Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 Page 7 Page 8 Page 9 Page 10 Page 11 Page 12 Page 13 Page 14 Page 15 Page 16 Page 17 Page 18 Page 19 Page 20 Page 21 Page 22 Page 23 Page 24 Page 25 Page 26 Page 27 Page 28 Page 29 Page 30 Page 31 Page 32 Page 33 Page 34 Page 35 Page 36 Page 37 Page 38 Page 39 Page 40 Page 41 Page 42 Page 43 Page 4432 IREC Farmers' Newsletter No. 197 — Autumn 2017 Drift of Group I herbicides Let’s just consider herbicide drift, and more specifically Group I herbicides such as 2,4-D. This gains plenty of media coverage every year for its impact on cotton crops. Some may say the main problem is that cotton is very sensitive to 2,4-D and this is true. For me, the issue is that we are clearly doing something wrong with the application to get the product into the air to start with. It is now twelve years since specific drift warnings were included on the labels of Group I herbicides and it would appear that it has made no difference to the incidence of spray drift. So what are we missing? What is the problem? In my experience, the vast majority of spray drift cases (probably 90% or more) is the result of ‘inversion drift’. That means the drift has not come from an adjacent sprayed area, it has come from one or more sources that are some distance from the site of damage. The distance between the sprayed site and the location of the damage may vary dramatically, from a few kilometres to tens of kilometres. Why is there so much inversion drift when Group I labels specifically prohibit use of the products under surface temperature inversions? Many may argue that it is a blatant disregard of the label by a few applicators. I do not agree this is the main problem. I believe the problem is a lack of understanding about how to tell when there is an inversion and particularly not knowing how ‘day wind’ moves differently to ‘inversion wind’. I continue to see good farmers/ applicators doing what they believe to be the right thing but it is not. These are people very concerned about minimising spray drift — they honestly do not think they are doing anything wrong. What is ‘day wind’? After sunrise, the sun begins to heat the ground, the warm ground then heats the air close to the surface, and this air then rises. As that warm air rises, cold air from above sinks down to replace it. The ground then warms this cold air and it rises. This cycling of warm air rising and cold air sinking creates turbulence and then wind. This is a good thing; turbulent wind movement is much safer for spraying. ‘Day wind’ has a turbulent motion and is much more likely to pull any fine droplets to the ground within a reasonable distance. During the day, we can predict which direction and how far fine droplets will travel. What is ‘inversion wind’? As the sun sets, the ground begins to cool quickly and this in turn cools the air next to the ground. As we all know, cold air does not rise and warm air does not sink. This means there is a layer of cold air trapped close to the surface and a layer of warm air above it. The result is no turbulent movement or mixing of the air. The air may become quite still and this is often observed around sunset when the daytime wind ceases or drops off. What happens next is where the real danger occurs for spraying. As the night progresses and the ground cools more, the cool air close to the surface becomes colder and therefore denser, particularly from midnight onwards. This cold dense air then begins to move across the landscape, often down slope and in very unpredictable directions. Remember this air is not turbulent, there is no mixing, it has layers of air, something like layers in plywood, and it flows parallel to the ground. Any fine droplets released into these layers of cold non-turbulent air will simply move sideways across the surface until the sun rises and heats the ground again. This is when the fine droplets are released from the layers and they come to ground, often in the lower parts of the catchment and a long way from the site of application. Puckering of leaf margins and ‘bubbling’ of the leaves are less severe symptoms of damage by Group I herbicide on cotton. ‘Witches’ hand’ is a typical symptom of more severe damage on cotton caused by Group I herbicides. Group I herbicides are synthetic auxins (growth regulators) and symptoms will be observed in the growing points of plants.