Is Mineral Nutrition of Glyphosate-resistant Crops Altered by Glyphosate Treatment?

Claims have been made that glyphosate application to glyphosate-resistant (GR) crops can result in deficiencies in certain mineral elements in those crops and that this is a cause of increased plant diseases. Strong evidence from multiyear and multisite studies has not verified these claims. Furthermore, these studies and others have found that glyphosate has no effect or a slight stimulation of yield of GR crops. Crops made resistant to the herbicide glyphosate represent about 80% of the acreage of transgenic (GM) crops grown worldwide. Their continued phenomenal success over the past 23 years has resulted from the ability to use perhaps the best herbicide yet devised with high-yielding varieties of soybean, maize, canola, sugarbeet, alfalfa, and cotton. Yet, there has been controversy over whether glyphosate adversely affects mineral nutrition of glyphosate-resistant (GR) crops. These claims have been linked to claims that GR crops are more susceptible to some plant diseases due to manganese deficiencies and other causes and to yield decreases. Two proposed mechanisms of purported glyphosate effects on plant mineral nutrition have been proposed: 1) direct effects by chelation of mineral cations, especially divalent cations such as Mn++ and 2) toxic effects on rhizoshere microbes involved in plant mineral assimilation. Although, an analysis of all the literature on this topic concluded that most of the literature did not support the view that glyphosate use in GR crops caused these problems, these claims of adverse effects have received considerable attention from farmers and the general public, and they persist in reviews and on websites. More recently, even stronger evidence refuting claims of altered mineral nutrition in glyphosate-treated GR crops has been published, while virtually no findings to the contrary have been reported. This short review discusses these new papers and the strong case for a lack of an effect.