Top sprayer tips: How to maximise efficacy of spring grassweed herbicide sprays in wheat

The heavy lifting of a grassweed control programme in wheat, in theory, should have been done before we reach the spring through a combination of cultural controls, pre-drilling Roundup (glyphosate) and pre-emergence and / or early post-emergence residual herbicides.

But depending on the success or otherwise of those measures, the season and the field population size, contact-acting spring herbicides are a crucial, almost last, opportunity to control grassweed problems, such as black-grass, ryegrass and bromes, and importantly reduce seed return for following seasons.

The target can vary in size, depending on when the application takes place, from small, just emerged to well-tillered over-wintered weeds.

To get the best from mesosulfuron-based products, such as Atlantis Star (mesosulfuron + iodosulfuron + thiencarbazone) or Pacifica Plus (mesosulfuron + iodosulfuron + amidosulfuron) go when the weeds are small, advises Mark Hemmant, technical manager for Agrovista.

“You might not have the perfect growing conditions and warm soils, but if your grass weed is growing and you can travel – avoiding a frost period – then I would be spraying in February or early March.”

That’s even more important after a mild winter where the grassweeds are likely to be well-tillered. “You don’t want the weeds any bigger because the efficacy will diminish rapidly where there are resistance issues.”

Mesosulfuron products are less temperature dependent than pyroxsulam-based products, he notes. If you are spraying in February, check labels of mesosulfuron-based products carefully as the rates of some products differ depending on whether it is a February or March application.

The target in later winter / early spring is likely to be mostly prostrate rather than the small vertical target of an early post-emergence spray in the autumn. Even so, Agrovista’s application research with mesosulfuron products hasn’t seen a difference in how to apply it in the autumn and early spring, says Mr Hemmant.

That’s not to say application doesn’t have a massive influence on performance – it does – just that the best application technique for the autumn still holds for the spring.

“Fundamentally you want to get as fine a spray as you can onto the target. Sometimes that might mean using a different nozzle to get it onto the target rather than drift away.

“We’ve tended to find flat fan nozzles work better than air-inclusion, and smaller flat fan nozzles, work better than larger flat fans as long as it’s not too windy. Our most consistent results over lots of years are with blue flat fans. But if the wind increases, a red might be better, while if it is less windy a yellow might be better.”

As grassweeds get bigger the difference between flat fans and small air-inclusion nozzles narrows, but performance still doesn’t match the flat fans, he says.

Angling nozzles is also worth considering. Agrovista compared nozzles all angling straight down, all forward, all backwards, alternately back and forward or alternating down and forwards for black-grass control in a trial a few seasons ago when mesosulfuron efficacy was generally higher.

“What we saw all forward was better than straight down, all backwards or alternating backward and forwards (see table). The theory is if they are all forwards you get better coverage on the target.

“But where we alternated forwards and down we got significantly better control.

“There are two reasons why that might work. First, because all the nozzles are not pointing in the same direction you break up to the spray sheet and reduce the drift.

“Second, it might encourage a little bit of positive drift, so instead of negative drift where the spray misses the target, this configuration at ground level seems to almost drift more onto the target, so better coverage and control.”

If using this configuration, he suggests using a nozzle with an 80 degrees pattern for the forward nozzle, and 100 degrees for the downward facing nozzles. “That narrower angle is less drifty and gives better matching of the spray pattern at the right boom height.”

Black-grass control with different nozzle angle configurations using mesosulfuron products

Source: Agrovista

The research also found that lower water volumes – more concentrated sprays – tended to work better. “So 100 L/ha is better than 200 L/ha, which is different to the residual sprays where 200 L/ha is better.”

That’s not always going to be possible depending on nozzle choice and forward speed, he points out. “A 100L/ha with a blue flat fan is 3.0 Bar with a forward speed of 14 km/h. But if your booms are flapping up and down at 14 km/h because the ground is not flat, then don’t do it as you will get poor control. It maybe you have to drop down to 10 km/h in which case you’re delivering 150 L/ha.”

Forward speeds of 10-12 km/h are much more likely than 14 km/h in most cases, with the need to maintain a constant boom height of 50cm above the target critical for minimising drift and maximising efficacy.

With the option of swapping flat fans for air-inclusion nozzles difficult because of the impact on performance, Mr Hemmant suggests the addition of a drift retardant to help reduce drift in more marginal spray conditions can be considered.

“They don’t make a medium spray into a coarse spray, but do manage drift out and we have seen some improvements in control.”

Most of Agrovista’s research has been carried out investigating black-grass control, but the same principles should apply for other grass weeds, such as bromes and ryegrass, he adds. “With brome having a hairy leaf, it’s going to be even more important to get as much of the spray onto the leaf as possible.”

Why spray application is important

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.

Minimising those losses and maximising the amount reaching the intended target will make a big difference to the success of a spray.

In this series of blogs, we will look at six common application tasks for arable growers and delve into the best current advice for that application, plus a look at important considerations for setting up a sprayer for success.

Available now, the next blog in the series of Top Sprayer Tips for the Season Ahead focuses on pre-season sprayer set up.