Friday, April 19, 2024

An early herbicide mistake can plague you all season

Dr. Mo Way
Rice Research Entomologist
Muthu Bagavanthiannin
Rice Weed Scientist

We know some of you do a lot of fall/winter tillage to control weeds. However, a spring burndown herbicide application or tillage just prior to seeding can help achieve a clean seedbed. Practicing a stale seedbed can also help reduce the weed seedbank and lead to less weed pressure into the season.

We recommend applying a residual pre-emergence herbicide, such as clomazone, to give you prolonged early season weed control. This is important because research has shown that not controlling weeds in the first few weeks of crop emergence can lead to substantial yield loss.

Once you apply your pre-emerge, activate it with a flush or rain (if you are lucky) ASAP. About 0.5 to 1 inch of rain/irrigation should be sufficient for activation.

This ensures you get the most activity from your herbicide.

Remember, soil organic matter and clay can tie up herbicides, so use rates per label instructions, specific to your soil type and situation. Sandier soils do not tie up as much herbicide, so be careful on sandy soils or severe rice injury may occur.

Last year, I experienced herbicide phytotoxicity to rice in one of my experimental blocks because I used too high a rate of clomazone for our soil type. The rice never fully recovered, which was reflected in atypically low yields.

This shows that a mistake with an early applied herbicide can affect your crop all the way to harvest. If you are going to ground-apply your pre-emergence herbicide, double check your rate calculations, calibrate your spray rig correctly, and make sure all your nozzles and other spray equipment are in top-notch shape and clean. If you make a mistake at this early stage, trouble simply compounds itself through the growing season.

The same is true for planting equipment. One farmer told me several years ago that he started planting and noticed one of his drill rows was lacking seed. When he checked (after planting a couple acres), he found a mud dauber nest clogging the opening.

Such planting issues will lead to poor rice stands in the field, which will allow more weeds to show up. Dense rice stands, on the other hand, can outcompete weeds for light, nutrients, water and space. Thus, obtaining a dense, healthy rice stand is an important step in early season weed control.

You can achieve this through varietal selection, seeding rate, proper fertilization and timely irrigation. For instance, applying a relatively early permanent flood can help with weed control and reduce potential water stress to your crop.

Also, you should take good notes regarding the weed history of each field so you can tailor your weed management program to the projected weed problems you most likely will encounter. Here is where I put in a plug for hiring a crop consultant, who can do this for you…in consultation with you.

Finally, I’m not sure if I have a good answer to the following situation I have observed several times: A field is about to go to permanent flood and is relatively weed-free. Should the farmer apply a pre-flood herbicide(s)? If any reader has a good answer, please contact me at 409-658-2186 or

Spend money wisely up front to save on costs later.

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