The selectivity of a herbicide varies between and within mode-of-action groups. For example, products such as glyphosate and paraquat are non-selective, that is, they kill most plants they contact. On the other hand there can be a lot of variability in selectivity within a mode of action. For example in Group 1, haloxyfop will kill most grasses, while others such as diclofop will selectively kill grass weeds in grass crops. There are also broadleaf herbicides, such as within Group 2 and Group 4, that will kill most broadleaf plants, while others in the same mode of action can control broadleaf weeds in certain broadleaf crops.

This selectivity depends on the plant’s ability to break down the herbicide before it gets to the target site in sufficient dose. Crop selectivity is often because the crop can break the herbicide down faster than the weed. Similarly, some weeds are able to rapidly detoxify the herbicide and hence they survive the application, while other weeds have poor ability to break down the herbicide and hence they are controlled. However, with trifluralin and some other pre-emergent herbicides, the crop may be just as sensitive. as the target weeds, so a physical separation of the crop and the herbicide is the mechanism for selectivity.

For some herbicides it may be possible to include a 'safener' in the formulation. This is a different chemical which increases the speed by which the crop can detoxify the herbicide, and hence improves crop selectivity. 'Safeners' can be found in several Group 1, Group 2 and Group 27 formulations, while a 'safener' is usually applied to sorghum seed to increase tolerance to s-metolachlor herbicides..

Often herbicide selectivity is reduced when plants are under environmental stress, such as waterlogging or low temperatures, as the crop cannot break the herbicide down fast enough before being damaged by the herbicide.

Crop tolerance can also be affected by herbicide tank mixes. For example, mixing a Group 1 with a Group 4 herbicide increases the metabolism of the Group 1 herbicide in the grass weed and reduces the level of control. Similarly, the addition of a Group 4 herbicide to several Group 2 herbicides may also increase the rate of metabolism of the Group 2 herbicide. In this case this is generally expressed as increased crop safety to the cereal crop.

Modes of action

Herbicides are grouped into modes of action. The mode of action refers to how the herbicide kills the plant, that is, the location on a specific biochemical pathway that is being disrupted. All herbicides within a single mode of action kill the weeds by targeting the same specific location.

The mode-of-action system was initially developed to help manage the development of resistance to herbicides, however by encouraging rotation or mixing of herbicides from different groups. Prior to 2021 Australia used a letter-based system to identify mode of action groups, however this has now been aligned internationally using a number based system.

For many herbicides, knowing a herbicide’s mode-of-action group will give users insight into what environmental and spray conditions are required for the optimum performance of that group. For example, poorly translocated herbicides such as bromoxynil (Group 6), glufosinate (Group 10), most Group 14 herbicides and paraquat and diquat (Group 22), and those with poor translocation like clethodim (Group 1) will require better levels of spray coverage than well-translocated herbicides such as glyphosate (Group 9) and 2,4-D (Group 4).

Some herbicides are not compatible in a tank mix due to how the different modes of action work. For example, tank-mixing glyphosate with trifluralin creates the problem that glyphosate relatively slow to enter the leaf and will be subject to rapid binding to dust or soil thrown onto the leaf surface post application, while trifluralin is highly volatile and needs soil incorporation within 4 to 24 hours depending on the rate applied. If incorporation by sowing occurs soon after application, glyphosate leaf uptake will be reduced by soil binding to herbicide residues still on the leaf surface. Conversely, if planting is delayed to allow time for the glyphosate to enter the leaf, then unacceptable trifluralin volatility losses may occur.

For growers applying trifluralin immediately in front of the planter a much better tank-mix partner is paraquat or paraquat plus diquat, due to their rapid absorption and activity in weeds. If using a paraquat based knockdown partner, incorporation by sowing can commence within about 15 minutes of herbicide application. 

Find out more

• See Croplife Australia https://www.croplife.org.au/wp-content/uploads/2023/07/2023-UPDATES_HerbicideMOATable.pdf

• GRDC In crop-herbicide guide.