Effect of crop canopy on spray coverage
by Bob Hartzler

June 15, 2001 - The frequent rains this spring have delayed application of many postemergence herbicides for corn. This situation creates several problems for applicators, one of them being obtaining adequate coverage of the target. The developing crop canopy intercepts much of the spray, therefore decreasing coverage of the weeds and exposing the crop to higher-than-label rates. Spraying fields with large crops present (>10 inches) can, therefore, result in poor weed control, crop injury, or both.  While most labels recommend the use of drop nozzles in these situations to minimize problems with coverage, the inconvenience of using drops results in many people ignoring this precaution.

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. Delays in applications may result in the crop canopy being significantly above the intended target—the weed canopy. 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 the height differential between crop and target on the potential rate the crop is exposed to is shown in the table and figure. For this data set, it is assumed that the target (weed canopy) is 4 inches above the soil surface and that the boom is set up to be operated 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 somewhat less herbicide than listed in the table. In this example, 12-inch corn treated with 0.67 oz/acre of Accent could potentially receive a dose of 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. Raising the boom height will do little to alleviate problems with coverage but will increase the potential for herbicide drift into adjacent fields.

Influence of crop height on interception of spray solution.

Article orignially appeared on June 15, 1998.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University