Two field studies were conducted in 2004 and 2005 to evaluate the response of glyphosate-resistant 'TV52R42' and 'TV52RI4' soybean to glyphosate at 0, 458, 916, 1,375, 1,833, or 2,749 g ae/ha applied at the R4 reproductive growth stage and at R4 followed by the same rates at R6. A single glyphosate application at the R4 stage did not injure soybean or cause reductions in soybean height, nodes per plant, branch pods, main stem pods, or yield, regardless of application rate. Likewise, sequential applications of glyphosate at the R4 followed by R6 growth stages resulted in no adverse effects on growth parameters or yield. Preliminary findings indicate excellent tolerance to glyphosate application at reproductive growth stages beyond the currently labeled R2 growth stage restriction in varieties evaluated. Research findings warrant evaluation of an expanded range of germplasm under a variety of environmental conditions and potential effects on seed quality for establishment of tolerance levels and potential herbicide label amendments.
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