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ISSUE 16   August 23, 2001



Several extension colleagues from across the Midwest have indicated common "myths" they hear each year about weed management and herbicides. Several themes seem to appear.

Weed Management

"We can eradicate weeds."

On a field scale, we will never eradicate weeds. Eradication may be possible with intense efforts on isolated infestations of specific weeds, such as a newly introduced noxious weed.

"Weed seed live in the soil for over 100 years."

Most weed seed either sprout or decay within several years, especially grass weed seed. It is well documented that weed seed survival increases the deeper placed in the soil. However, In artificial burial studies, a few weed seeds have remained viable for several decades and these results have contributed to this myth.

"Herbicides are the only way to control weeds."

Competition exerted by the crop is often overlooked and under-appreciated. Shading by crops can be very effective - just look at most any skip in a field. Crop competition combined with other weed management techniques constitute useful tools.

"The herbicide I used grew a good crop"

The herbicide controls weeds but soil, hybrid, variety selection, and crop management imparts yield potential. The herbicide aids in protecting crop yield. More herbicide does not increase yield.

Scouting weeds

"Scout for weeds? Why? The herbicide has a guarantee!". Also, "I have the same weeds that my neighbor does."

Scouting is important to match the right herbicide or herbicide combination with weeds present in the field. If it is true that you have the same weeds as your neighbor, do they know which weeds they have?

Postemergence applications

"Iíll wait until all the weeds are up before I spray."

This strategy may not protect your crop yield because of early season weed competition. The yield of most crops like corn, wheat, sunflower, dry beans are affected more by early season competition. Soybean are more resilient and can tolerate some weeds early. Some weeds exert more competition than others.

"The weeds were only 4 inches tall when they were sprayed."

Sometimes this can be a hard one to believe until you see the actual size of the dead weeds in the field. I think weeds end up being taller than expected because they grow so fast (rather than people using defective rulers).

Herbicide drift

"There was no wind: it couldnít have drifted."

Wind inversions and even small amounts of wind in combination with herbicides and a nearby sensitive plant or crop can result in crop injury or death. With calm conditions, wind currents can change directions several times during an application and move herbicide particles in unexpected directions. A light breeze is actually more desirable because the direction of any drift is more predictable. If there is a sensitive site downwind, the application can be delayed until the wind is blowing away from the site.

Crop injury

"Any crop injury is from herbicide."

Herbicides may or may not be involved in injury symptoms exhibited. All factors including environment, fertility, disease, compaction, insects or combinations of any of the these must be examined to determine the cause.

"I always rinse out my spray tank." and "There wasnít much left in the tank."

Tank contamination is a common cause of herbicide injury. Some herbicides need more than a quick rinse to remove herbicide residues. Plus, a little chemical residue can go a long way in contaminating the next load.

Rotational restrictions are on the label only because of lawyers."

It may be true that lawyers approve label wording and labels are legal documents. However, many rotational restrictions exist to protect sensitive crops from injury. This is especially important for the many minor crops grown in ND. There may be restrictions for other rotational crops because tests have not been conducted to determine extent of residue in the crop. In either case, directions on the label for rotational crops must be followed.

Other comments

"Sprayer calibration is not that critical."

Ever try spreading a stick of butter (4 oz) on a slice of bread equivalent to 1 acre (43,560 sq ft)? That is the same difference with what we do with herbicides. Sprayer calibration is the only way we can do this successfully. Proper calibration can save money, protect against crop injury and ensure effective weed control.

"If a little bit is good, more must be better."

Sometimes herbicide applications are made out of retaliation and retribution to punish weeds for existing. Cost of crop production is high enough without making weed deader. Late applications on large, somewhat tolerant weeds force rates at the high end if a rate range is given but any time rates exceed label risk of crop injury increases.

"All adjuvants are created equal."

Despite what the sales and advertising folks say about the performance of a product it is best to examine reliable, unbiased data. Under optimum conditions with high herbicide rates adjuvants may perform near equal. However, with large, somewhat tolerant, and or stressed weeds with low herbicide rates creates an ideal situation where enhancement of herbicide from adjuvants can be measured.

Richard Zollinger
NDSU Extension Weed Specialist

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