Effect of glyphosate and fungicide combinations on weed control in soybeans

Field studies were conducted in 2005 and 2006 along the upper Texas Gulf Coast near Yoakum, Texas and in 2005 near Tifton, Georgia to determine weed control and soybean response to glyphosate plus fungicide tank-mixes. In Texas, glyphosate at 0.77 kg a.e./ha in combination with azoxystrobin, tebuconazole, pyraclostrobin, and tetraconazole applied at the V3 or R1 soybean growth stage controlled Amaranthus palmeri and Panicum texanum at least 97% control while glyphosate alone controlled both weeds 100%. In Georgia, glyphosate applied alone to a mixed stand of annual grasses [P. texanum L., Digitaria ciliaris (Retz.) Koel., and Dactyloctenium aegyptium (L.) Willd.] at the V3 stage of soybean development provided only 68% control and at the R1 stage provided 94% control. When rated approximately four weeks after treatment application, increased control of the mixed stand of annual grasses was obtained with glyphosate plus azoxystrobin or pyraclostrobin applied at the R1 stage rather than the V3 growth stage. No yield results were obtained at the Georgia location; however, at the Texas location, soybean yields were higher when glyphosate plus azoxystrobin or tetraconazole was applied at V3 compared with the R1 stage. In conclusion, tank mixing of glyphosate with a fungicide is a good option and can be used to reduce production costs. If glyphosate applications are delayed too long, yield reductions can occur due to early season weed competition.