Time of weed removal with glyphosate affects corn growth and yield components

Growers need to be aware that corn (Zea mays L.) develops leaf area slowly and competes poorly with early emerging weeds in the northeastern USA when planning a total postemergence program in glyphosate-resistant corn. We applied glyphosate at early postemergence (EPOST), third to fourth leaf stage (V3-V4) of corn growth; mid-postemergence (MPOST), V5 to V6 stage; and late postemergence (LPOST), V7 to V8 stage, in New York in 2003 and 2004 to determine how initial glyphosate timing affects growth, development, yield, and yield components of corn. The EPOST treatment, which received glyphosate when weeds were 10 cm or less in height, and a weed-free treatment silked on the same date, had similar dry matter (DM) accumulation (738 g m(-2)) and leaf area index (LAI) at silking (similar to 3.50), kernels per plant (528-549), and grain yield (11.1 Mg ha(-1)). The MPOST treatment, which received glyphosate when some weeds were 18 to 35 cm tall, silked 2 d later, had 35% less LAI and 39% less DM accumulation at silking, 21% less kernels per plant, and 25% less grain yield. Although the LPOST and untreated control, which had 1363 and 653 weeds m(-2) at the V5-V6 stage in 2003 and 2004, respectively, had the same LAI at silking (1.75-1.87), the LPOST treatment yielded higher (6.4 vs. 3.2 Mg ha(-1)). Results from this study indicate that growers in the northeastern USA should apply glyphosate by the V3-V4 stage to avoid yield losses from early season weed competition.