Soybean response to late herbicide applications

How late can you apply post emergent soybean herbicides without causing significant injury and yield reductions? Most herbicide labels specify a maximum crop stage (Table 1). A three year study by Dr. Peter Sikkema reveals that of all the herbicides tested, Pursuit (imazethapyr) and Pinnacle SG (thifensulfuron) should not be applied at advanced crop stages (between the 5th trifoliate stage and  early flowering) because they caused significant visual injury, height and yield reductions (Table 2).

Table 1. Application window for post emergent broadleaf herbicides in soybeans.

Herbicide Application Window
Basagran Forte unifoliate to 2nd trifoliate
Classic + non ionic surfactant + UAN 1st to 3rd trifoliate
FirstRate + non ionic surfactant + UAN prior to flowering
Pinnacle + non ionic surfactant + UAN 1st trifoliate to pre flower
Pursuit + non ionic surfactant + UAN up to 2 leaf weed stage
Reflex + Turbocharge 1st to 2nd trifoliate
glyphosate1 until flowering

1RR (glyphosate tolerant) soybeans only

Table 2. Injury, height reduction and yield of soybean following applications of post emergent herbicides applied at twice the maximum labelled rate (overlap rate) and between the 5th trifoliate and early flowering stage of soybean.

Herbicide Injury(%) Height Reduction (%) Yield (bu/ac)
None 0 0 56
glyphosate1 2 3 54
FirstRate + Agral 90 + 28% UAN 11 13 56
Basagran Forte 10 2 56
Classic + Agral 90 + 28% UAN 13 12 54
Reflex + Turbocharge 13 2 55
Pursuit + Agral 90 + 28% UAN 26 19 50
Pinnacle + Agral 90 +  28% UAN 47 28 51

1RR (glyphosate tolerant) soybeans only

Source: Three trials, Dr. P.H. Sikkema, University of Guelph (2007-2009)