Course: Grassweed management in cereals | Last Updates: 30th October 2015
Autumn spraying to control grassweeds, particularly post-emergence, demands the best possible spraying technique – more so than at any other time in the spraying calendar.
Not only are grassweeds extremely competitive, but some, especially blackgrass, are becoming increasingly difficult to control as resistance spreads. If that weren't enough, a small grassweed is also a difficult target to hit.
What should the autumn-applied sprays achieve?
The key aim with autumn grassweed control is to get the right amount of product onto the target – the correct dose transfer. This is probably more important than coverage – the proportion of target area that has chemical on it, although when applied to small targets, sprays that will give the highest deposits will also give good coverage.
So the aim is to get enough active ingredient to each weed – subsequent absorption and translocation will ensure the chemical does its job. Choosing the right equipment and using it at the right time will go along way to meeting that aim.
What nozzles should you use in the autumn?
This depends on the timing. With pre-emergence sprays, there is evidence from field trials that weed control is not sensitive to the type of nozzle used although those giving larger droplets such as air-induction nozzles will reduce the risk of drift.
Results from laboratory studies have shown that angled nozzles and fine sprays can help sprays wrap around clods.
But with post-emergence graminicides the application method is crucial. This is the most important difference between the two timings.
A 110° blue (03) conventional flat fan nozzle operating at a pressure of 3.0 bar to produce a medium/fine spray will put in the order of 50% more deposit on a grassweed leaf than for example, an air-induction nozzle giving a droplet size in the middle of the range for this type of nozzle design. This is because air-induction nozzles produce larger droplet sizes that are less well retained on small targets.
Some horizontal spray movement, particularly of the smaller droplet sizes, is useful when trying to hit a small grassweed with upright leaves. Such movement can be generated by the wind but it is then really important to adjust boom height according to the manufacturer's recommendations to minimise the risk of drift.
Angling nozzles will also generate horizontal spray movement but again care is needed to control the risk of drift by keeping the boom as low as possible. The minimum boom height will be lower when nozzles are angled than when they are directed straight down. Alternating angled nozzles also reduces drift when compared with those all angled forwards or backwards.
Once grassweeds get to about the four-leaf stage, air-induction nozzles can be used.
Does boom height matter when spraying bare soils?
Boom height is critical at any time, but especially so when applying pre- or peri-emergence when sprays are particularly prone to drift. With 110° flat fan nozzles the boom should be no more than 0.5m above the soil surface.
This will always be preferred practice, as the nozzles are operating near the ground where wind speeds are slower and more predictable. And, all other things being equal, if boom height is doubled, drift increases by a factor of between 5 and 10.
Are high speeds bad news?
Going slowly certainly helps boom stability, but timeliness will suffer. Going beyond 15kph risks running into a new set of problems.
The faster the sprayer travels, the more turbulence or "wake" it creates behind it. Small droplets tend to be pulled into areas of high turbulence, creating drift, and these areas also produce greater levels of deposition on to plant targets.
This results in increasingly uneven distribution as speed increases. That's bad enough with pre-emergence treatments, but it can have a much bigger effect when trying to hit small grassweeds with the recommended medium to fine quality spray.
Maintaining high speeds across a field can be difficult, particularly close to field boundaries and where travelling conditions vary. With rate controllers, reduced speed translates to lower pressures that can change the nozzle pattern and result in an uneven distribution.
Generally the standard recommendation for spraying speed of 12-14kph looks robust.
What pressure should you use?
For most nozzles, there is no need to exceed 5 bar. High pressures should be avoided – they generally create smaller droplets which can cause drift problems. Perhaps more importantly, pressures below the minimum recommended for the nozzle design should not be used since this will influence the spray pattern from the nozzle.
The most important things to get right are the nozzle and droplet size – this will dictate operating pressure, which will usually end up being between 2 and 3 bar.
Variable pressure nozzles can operate effectively over a wider pressure range enabling a wider speed range to be used with rate control systems. But some designs produce smaller droplets and may be more prone to drift, so these need using with care.
What about water volume?
Many herbicides work better at 100 litres/ha than at 200 litres/ha. It has been shown that more active ingredient will be retained on small targets at 100 litres/ha compared with 200 litres/ha and so should be used unless there is a clear reason for using higher volumes. Using the lower volume will also give advantages in work rate and timeliness.
However, going too low may involve the use of small nozzles that are prone to blockage. So the simple message when controlling grassweeds pre- or post-emergence is to stick to a water volume of 100 litres/ha.
How important is timeliness?
Good spray days are few and far between in the autumn, especially later in the season. So making the most of them is vital – applying sprays at the right soil moisture level, or when leaves are dry enough, or when wind speed and temperature are within tolerable limits can make a big difference to spray efficacy.
Choosing suitable speeds for a given sprayer and using low water volumes can all help, as do fast turnaround times. It is important to develop a system that fits well with the farm scale and layout and enables the maximum number of tankfuls of spray to be applied in a working day.
Establishing a good routine will help to ensure the sprayer is in the field at the right time.
- Use a fine/medium spray to treat grassweeds early post emergence
- Consider using angled nozzles to treat small grassweeds
- Apply sprays in 100 litres of water per hectare
- Maintain the lowest boom height for the nozzle you are using
- Travel at speeds where the boom becomes unstable
- Operate at pressures outside of the specified range for the nozzles you are using
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