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.

HerbicideApplication Window
Basagran Forteunifoliate to 2nd trifoliate
Classic + non ionic surfactant + UAN1st to 3rd trifoliate
FirstRate + non ionic surfactant + UANprior to flowering
Pinnacle + non ionic surfactant + UAN1st trifoliate to pre flower
Pursuit + non ionic surfactant + UANup to 2 leaf weed stage
Reflex + Turbocharge1st to 2nd trifoliate
glyphosate1until 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.

HerbicideInjury(%)Height Reduction (%)Yield (bu/ac)
None0056
glyphosate12354
FirstRate + Agral 90 + 28% UAN111356
Basagran Forte10256
Classic + Agral 90 + 28% UAN131254
Reflex + Turbocharge13255
Pursuit + Agral 90 + 28% UAN261950
Pinnacle + Agral 90 +  28% UAN472851

1RR (glyphosate tolerant) soybeans only

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