Georgia: 5 Thoughts on Herbicides Following the Big Rain

Wet field, recently planted.

Wet field, recently planted.

Most of Georgia received a significant rainfall event on April 24, 2021 (Figure 1).  Some areas received more than than 6″.  Consequently, my phone has been ringing off the hook about the potential effects of this precipitation event on herbicides.  Here are a few thoughts:



1. The two biggest herbicide concerns with heavy rainfall events such as this are runoff and leaching.  

2. If a field received enough rainfall to physically move the soil from the field, herbicides were likely moved with it.  In this scenario, residual weed control would be reduced.  Tough luck here and nobody can prevent/predict an Act of God!!!

3. Herbicide leaching is a complicated interaction of many factors including soil type, OM, and certain chemical properties (i.e. water solubility and soil adsorption).  Very generally, herbicides with higher water solubility and lower soil adsorption characteristics are more likely to leach from the soil profile.  Check out Table 1 for a list of these properties.

Note that dicamba has a very high water solubility and very low soil adsorption so it is very prone to leaching.  Although paraquat and glyphosate are very water soluble, they are strongly adsorbed to soils.

4. Most of the questions that I have been getting have had to do with Prowl (pendimethalin).  You will note in Table 1 that Prowl is not very soluble in water and is strongly adsorbed to soils.  Thus, it is less likely to leach in comparison to other herbicides so it should still be around in most situations.  

5. If a grower has already applied Prowl (on peanut land) and has observed significant soil movement/washing, I would suggest that he/she consider adding an additional 16 oz/A of Prowl in combination with Valor immediately after planting (i.e. behind the press wheel).  I would also suggest this in peanut fields where prepared beds must be reworked/remade/ refreshed before planting.

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Table 1.  Herbicide Water Solubility and Soil Adsorption Properties for Common Herbicides Used in Georgia.

Trade Name

Common Name

Water Solubility (ppm)

Soil Adsorption (Koc)

Aatrex

atrazine

33

100

Callisto

mesotrione

2200

14-390

Dual Magnum

s-metolachlor

488

200

Engenia/Xtendimax

dicamba

4500

2

Gramoxone

paraquat

620,000

1,000,000

Impact

topramezone

0.69

22-172

Laudis

tembotrione

28.3

66

Liberty

glufosinate

1,370,000

100

Prowl

pendimethalin

0.275

17,200

Reflex

fomesafen

50

60

Roundup (IPA Salt)

glyphosate

900,000

24,000

Sonalan

ethalfluralin

0.3

5,000

Tricor

metribuzin

1100

60

Valor

flumioxazin

1.79

557

Zidua

pyroxasulfone

3.49

57-114

Source URL: http://ugaweedscience.blogspot.com/2021/04/the-big-rain-and-herbicides-prostko.html