Care Required with Residual Herbicides01 July 2014
AUSTRALIA - With the winter season now well underway across much of the central west, weed control is high on the priority list for many growers, writes Penny Heuston, GRDC Northern Region.
Well managed weed control plays a vital role in the production of profitable winter crops as weeds can have a significant impact on yield by competing for moisture, nutrients, space and light as well as harbouring diseases.
It’s one of the key reasons that 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.
Most growers understand that weed control needs to be considered on a whole-of-farm basis, being particularly conscious of the cropping and chemical rotations where residual herbicides are applied.
While residual herbicides are an integral part of successful integrated weed management programs, particularly in minimum and zero till operations, their use can hold important implications for the germination and vigour of successive crops and residue issues can affect crop and varietal selection.
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 best utilised in conjunction with other control strategies such as such as growing strongly competing crops, mechanical control, strategic tillage, brown or green manuring and strategic burning.
Breakdown of residual herbicides within the soil relies on rainfall and/or microbial degradation. In some cases at least 200mm is required for this breakdown, but it is also affected by soil type, soil pH, soil health, soil temperature, microbial activity and the chemical used. So breakdown is quicker in the warmer, wetter months and slows when the soils cool in the winter.
With drought affecting large areas of the northern cropping belt over the past year, the impact of residual herbicides on agronomic and crop management decisions has come to the fore. Plant backs will be lengthened in drier conditions and the label recommendations need to be consulted with this in mind
Before deciding on whether or not to include a herbicide in a control program, growers need a thorough knowledge of the target weed including weed identification, biology and susceptible growth stages; the herbicide including its mode of action (chemical grouping) and application methodology; and site conditions such as soil moisture, surrounding plants, weather conditions and soil characteristics.
Growers need to be WeedSmart and implement a chemical rotation plan between seasons that rotates between chemical groups as the bane of herbicide resistance becomes a reality for many producers in the north.
It is important for growers to have a clear rotational plan for their property going forward at least two to three years.
This will enable farmers to have a clear picture of what crop will be in what paddock the following year and what chemicals they can or cannot use within the current season.
To determine what crop can be safely sown following a residual herbicide application, growers need to calculate the herbicide rate, quantify rainfall from date of application up until sowing the plant back crop (excluding isolated storm events during summer) and carefully consult product label recommendations.