Be mindful of herbicide plant back restrictions
When planning this year’s cropping rotation and starting date of seeding, it is important to be aware of any plant back restrictions from herbicide applications. These plant back restrictions can be from herbicides applied last season or from a summer spray application.
Herbicides break down by either microbial or chemical degradation. Both processes are increased when moisture is high and temperatures are warm. They are both influenced by soil type, pH and organic matter.
Last season’s dry spring has meant there are herbicide applications that would not meet the minimum rainfall requirement for particular herbicides to break down. A common example would be the application of certain Group B imidazolinones herbicides that may require up to 250mm of rainfall post application before a sensitive crop can be grown.
Some summer spray herbicides can have short residual plant back periods. An example of this is 2,4-D Ester 680, which has a short plant back period. This is particularly important for pulses. The ester plant back does not start until 15mm of rain has fallen. This has to be taken into account especially if no rain occurs from spraying until planting as delaying seeding the crop would be necessary to avoid damage.
Another trap can be herbicides that are registered for use on a particular crop but have plant back periods for applications prior to planting the crop. For instance, metsulfuron which can safely be applied in wheat and barley but has a plant back period of six weeks for barley and 10 days for wheat due to seedling sensitivity.
Knowing the plant back restrictions of different herbicides can help avoid serious crop injury or costly delays in sowing. It may be possible to change crop or variety choice to overcome any problems with herbicide application from the previous season. For summer/autumn herbicide applications, it may be possible to change the herbicide mixture to one that has no plant back restrictions. Either way, reviewing herbicides applications would be time well spent prior to seeding this year.
The Smart Agro