Herbicides are an exceptional tool for farmers when used correctly, but like any tool, they can’t work if they’re not used properly. Here are 10 common mistakes that farmers make when using herbicides.

1. Herbicide applied when weeds are too big

This is the most common mistake when applying herbicides. Herbicide labels state a range of weed sizes for which the herbicide will be effective and the recommended rates. If you apply postemergence herbicides to weeds that are bigger than what the label states, expect less than complete control.

2. Rate of herbicide application is too high

In general, the ability to kill weeds or prevent weeds from growing while not damaging desired crops involves the ability of the crop to metabolize or detoxify the herbicide. Under certain conditions, such as overlaps or applications of very high rates, the crop's ability to metabolize or detoxify the herbicide may be overwhelmed. Crop injury often occurs in this situation.

3. Herbicide carryover

Many herbicides are broken down by microbial action. Anything that affects that activity will slow the breakdown of the herbicide and can lead to carryover and potential injury to subsequent crops if they are sensitive to that herbicide. Drought, low pH, soils with low microbial activity to begin with, and other factors that reduce microbial activity will increase the risk of carryover.

4. Effectiveness is lost

A herbicide also can have reduced longevity due to increased microbial activity in the soil. Wet, warm soils and leaching rains for extended periods can lead to premature loss of residual activity in herbicides.

5. Rate of application is too low

Many control problems are due to applying herbicide at a rate that is not high enough. It is important to read the label for soil restrictions and soil recommendations with residual herbicides.

6. No water to activate

A herbicide that requires water to activate will not work if there is no rainfall or irrigation.

7. Weeds are stressed

Postemergence herbicides work poorly on weeds that are stressed or are not growing normally. Why? These weeds have partially shut down certain physiological activities so the herbicides may not be taken up or moved to sites of action in the stressed plants.

8. Improper adjuvant used

Use of proper adjuvants is critical for many postemergence herbicides. Surfactants, oils, and other additives may be necessary for them to work effectively.

9. Application equipment mistakes

Misapplication or misuse such as improper cleaning of sprayer tanks and lines, using products not labeled for a crop, mixing up herbicides when filling tanks, spraying when conditions are poor, and excessive overlapping are just some of the reasons why a herbicide won’t work as intended.

10. Poor crop growth

Weed control in a crop is often dependent on both the activity of the herbicide and the development of a crop canopy to shade any weeds that might germinate later. Anything that affects the development of a crop canopy can reduce weed control.