Spray drift is of great concern for sensitive crops and environments, along with the fact that if the spray doesn’t hit the intended target, crops can be damaged while weeds live.
Nufarm Australia spray application consultant Bill Gordon said the focus of spraying herbicide needed to be on doing the job right so the weeds received the correct dose and died, and this included reducing airborne spray to a bare minimum.
‘‘In many cases this means not spraying at night if the wind speed is too low,’’ he said.
‘‘Many product labels prohibit night spraying due mostly to the risk of spray drift.
‘‘Our studies have shown that with a coarse spray quality, drift may travel up to 300 to 400m during the day after any inversion has broken, but spraying at night can leave up to five times as much chemical in the air using the same products, nozzles and ground speed.
‘‘This can result in spray drifting 10 to 20km or more at night, and this is unacceptable for other farmers, the community and the environment.’’
Mr Gordon said most growers were implementing best spray practice when it came to boom height and nozzle selection but the temptation to spray at higher speeds and into the night could override commonsense at times.
‘‘The flip side of this is that it is becoming more evident that using Delta-T as the main determinant of safe day-time spraying conditions may not be the best approach,’’ he said.
‘‘What really matters most is the wind speed and whether the weeds are stressed or not.’’
When environmental conditions are borderline in terms of wind turbulence, the safe spray window can be extended slightly through the use of coarser sprayer quality where the proportion of droplets less than 150 microns is 10 per cent or less, keeping drift to a minimum.
The trade-off, however, is reduced efficacy when using very coarse droplet size, particularly when the target is small, vertical or hard to wet.
Mr Gordon’s rule of thumb for effective weed control when applying fully translocated products (e.g. glyphosate and Group I) is a minimum of six to eight per cent coverage, while coverage of 10 to 12 per cent or more is required for contact herbicides.
Bill Gordon’s 10 tips for reducing spray drift
1.Choose all products in the tank mix carefully.
2.Understand the product mode of action and coverage requirements.
3.Select (and check) the coarsest spray quality that will provide effective control.
4.Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. Do not spray.
5.Use weather forecasts to inform your spray decisions.
6.Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4-5km/h for more than 20 to 30 minutes, and clearly blowing away from any adjacent sensitive crops or areas.
7.Set the boom height to achieve a double overlap of the spray patterns.
8.Avoid higher spraying speeds.
9.Leave buffers unsprayed if necessary and come back.
10.Continue to monitor conditions, particularly wind speed, at the site during the spray operation.