Soybean growth and development as affected by glyphosate and postemergence herbicide tank mixtures

Field research was conducted to evaluate the effect of glyphosate [N-(phosphonomethyl)glycine] and postemergence herbicide tank mixtures applied to soybean [Glycine max (L.) Merr.] at the V5 growth stage on growth, development, and grain yield of glyphosate-resistant and nonresistant cultivars. Glyphosate at 840 g a.e. ha(-1) did not affect vegetative growth, reproductive development, leaf area index (LAI), or grain yield of glyphosate-resistant soybean compared with nontreated glyphosate-resistant soybean. Soybean injury 21 d after treatment (DAT) was 14 to 18% when herbicide tank mixture treatments mere applied. Soybean injury caused by postemergence herbicide tank mixtures resulted in delayed vegetative development when measured 7 DAT, delayed reproductive development at 20 and XO DAT, reduced season-long height of 'Asgrow A1900' and 'Asgrow AG1901', and reduced aboveground dry weight at 35 and 56 DAT compared with nontreated plants. Leaf area index was reduced by postemergence tank mixtures up to 52 DAT but was greater 70 and 80 DAT, depending on the cultivar. The ratio of red to far red light that reached the soil surface 28 DAT was dependent on cultivar and herbicide selection. Yield of soybean treated with herbicides other than glyphosate timed for weed control and disease suppression was reduced at least 200 and 130 kg ha(-1) in 1997 and 1998, respectively, in the absence of weeds and disease.