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Effect of crop canopy on spray coverage
by Bob Hartzler
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Updated May 31, 2000 -  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 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 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. The use of drop nozzles will increase weed control while reducing the risk of crop injury.   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.

Crop height
(in inches)
% of desired application rate intercepted by crop canopy
4 100
8 117
12 140
16 175

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Article orignially appeared on June 15, 1998.

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

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.