Top sprayer tips: How to maximise grassweed control from pre-emergence herbicides in cereals

Grass-Weed Control Key Considerations

Perhaps more than any other application, risk management is a key consideration when applying pre-emergence herbicides.

Neither the crop nor weed has emerged, so decisions around risk are required to manage the field appropriately to the conditions and perceived levels of weeds, says John Cussans, NIAB weed biology expert.

“You have to have the right level of herbicide for the weed spectrum and pressure, which can only be judged on the history of the field, crop being grown, cultivations used and drilling date. And then the conditions around drilling, drill system, and weather around application determine crop safety,” he says.

Increasingly larger stacks of residual herbicides are being used at pre-emergence for grass-weed control in cereal crops, he notes, with recent additions to the armoury perhaps having narrowed the crop safety margin, while offering greater efficacy.

Crop safety of many residual herbicides is based around separating the crop seed from the layer of herbicide through which weeds grow and come into contact with the herbicide.

Risk increases where good soil coverage of the seed hasn’t happened, which is why most residual herbicide labels advise that applications should be made to fine, firm, consolidated seedbeds with seed drilled to a particular depth, usually at least 30mm. Heavy rainfall after application which washes the herbicide down the soil profile to where the seed was drilled can also increase crop damage.

Agronomic decisions can either increase or decrease the crop safety risk and likely efficacy on a field-by-field basis.

Most residuals need a degree of soil moisture to activate the active ingredient and provide efficacy against the target weed. That moisture is typically more likely to be present with later drilling, which also gives more time to target grass-weeds before the crop is drilled and reduce the in-crop burden, explains Roger Bradbury, Bayer market development manager for cereal crops.

“It’s also important not to cultivate or roll once the residual have been applied to avoid disturbing the layer of chemical protection formed.

“From a grass-weed management perspective early drilling where grass-weeds are present, especially when dry, increases the risk of poor performance from residuals,” Mr Bradbury adds. “That will have implications for potentially increased input costs trying to get on top of grass-weeds, reduced yields, more seed return and even spraying crops off and re-drilling.”

In very dry conditions getting the necessary soil coverage with low disturbance establishment methods can be more challenging, Mr Cussans adds, with dry soils hindering the ability of the coulters of some tine-based drills to drill seed to a consistent depth.

In wetter conditions, some disc-based drills might struggle to close the slot created for the seed to be placed, again creating a crop safety risk, as will waterlogged soils or an extreme rainfall event after application.

“In both dry and wet you have risks for crop safety or performance, and you have to think about in dry conditions whether patience might be the best solution, or moderating the level of herbicide you are applying in crops where you have drilled and it’s turned wet.”

With applications doing the basics right is critical before attention turns to fine-tuning, Mr Cussans stresses. That starts with sprayer set up – making sure the sprayer is calibrated and correctly maintained.

“That’s your instrument for delivering weed control and you need that to be functioning to the top of its ability.”

Next on Mr Cussans’ list of essentials is operating at a sensible forward speed of between 10-12 km/h. “That facilitates the next important element, which is a low boom height of 50cm or lower above the soil surface and good boom control. You can’t achieve that if your forward speed is too high.”

Attention can then turn to more application specifics around nozzle choice and water volumes. Recent advice from some quarters has promoted the use of drift reduction nozzle technology and higher water rates for pre-emergence applications, but Mr Cussans suggests that lower water volumes and fine-medium spray quality are equally valid to use.

Drift reduction nozzle technology produces a much coarser spray, reducing droplet coverage per square metre. Increasing water volumes increases coverage and mitigates the use of drift reduction nozzles.

“The two are linked together,” Mr Cussans stresses. “What grower must not do is use drift reduction nozzles and very low water volumes together.”

But is perfectly possible to use low water volumes with the correct nozzle technology, such as a flat fan, to produce good coverage, he says.

Ultimately growers will choose what works best in their system – for some maximising work rates and efficiency using lower water volumes will be more important, for others lower work rates but more flexibility over spraying in marginal conditions, or better drift reduction will be crucial.

“Farmers will have different drivers in their decision-making.”

Angling nozzles has been another area of debate. Some have suggested that angling can help cover all sides of clods in a cloddy seedbed, but Mr Cussans says even if that’s true it doesn’t make any difference to weed control if the clod breaks open.

“You spray one surface and then the clod breaks down and you have another surface that hasn’t been treated. There’s not much you can do about that.”

But angling can be helpful for counteracting the movement of the sprayer. “The sprayer is moving forward, so having at least some of the nozzles angling forward, and the rest down, is counteracting that.

“The thing to avoid is backward pointing nozzles as these will contribute to drift, creating a tendency for the spray to ripple out the back and reach a velocity where it will not contact the ground where you want it to.”

Most manufacturers recommend sprays are applied within 48 hours of the crop being drilled. Mr Cussans suggests this is a good strategy to ensure the treatment is applied as a true pre-emergence of the crop and weed, where efficacy is likely to be highest.

Organising labour and machinery to help achieve this is worth doing, Mr Bradbury suggests. “Prioritise the order of drilling fields so the worst fields are drilled later, and only drill when moisture is present or forecast, and then roll to consolidate the seedbed if appropriate and conditions allow, before spraying immediately afterwards. Use contractors if necessary. Evidence from the field suggests that delays in application risk either missing the timing and / or compromising performance.”

Splitting applications of large stacks of residual herbicides between pre-emergence and peri-emergence can also be helpful in increasing crop safety margins and hedging bets on efficacy, Mr Cussans says.

He defines peri-emergence as when cotyledons begin to emerge through the soil surface, with post-emergence defined as when the plants produces leaves.

“In some situations, such as when it is bone-dry when you drill, peri-emergence applications can be as effective as pre-emergence, but pre is virtually always best. As soon as the weeds reach post-emergence performance drops off a cliff.

“A sequence of pre and peri-emergence can be optimal for both weed control and crop safety, but the window for achieving this is very tight,” he concludes.

What do sprayer operators do currently?

The vast majority of sprayer operators are heeding Mr Cussans advice of travelling at no more than 12km/h, according to our survey of sprayer operators early in 2022, with 92% stating they travel at 8-12 km/h for this application.

Most were using higher water volumes – 58% at 200 L/ha, with a further 29% at 150 L/ha, which mostly ties up with a greater proportion using air induction nozzles (57%). Just over three-quarters were also using some form of angling, whether with a flat fan or an air-induction nozzle.

4% of the respondents were potentially compromising coverage by using lower water volumes with air induction nozzles in the survey.

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.