap, pyrasulfotole, isoxaflutole I 2,4-D, clopyralid, dicamba, MCPA, mecoprop, picloram, triclopyr J 2,2–DPA, flupropanate, molinate, prosulfocarb K napropamide, dimethenamid, metolachlor, propachlor L diquat, paraquat M glyphosate N glufosinate O dichlobenil, isoxaben P naptalam Q amitrole, clomazone R asulam Z flamprop, endothal, DSMA In Australia, the letters (A, B, C etc) are used to identify the A key method for managing resistance is keeping chemical different Mode of Action groups whereas overseas, the use records, which can be used to monitor what Modes of numbers (1, 2, 3 etc) are used. Action have been used. It is compulsory to make specified records within 48 hours of using an agricultural chemical Unfortunately, if a weed becomes resistant to a herbicide, it product, and keep these records for a period of two years. often becomes resistant to other herbicides in the same This applies to all agricultural chemicals used, including Mode of Action group. Sometimes, it can even become poison baits used for pest animal control. This requirement resistant to all other herbicides in the same group. came into effect on 24 July 2007 and excludes the use of It is also possible for a weed to become resistant to household or home garden products. herbicides from more than one Mode of Action group. This An example of herbicide resistance is Annual Ryegrass is called ‘multiple herbicide resistance’ and is very, very (Lolium rigidum), found in broadacre cropping areas of difficult to manage. Victoria. Annual Ryegrass is frequently found to be How do selective herbicides work? resistant to some or all Group A herbicides. This means Further information that careful selection of herbicides from different Mode of Action groups is required to achieve effective chemical Australian Pesticides and Veterinary Medicines Authority control of this weed. Chemical control alone is not a long website: www.apvma.gov.au term answer. Chemical manufacturers There have also been several documented cases of Chemical resellers and agronomists glyphosate resistance in annual ryegrass and more recently, in barnyard grass. DEPI Chemical Use website: www.depi.vic.gov.au/chemicaluse Integrated Weed Management (IWM) DEPI Chemical Standards Officers To ensure sustainable weed control, Integrated Weed Email: [email protected] Management (IWM) should to be implemented. IWM involves the use of a range of different strategies against Fax: (03) 5430 4590 the weed, not just chemicals. DEPI Chemical Standards Officers Other IWM strategies available include: Statewide Physical control Steve Field (03) 5430 4463 Cultural (management) control Loddon Mallee Genetic control Alex Perera (03) 5430 4591 Biological control Hume Quarantine Felicity Collins (03) 5833 5203 Grampians Neil Harrison (03) 5336 6616 Gippsland Jane Rhodes (03) 5147 0832 Enquiries from other regions should be directed to the nearest of the above-named regional officers. © The State of Victoria Department of Environment and Primary Industries Accessibility Melbourne 2014 If you would like to receive this publication in an alternative format, please telephone DEPI Customer Service Centre 136 186, email [email protected], via the National Relay Service on 133 677 www.relayservice.com.au. This document is also available on the internet at www.depi.vic.gov.au This work is licensed under a Creative Commons Attribution 3.0 Australia licence. You are free to re-use the work under that licence, on the condition that Disclaimer you credit the State of Victoria as author. The licence does not apply to any This publication may be of assistance to you but the State of Victoria and its images, photographs or branding, including the Victorian Coat of Arms, the employees do not guarantee that the publication is without flaw of any kind or is Victorian Government logo and the Department of Environment and Primary wholly appropriate for your particular purposes and therefore disclaims all liability Industries logo. To view a copy of this licence, visit for any error, loss or other consequence which may arise from you relying on http://creativecommons.org/licenses/by/3.0/au/deed.en any information in this publication. ISBN 978-1-74146-408-5 (Print) ISBN 978-1-74146-409-2 (pdf) www.depi.vic.gov.au .