Dicamba Residue: What does ppb (parts per billion) mean in the field?

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I posed this question to my colleague Dr. Tom Mueller after receiving a lab report from a tissue sample submitted from a field that had suspected dicamba injury.  I wish to thank him for his thorough explanation which helps make sense of the report.  His response is below:

What does it mean in ppb?

As we get numbers on dicamba concentrations in various plant samples, one asks the question, so what does a certain ppb really mean?

For sure, there are many factors that affect a plant’s response to any external stressor, including herbicides.  But, some frame of reference would be helpful. So, let us consider the following question:  If data shows that 1/20,000 of a 1x rate (0.5 lb ae/ac) causes injury symptoms on soybeans, what does that equate to in parts per billion?  Or ng per treatment?

Whenever one tries to answer these questions, one has to make assumptions. So, here we go:

0.5 lb active / acre

one soybean plant per 6 inches of row

30 inch rows

top trifoliate leaves weigh 5 grams

0.5 lbs = 227 grams                                                                                                                                                     (Convert to grams)

227 grams / 20000 = 11350000 nanograms per acre                                                                   (calculate dilution, change units)

area for one soybean = 0.5 ft*2.5 ft = 0.00002869 acres                                                                    (How much in area of 1 plant)

11350000 ng* 0.00002869 A = 325 ng per area of soybean                                                             (How much on 1 plant)

325 ng / 5 grams = 65 ng/g or ppb                                                                                                            (What is concentration in top leaves)

NOTE: any of these assumptions could lead to error, but at least this might be helpful

So, a possible guess on what does it mean in ppb is that if 1/20,000 is on the top of the plant, the initial concentration is ~ 65 ppb.

By the way, as soon as the dicamba is contacted by the plant, it starts to move (translocate) and be degraded (metabolized) into breakdown products.  It is quite possible that a soybean plant is more sensitive to dicamba than any analytical tool.  Perhaps that is a reason dogs are used to smell for minute traces of drugs or explosives instead of walking around with a chemical detector.

1 pound = 454 grams

1 gram = 1000000000 nanograms

1 acre = 43,560 ft2

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