Tolerance of Bradyrhizobium strains to glyphosate formulations

Application of glyphosate to soybean plants, resistant to this herbicide, may be harmful to Bradyrhizobium, the symbiotic bacterium. This study compared the effect of two glyphosate commercial formulations, ZappQi® and Roundup Transorb® with the standard N-(phosphonomethyl) glycine on the growth of four Bradyrhizobium strains under laboratory conditions. Commercial formulations were applied at a concentration of 43.2 μg L−1 of a.e. and the strains were inoculated in yeast extract mannitol (YEM) broth used with or without a mixture of aromatics amino acids (phenylalanine, tyrosine, tryptophan). Herbicide effect on the growth of the Bradyrhizobium strains was assessed by optical density reading in a spectrophotometer. Roundup Transorb® had the biggest effects, probably, because of the presence of ethylamine as a surfactant. Pure glyphosate had minimal effects on Bradyrhizobium strains when aromatics amino acids have been added to YEM broth. Glyphosate application as Roundup Transorb® in the initial stage of soybean development may affect the nodulation process by Bradyrhizobium.