Most weed control failures are not due to resistance. Before assuming weeds surviving an herbicide application are resistant, eliminate other possible causes of poor control:

1. Herbicide application

a. Inadequate rate

b. Poor spray coverage and/or incorporation

c. Improper timing of application of postemergence herbicides (after weeds are too large to control)

d. Failure to use an adjuvant (if needed)

e. Excessive dust with post-emergence applications

f. Spray intercepted at application by wheel traffic

g. Antagonism between two or more herbicides

2. Soil and/or climatic conditions

a. Excessively wet or dry soil

b. Seedbed condition (clods, etc.)

c. Herbicide adsorption to soil particles or organic matter
d. Stress conditions, such as hot and dry

e. Lack of timely rainfall for activation

f. Wash-off of postemergence herbicides

3. Weed characteristics

a. Weeds emerging after application of a short or
non-residual herbicide
b. Extremely high infestations
c. Weeds not susceptible to the herbicide

Once other possible causes of poor control have been eliminated, the following are common features that characterize fields where resistant weed populations occur

The weed in question is initially confined to small patches within the field. After a few years, the resistant biotype becomes established throughout the field;
Herbicide performance was poor on one species whereas other species were controlled well, and the herbicide would normally be expected to kill the species that was poorly controlled;
Healthy plants of the species in question are interspersed among plants of the same species that were killed;
Heavy reliance was placed on the herbicide with little or no use of other MOAs, cultivation, or other non-chemical control methods; and,
The field has a history of extensive use of the herbicide in question or herbicides with the same mechanism of action in the absence of other equally effective herbicides with different mechanisms of action.

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