Several new residual broadleaf herbicides have been registered recently for use in cereals, canola and pulses, with additional chemicals in late-stage development. However, as new products they generally come at a significantly greater cost than the older, post-emergent herbicides traditionally used. Therefore, these products will most likely be used strategically in the cropping programme.

The following table outlines some of these products:

The following factors should be considered when deciding to use one of these products:

Improve spraying logistics - residual herbicides should reduce the need for EPE herbicide applications, allowing other paddocks to be sprayed in a timely fashion.
Less spraying passes - the reduced number of spraying passes will reduce the cost of herbicide application including labour, machinery depreciation, repairs and maintenance plus fuel usage. If contractors are used this will result in significant cost savings.
Mitigating adverse weather conditions - the waterlogging experienced in 2021 is one way that weather conditions can hamper timely spraying and control of weeds. Residual herbicides reduce the requirement for spraying, therefore reducing the risk of adverse weather conditions impacting spraying operations.
Weed populations - paddocks with a high broadleaf weed burden are a good place to start with these products, as it reduces the burden placed on post-emergent herbicides to control all weeds. A follow-up application can then be used to control any survivors or later germinations of weeds. Alternatively, paddocks with low populations of weeds may not require any broadleaf weed control in-crop after a residual product is applied.
Increased chemical diversity - new modes of action will help combat herbicide resistance.
Reduced tank mixing – if broadleaf weeds are controlled by residual chemicals, there will be less need to apply antagonistic broadleaf herbicides when using selective grass sprays and fungicides during post emergent spraying.
Grazing Withhold Periods - residual products generally have longer Grazing Withhold Periods than post-emergent herbicides, therefore when applying to grazing crops the duration to expected first grazing will need to be considered.

Rural Management Strategies conducted a trial using Voraxor herbicide across four paddocks in 2021. All paddocks had significant broadleaf weed populations germinate, however these were not able to be sprayed at the preferred time due to waterlogging. The Voraxor in three of the four locations significantly reduced the broadleaf weed populations to the extent that no further weed control was required, similar to the experience of clients who applied the product across whole blocks in 2021. The following pictures demonstrate the level of weed control achieved by Voraxor 8 weeks post application.

The use of residual broadleaf herbicides can provide many advantages, particularly to large scale growers through reduced spraying passes, reduced use of resources, greater timeliness of operations and a reduction in the likelihood of resistance developing. This is in conjunction with achieving as good or better weed control compared to conventional post emergent chemistry. This is offset by increased up-front chemical costs. The registration of new herbicide groups is very positive for the industry and growers should use this opportunity to drive down weed burdens and increase the efficiency of their production system.

New Generation Broadleaf Residual Herbicides