How cultural grassweed controls can deter glyphosate resistance

Applying glyphosate to wheat stubble

©Tim Scrivener

Growers are urged to exhaust all cultural grassweed control options pre-drilling of cereals this autumn to reduce the risk of resistance developing to the vital weedkiller glyphosate.

At present, the non-selective herbicide is the only viable chemical option for grassweed control in stubbles and stale seed-beds and its loss could be devastating for those with a significant grassweed problem.

Contact herbicides such as diquat (Retro) and carfentrazone (Shark) have approval for use in stubbles, but are only effective against a range of broad-leaved weeds.

It is estimated that the potential loss of glyphosate could hit crop yields by 20% if it was rendered ineffective through resistance or banned by regulators.

See also: How to create the perfect blackgrass stale seed-bed

Jim Orson

Jim Orson ©Tim Scrivener

With the scale and frequency of use rising sharply in recent seasons as key grassweeds such as blackgrass and Italian ryegrass become resistant to key in-crop herbicide chemistry, it is feared that multiple applications will hasten glyphosate resistance development.

New Weed Resistance Action Group (Wrag) guidelines launched at Cereals 2015 provide growers with best practice advice on negating the rise of glyphosate resistance.

One of the key components of the Wrag advice urges growers to use alternative measures to glyphosate, whether that is chemical or cultivation.

Niab Tag weed expert Jim Orson believes that where growers aren’t ploughing, using as many shallow cultivation passes as soil conditions allow ahead of using glyphosate is the best option.

“Although we are unsure of the true risk of glyphosate resistance, it needs to be treated as a very real threat and cultivation is much better than spraying multiple times with glyphosate,” he explains.

“The key is not to cultivate too deep and bring up old seed. Go no deeper than 5cm, as blackgrass won’t emerge from any depth greater than that.”

Allelopathic effect

It has been touted that blackgrass releases root exudate that has an allelopathic effect, preventing surrounding seed from germinating and reducing the effectiveness of pre-drilling control.

Getting the best out of glyphosate

Growers should treat stubble and stale seed-bed glyphosate applications with the same care and attention given to controlling septoria in wheat, according to Monsanto’s Barrie Hunt.

“People need to get out of the mindset that ‘it’s only treating stubbles’. It isn’t.

“Growers targeting weeds such as blackgrass can get very good control with glyphosate at a much cheaper cost than herbicides applied in the crop. Why compromise that?

“It’s a great opportunity to get things right,” he explains.

With a huge number of glyphosate products on the market Mr Hunt says the first step is to choose a modern formulation from an established manufacturer.

He adds that there can be an advantage adding an adjuvant to some glyphosate products, but newer types will have wetters and anti-drift agents already in the can.

For the autumn stubble and stale seed-bed season, when blackgrass is unlikely to be particularly large, a minimum of 540g/ha of active ingredient is advised to ensure effective kill and, more importantly, minimise the risk of survivors.

“Use a spray quality on the finer side of medium if conditions allow and between 80-200 litres/ha of water depending on the product,” explains Mr Hunt.

Niab Tag’s Jim Orson adds that the company’s trials have proved a water rate of 100 litres/ha is optimum in most instances.

Mr Orson argues that this isn’t the case, with no scientific evidence the allelopathic effect exists. Instead, he believes there is soil surface shading from emerging blackgrass seedlings.

“Blackgrass likes light to germinate and cultivation may not be as reliable for killing blackgrass, but it will encourage further germination and counter the shading effect,” he says.

Last blow

Mr Orson suggests using as many passes as possible where soil conditions and moisture are adequate, timing each pass when germinated plants begin to shade the soil surface.

One robust hit of glyphosate can then be used just prior to the final seed-bed preparation, reducing chemical applications and subsequent resistance selection pressure.

“Don’t expect too much though. If you have a hell of a problem, even the best stale seed-bed techniques and delayed drilling aren’t going to get you out of a fix.

“If you are having to do multiple passes with a cultivator or glyphosate, can you get number to acceptable levels in the following cereal crop?

“You can’t put too much pressure on the pre-emergence herbicides, because if it comes dry, you could have a disaster on your hands, so consider whether you should establish a winter cereal at all,” says Mr Orson.

Green bridge

Monsanto’s technical development manager Barrie Hunt agrees that there is a necessity to combine glyphosate application with cultural methods and says two sprays would be the optimum.

“There was talk of people using four or more applications of glyphosate last autumn and that’s clearly not a good idea, but twice is a good target.

“There is also the option of including glyphosate in the pre-emergence herbicide mix to hit any weeds that emerge just before the crop,” says Mr Hunt.

He adds that while many will focus on annual grassweeds such as blackgrass, breaking the “green bridge” to prevent the build up aphids and crop diseases and controlling perennial weeds such as couch grass will also be a key consideration.

“Always look at how successful your treatment has been and if there are any surviving grassweeds, ask the question why, but if you get product dose, timing and application right, you should be OK,” he adds.