Atrazine - Still a Valuable Tool
By Hugh Martin, Weed Management Specialist, OMAFRA

At this time of year, we start to look around to see what herbicides will be new for the 2001 market. We will also continue to see one of our oldest herbicides, atrazine. It was introduced to Ontario corn growers over 40 years ago. In the seventies, we used it at high rates in continuous corn. Spring and fall applications, long-lasting residuals in the soil and high rates of erosion meant that it was found in surface waters of creeks and rivers. On sandy soils, it could leach into shallow aquifers. Long residuals in the soil also meant that it would injure many crops and alter crop rotations.

A herbicide with so many problems: why is it still useful? After many claims, there is still no evidence that it causes human health problems. A recent review of the research recommended to the U.S. Environmental Protection Agency that atrazine be declared 'not a carcinogen'. Simply put, there is no evidence that it causes cancer, a frequent claim of environmentalists.

But we do not use it the way we used to. We use less than one-third of what we used to. Instead of applying it at rates of 2-4 kg/ha per season to control quackgrass, we now apply less than 1 kg/ha for specific weeds. Quackgrass is now controlled with glyphosate, or with newer ALS herbicides such as Accent or Ultim.

More conservation tillage and especially no-till means that less atrazine is lost into surface water runoff. Fall applications are no longer used: this means that the herbicide is applied in season when it is most useful.

Atrazine is one of our workhorse herbicides. It fills in the gaps that many newer herbicides miss. Accent and Ultim are weak on yellow foxtail. Using a low rate of atrazine will improve the control of yellow foxtail. There can be some antagonism, however, so apply them separately or only tank mix as directed on the label.

Converge was registered last year, but only as a tank mix with atrazine which is sold together as a co-pack. Converge is an excellent herbicide, but atrazine makes it better by improving the yellow foxtail control and picking up the wild buckwheat and smartweeds that would escape if the atrazine was not used.

Dicamba is also a very popular herbicide that is effective on many broadleaf weeds, but it is weak on mustard. Marksman adds atrazine to the dicamba and picks up the mustard as well as improving the season-long weed control of several species.

Dual and Frontier are excellent soil-applied grass herbicides, but adding atrazine as a tank mix partner gives them extra strength for broadleaf weeds to protect the crop until a post-emerge broadleaf herbicide can be applied.

There are many corn herbicide situations where a low rate of atrazine improves the weed control of an almost excellent herbicide program. It is not surprising that this 'old' herbicide is still used on about 70% of our corn acres.

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