Residual herbicides have become an important tool in the war against weeds for many northern region growers in recent years.
Residual herbicides are applied to the soil in order to kill weeds by root/shoot uptake and can remain active in the ground for a few weeks up to a few years.
They are a critical part of modern farming systems - particularly minimum and zero till operations - and the Grains Research and Development Corporation (GRDC) continues to support a multitude of industry-led projects aimed at enhancing the effectiveness of integrated weed management programs while minimising crop damage.
However their use impacts on decisions across the entire farming system and may restrict cropping options or varietal selection, particularly in areas that have experienced a drier than expected summer and autumn.
Growers are urged to carefully consider their plant-back options for the 2014 winter season in country previously treated with in-crop or fallow residual herbicides as label recommendations for replant intervals usually require a minimum amount of rainfall to kick-start breakdown of the residual herbicide.
Potential damage is largely dependent on the soil type, soil temperature, seasonal conditions, microbial activity and importantly, soil moisture levels.
In areas which have not received the required rainfall to complete herbicide breakdown, there are a number of active ingredients which may pose a problem to some susceptible winter crop varieties including Picloram, Metsulfuron, Imazethapyr, Isoxaflutole, Chlorsulfuron and Atrazine/Simazine.
Double cropping sorghum country into chickpeas is likely to be a popular rotation for some Central Queensland growers this winter and residue potential needs to be considered prior to planting if crop damage and yield loss are to be avoided.
Growers looking to plant chickpeas need to be particularly careful if atrazine was applied to the preceding sorghum crop as atrazine, like other herbicide groups, takes longer to break down in dry conditions.
Commonly used Group B chemicals (imidazolinone) could also pose a risk to winter crops where summer/autumn rainfall has been limited. In most instances a minimum amount of rain from the date of application is needed to break down herbicide residues. This amount of rainfall is dependent on the crop to be planted but the speed of breakdown can vary according to soil type, soil pH, soil temperature, microbial activity and the residual chemical used.
Knowing the chemistry and mode of action of each chemical is paramount to deriving the best combination of weed control and crop safety, whilst also ensuring you are doing your best to minimise the chance of herbicide resistance.
To determine what crop can be safely sown following a residual herbicide application growers need to calculate the herbicide rate, quantify the amount of rainfall from the date of application up until sowing the plant back crop (excluding isolated storm events during summer) and carefully consult product label recommendations.
In some cases, herbicide tolerant crops may need to be considered and growers should contact their local agronomist to discuss their herbicide program and planting options.
The best sources of information on individual varieties are the GRDC-funded 2014 Wheat Variety Guides which are a collation of data from the NVT program conducted across the region. The website and brochure benchmark the performance of regionally important varieties together with individual disease and agronomic ratings, varietal information and recommendations on planting windows and herbicide sensitivity.
Further information on chickpea production as well as the Queensland Wheat Variety Guide are available via GrowNotes at www.grdc.com.au/GrowNotes or at www.grdc.com.au/WheatVarietiesGuide-QLD and www.nvtonline.com.au/crop-guides/nsw/
Kelly Becker, GRDC Northern Region Panellist, Theodore