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zine, simazine, cyanazine and metribuzine.
“Prevention is the key to stopping the spread of triazine-resistant weeds,” says Ron Ritter, Ph.D., University of Maryland, Extension weed scientist. “Triazine herbicides shouldn’t be used year after year and weed escapes should be controlled through cultivation and/or herbicides having a different mode of action from the triazines.”
Resistance to triazine herbicides develops through the genetic process of natural selection, explains Sandoz product development specialist William O’Neal.
“Triazine-resistant weed biotypes have always been present,” says Mr. O’Neal. “They just didn’t become numerous until herbicides killed off susceptible, nonresistant biotypes. Now weed species that were once susceptible to triazines at labeled rates can withstand 10 to 30 times recommended rates.”
“Normally, triazine herbicides kill weeds by bonding to the plant’s chloroplast membrane and preventing photosynthesis,” O’Neal explains. “In resistant weeds, a chemical change in this membrane structure alters triazine bonding, so treated plants continue to grow.”
Pesticide resistance is not a new phenomenon. Many instances exist of insects and fungi developing resistance to commonly used chemicals. Today, 43 plant species worldwide are documented as resistant to triazine herbicides.
94.5%