Keep revised WRAG guidance in mind when managing stubbles ahead of autumn drilling

Spraying stubble with Roundup – Norfolk, October

Matt Siggs, Bayer

Guidelines from the Weed Resistance Action Group (WRAG) underline the importance of integrating cultivation and chemistry to achieve the best overall results. “The basic guidance is for a maximum of two glyphosate applications after harvest and before drilling. If you have time to make two applications, use cultivation in between to eliminate any survivors” explains Matt.

“Rates and application technique are important too, make sure the dose is sufficient for the hardest to kill weed in the field. And, don’t go too fast and risk sub-optimal doses which could leave survivors requiring further treatment.”

Farmers using zero-till approaches need to be particularly careful around glyphosate applications as they don’t have cultivation available as a follow up method. Matt suggests that farmers need to look at a range of methods to manage weeds. Having a diverse rotation with varied establishment dates, possibly using cover crops can all help avoid over-reliance on glyphosate.

Some of the key risk factors to be aware of include:

Population dynamics

High weed populations tend to increase the risk of resistance development. Take action to drive down numbers before using herbicides to reduce the risk.

Lack of diversity in control methods

Reliance on few or only one method of control greatly increases the risk of resistance developing. This applies to chemical and cultural control methods, the more diversity across the cropping cycle, the better.

Cultivating oil seed rape stubble in Cambridgeshire, using Great Plains (Simba) SLD600 with Case 550 Quadtrac

Repeat applications to survivors

A small number of weeds often survive a herbicide application. In all likelihood, this is not due to resistance but because of application or field effects. In any case, take no chances, ensure survivors are controlled with another chemical mode of action or cultural method. Repeat applications of glyphosate to surviving weeds is a major resistance risk.

Low rates

Insufficient application rate for the target weed and poor application timing means too many weeds survive the application. These surviving weeds are a potential resistance risk and an immediate problem for farmers aiming for clean, high-yielding crops.

Spraying cultivated stubble with Roundup – Norfolk, October

Poor application technique

Incorrect sprayer set up and operation can result in under or overdosing the product, affecting performance in the field. In the long term, this can contribute to resistance development. Examples of poor application technique include:

• Poorly maintained machinery which is not routinely calibrated and serviced
• Incorrect selection of nozzles for the job at hand and/or not replaced when worn out
• Operating at the wrong boom height and travelling too fast (+12km/hr)

Additionally physical losses from drift, run-off and bounce, and chemical losses due to pH, hard water and light degradation can all reduce the quantity of active reaching the target hence causing below par control.

Incorrect timing

Adverse weather conditions and weed growth stage can both impair glyphosate performance. For example, poor downward translocation will occur during the stem extension phase of growth.

Further information on the Roundup product can be found on Bayer’s website.