Fomesafen Use Last Summer Could Cause Grain Sorghum Stand Issues

DR. LARRY STECKEL

JACKSON, TENN.
   It appears that grain sorghum will be a historically big acre crop this year. A number of these intended fields to be planted to grain sorghum were in soybean last year and had late applications of fomesafen (FlexStar, Prefix, Reflex, Rhythm, Dawn etc).
Soil samples were taken within the last couple of weeks from a few of these fields treated with 16 oz/A rate equivalent of fomesafen last July and tested to see if any of this herbicide could be detected. The results of those tests showed that those fields had in effect 8 to 4 oz/A rate of fomesafen still present.
   That much fomesafen would likely eliminate a grain sorghum stand if it was planted before June. The plant back to grain sorghum after a fomesafen application of 1.5 pts (0.375 lbs ai/A) is 10 months here in Tennessee (Region 1 on label). The reason for that plant back is that seedling grain sorghum is very puny when it is establishing a stand. Carryover of fomesafen could greatly hurt that process.
   How quickly will fomesafen in the soil break down this spring? The answer to that question depends upon the weather. Fomesafen is readily broken down in saturated warm soil. Last fall was relatively dry and cool which is a reason that fomesafen was showing up in those tests mentioned above. Had last fall been wet and warm the probability of this much fomesafen still present in the soil would have been remote. It, of course, has been wet in February and early March. However it has been too cold for much break down of this herbicide. If the rest of this month is warm with good rain then the fomesafen will likely break down rapidly. ∆
   DR. LARRY STECKEL: Extension Weed Specialist, University of Tennessee