When a spray boom is set up properly, it applies the desired rate of product uniformly across the width of the boom at a specific distance below the nozzles. Uniform application can only be achieved if the boom is set at the proper height for the target. For postemergence herbicide applications, the target height is the average weed height.
Delays in application may result in the crop canopy being significantly taller than the intended target. Under these conditions, the crop not only interferes with distribution of the herbicide onto the weeds but also the crop may be exposed to excessive herbicide rates. The effect of a height differential between crop and target on the amount of herbicide intercepted by the crop is shown in the figure. For this situation, it is assumed that the target (weed canopy) is 4 inches above the soil surface and that the boom is set to operate 28 inches above the target. Crop rows that are directly beneath a nozzle receive the maximum dose, whereas rows positioned to the side of nozzles would be treated with less herbicide. In this example, 12-inch corn treated with 0.67 oz/acre of Accent could potentially receive 0.94 oz Accent/acre, whereas 16-inch corn could be treated with up to 1.17 oz/acre.
The potential for spray coverage problems increases as height differential between the target and crop canopy increases. The only real solution to the problem is to use drop nozzles where the nozzles are placed directly over the row middle. The use of drop nozzles increases weed control while reducing the risk of crop injury. Raising the boom height does little to alleviate problems with coverage but increases the potential for herbicide drift into adjacent fields.
This article originally appeared on page 102 of the IC-484(13) -- June 12, 2000 issue.