Weed Wrapup, April 27, 2006

Chris Boerboom, Ext. Weed Scientist

UW Pest Management Field Day

This is a pre-announcement of our third annual Pest Management Field Day so you can mark your calendars.  The field day will be held on Tuesday, June 27 at the Arlington Ag Research Station.  The day will feature herbicide evaluation trials, weed management research, and herbicide demonstrations plus updates on the latest in UW plant pathology and entomology research.  Wagon tours will start at 8:30 am at the Public Events Building and a lunch will be served at noon.  You are invited review trials in the afternoon as well.  A complete announcement will be made in a future issue of the Crop Manager.

Soybean Burndown Addition

In the last issue, I reviewed some of the burndown options in corn and soybeans.  I should have also included a review of Synchrony XP, which can be used as a burndown herbicide in soybeans.  Synchrony XP at 0.375 oz/a provides the equivalent of 0.32 oz/a Classic and 0.03 oz/a Harmony GT.  With these ingredients, Synchrony will control several annual broadleaf weeds, but not grasses. As a consequence, Synchrony would typically be used with glyphosate, which would provide grass control. In turn, the Synchrony can improve the broadleaf control of the glyphosate, particularly for dandelion, horseweed, and giant ragweed.  Synchrony does not require a 7 day delay before planting soybeans like 2,4-D.

Crop Rotational Restrictions

Each year I receive calls asking “Is safe to plant crop X if I sprayed herbicide Y last year?” or “Can I recommend herbicide Y if a grower is planting crop X next year?”  The answers to these questions are usually easy, but sometimes not popular.  The reason the answers are easy is because most herbicide labels are quite specific in listing the rotational crops or intervals allowed AND the label is a legal document.  So, if the label states “Do not plant the following crops”, it means that it is not legal to plant that crop. 

The obvious reason that a rotational crop may not be allowed to be planted is because the herbicide may carryover and injure the crop.  Neither the grower nor the manufacturer wants this to happen.  The manufacturer conducts field studies to determine the risk of carryover to rotational crops in setting rotational intervals.

The second reason some crops may not be allowed as a follow crop is not visible like injury.  The EPA protects our food supply by only allowing acceptable levels of pesticide residues in food or feed.  This residue level is called a “tolerance” and as long as the concentration of that pesticide is below the EPA-approved tolerance, it is legal.  For example, sweet corn has a tolerance of 0.25 ppm for atrazine.  In a high percentage of sweet corn, atrazine is not even detected, but the tolerance allowing a trace amount of atrazine is the safe guard provided by EPA. 

If herbicide residues persist in the soil and are taken up by a rotational crop, that crop may contain illegal residues unless a tolerance exists or unless the manufacturer determined through field tests that no residues are detected in the rotational crop.  Certain rotational crops are not allowed because the manufacturer has not conducted the field tests to determine that the crop is free of herbicides residues, even though the crop is not injured. 

Whether the crop rotation is not allowed because of the risk of crop injury or because of potential herbicide residues, the answer to the question is the same.  Read and follow the label.  “Do not” or “Shall not” means do not.  In some causes, the labels offer the user recommendations or advice such as “Injury may occur if soybeans are planted the following year.”  In this case, it is the grower’s choice if they want to accept the risk of injury, but they are warned.  On labels, “should” is another word that is advisory.

Basically, if we want to have these tools available for our use and maintain the confidence of the public, we need to follow the label instructions.