Residuals a critical factor
THIS year’s residual herbicide applications may play a critical role in dictating 2019/2020 summer crop options for growers across the northern grains region, with dry conditions continuing to plague large tracts of New South Wales and Queensland.
Without adequate moisture, residual herbicides take longer to break down in the soil which may pose a potential risk to the germination and vigour of successive crops.
The persistence of residual herbicides can be difficult to predict as it depends on numerous factors including soil temperature and moisture, microbial activity, soil type, soil pH, soil organic matter and the chemistry of the herbicide used.
However, there are some key points for growers to keep in mind over the coming months:
• Soil microbial activity is critical for herbicide breakdown and is significantly affected by lack of moisture.
As a general rule of thumb, if the top 10cm of soil isn’t moist, then little herbicide degradation is occurring regardless of how many months have passed.
• Consider how rain has fallen since the herbicide application rather than simply calculating the rainfall total.
A single rainfall event with months of dry topsoil is less effective at sustaining microbial populations compared to the same rainfall occurring as a number of events that keep the topsoil wet for longer.
• Consider the mobility of the herbicide. Those with higher water solubility and lower affinity for soil binding can move down the soil profile after rainfall events and sit below the zone of optimum microbial activity.
This can increase persistence. • Adhere to label recommendations on plantback period to minimise the risk of crop damage
The Grains Research and Development Corporation is dedicated to helping growers and advisers extend their understanding of residual herbicide behaviour and performance in different soil types under different seasonal conditions, through investment in workshops and technical resources.
Workshop facilitator, Independent Consultants Australian Network senior consultant Mark Congreve, said keeping abreast of the latest information on pre-emergent chemistries could help guide rotation planning and paddock selection in response to changes in seasonal conditions.
“That’s certainly the case this season where growers may have applied pre-emergent herbicides expecting that adequate rain would fall during last summer and into this winter,” Mr Congreve said.
“In many areas, this simply hasn’t been the case which may trigger plant back implications for the upcoming summer crop.
“However, predicting herbicide persistence can be complex.
“When on the edge of the labelled plantback periods, understanding the soil type, the environmental conditions since application and the herbicide properties can give a higher level of confidence as to the likelihood of damaging residue levels remaining.”
The GRDC is hosting Herbicide Behaviour workshops in the lead-up to the summer cropping season at Moree on October 15 to 16, Toowoomba on October 17 to 18 and Gunnedah on October 31 to November 1.
For more information about the issue, visit the GRDC events page grdc.com.au/ events or to register yourself visit the website www.icanrural.com.au /herbicideworkshops.html.