Top sprayer tips: How to maximise glyphosate efficacy on autumn stubbles

Effective pre-crop management of grass-weeds is perhaps even more critical to overall weed control than what tactics are employed during the life of a winter cereal.

Alongside effective cultivation strategies and timing of drilling, the use of Roundup (glyphosate) on stubbles before drilling is perhaps the most important control measure in the season. No other herbicide is likely to give the same level of efficacy, so it is one to make sure is applied properly to maximise its impact, even if workloads are heavy.

There is also the risk of resistance developing to glyphosate – sadly a common problem in other parts of the world where glyphosate has been relied on too much and overused. In the UK, there have been some isolated signs of reduced sensitivity to glyphosate in bromes and Italian ryegrass, which serves as a warning sign about being careful in its use.

Practical application guidelines produced by the Weed Resistance Action Group were updated in 2021 to help growers reduce the risk of resistance development, with a key message around using a maximum of two applications in the window between harvest and drilling, explains Roger Bradbury, market development manager for Bayer.

“Two applications are regarded as an acceptable risk, provided there is some other mitigation – e.g. cultivation between applications.

“That’s to avoid applying repeat applications to surviving plants, as that is the biggest risk.

“And if you can’t do that, for example, in a no-till situation the advice is only to use one spray.”

The other key advice is around timing and appropriate dose for the size of weed, he says. “The optimum timing is the two- to three-leaf stage of the grass-weed, with the minimum dose being 540g/ha of glyphosate, rising to a minimum of 720 g/ha once weeds are tillering.”

Wait for a significant flush of weeds, if possible, before spraying, says Mr Bradbury. “To my mind, that’s hundreds of plants per sq m. If you’re making significant inroads into grass-weed populations and struggling to get that level of flush, which is a good thing, you might argue you only need one application.”

The final application before drilling, whether that’s a first or second spray, ideally should be within 7-10 days of planting, Mr Bradbury suggests. “If you’re a month ahead of planting, there is a significant risk of subsequently emerged weeds being present when you’re drilling, and even if you have some level of cultivation when drilling there is a risk of transplanting those weeds into the crop.”

As well as stewardship, getting the best return on investment from glyphosate should focus minds on application practice, he recommends . “If conditions are ideal, you might be able to get away with one or two things being suboptimal, but when conditions are against you, you don’t.”

“If the dose is too low, the timing is out, or the application practice is poor, that’s when you start to see variable performance in the field.

“The way product is applied influences how much of it gets to where it needs to be, and while the correct dose will likely have been recommended, it could be that a suboptimal dose is being applied if application practice is not up to standard.”

Travelling too fast can result in poor efficacy, for example, he says, through the influence of excessive boom movement, creation of turbulence and drift reducing deposition on the target.

“While growers know stubble applications is a job they need to do, their minds might be focused on drilling and other tasks, and it can be rushed to tick it off the list and not necessarily given the thought to optimise the application.”

Spray application is crucial, Agrovista’s Chris Martin agrees. The target is usually a small grass-weed, for which he advises a medium spray quality with the addition of an application aid to manage and mitigate drift.

“When you go from a medium spray quality of around 200 microns to a coarse quality of around 400 microns, you have eight times fewer droplets for the same volume. If you’re trying to hit 1-2 leaf black-grass plants with fewer droplets, you’re not going to have the same coverage.”

New automatic boom control technology should also help reduce drift, he notes. “As an industry, we get fixated on low drift nozzles, but they do impact the efficacy of any contact product by dropping the number of droplets.

“Using lower boom heights with the technology available, and application aids which won’t increase the size of your droplets significantly while getting rid of the really fine ones of less than 100 microns diameter which are prone to drift, will help get the best efficacy of the product.”

He suggests setting up the sprayer with an 03 blue nozzle, travelling at 10 km/h at 3 bar pressure and 150 L/ha of water.

What do sprayer operators do currently?

Our survey of around 50 sprayer operators in spring 2022, suggests that autumn stubble application is one where operators feel happier to go more quickly with 19% travelling at 14km/h or more. Only fungicide applications and harvest management sprays in wheat have a higher proportion going that quickly.

Most sprayer operators, however, are sticking to the slower speeds with 62% at 12km/h and 19% at 10 km/h. Water volumes are typically in the 100-150 L/ha region – 92% use these, while air induction nozzles are most popular (61%). Most (55%) use some form of angled nozzle.

Why are we doing this series?

While a lot of attention is rightly placed on product choice, it’s important to maximise the output from this investment – the difference between good and poor spray application can potentially have a significant influence on the performance of the treatment

There are a lot of factors that will influence that success – some within the control of the sprayer operator and others outside. These include physical losses of whatever you are trying to apply through things like drift, run-off and bounce, and chemical losses caused by hard water, pH and light degradation, for example.

On top of that are field factors, such as the target’s characteristics like shape, structure and size, and weather that all might affect performance. By the time the spray reaches its intended target the final dose could be very different to what it was when put into the sprayer.

The final blog in the series of Top Sprayer Tips for the Season Ahead will be available from September 16th, 2022.

Blog 8 in the series of Top Sprayer Tips for the Season Ahead focuses on How to maximise grassweed control from pre-emergence herbicides in cereals.