Manganese fertilizers for glyphosate resistant crops - are they worth it?
University of Idaho's current position
There has recently been some speculation that glyphosate resistant sugarbeets are more susceptible to manganese (Mn) deficiencies than conventional sugarbeets, due to 1) genetic alterations in the plant, 2) interactions between glyphosate and Mn within the plant, and 3) interactions between glyphosate and Mn in the soil.
Much of this speculation is based on a small number of studies on soybeans, summarized by Bob Hartzler in the Iowa State University Extension publication "Glyphosate-Manganese Interactions in Roundup Ready Soybean", which can be downloaded at http://www.weeds.iastate.edu/mgmt/2010/glymn.pdf.
As reported by Hartzler, these studies illustrated increased incidence of Mn deficiency in some (not all) glyphosate resistant varieties in comparison to conventional varieties grown on Mn-limiting soils.
Because not all glyphosate resistant varieties tested in these studies illustrated Mn deficiency, Hartzler suggests that the Mn response is not directly related to the glyphosate resistant trait. Also, in soils that had sufficient Mn, no difference was detected. In other words, the relationship between Mn deficiency and the glyphosate resistant gene in soybeans has been greatly exaggerated.
In regards to interactions between glyphosate and Mn within the plant, Hartzler lists three separate studies that did not identify any differences in Mn absorption, accumulation, and availability between glyphosate and non-glyphosate treated plants, with only one study that did report decreased Mn adsorption.
Needless to say, the current research on Mn deficiencies in glyphosate resistant soybeans lacks scientific documentation, at best.
To relate these studies to sugarbeets, a big assumption has to be made that the genetic alterations for glyphosate resistance will alter other traits in sugarbeets similarly as soybeans.
This is highly unlikely, especially since the evidence with soybeans is very weak. Due to the fact that growers have been using glyphosate resistant sugarbeet varieties for only a few years, there has been no extensive research completed on sugar beets.
However, university researchers from North Dakota, Michigan, Wyoming, and Idaho have reported that Mn deficiencies have not been detected in glyphosate resistant sugarbeets in their respective regions.
Finally, addressing interactions in the soil, Hartzler mentions that while glyphosate has the ability to chelate cations in the soil, it is far more likely that glyphosate will chelate cations in the greatest abundance, such as calcium and magnesium, as opposed to Mn and other micronutrients.
Because calcium and magnesium concentrations in Idaho and other dry climate soils far exceeds plant requirements, deficiencies from glyphosate chelation would be highly unlikely.
The best recommendation is to manage nutrients for glyphosate resistant sugarbeets similar to conventional varieties.
In other words, if you detect limiting concentrations of Mn in the plant tissue (between 4 and 20 ppm in the leaf blade), apply Mn fertilizers.
(It is not recommended to apply glyphosate in tank mixtures with Mn or other cationic fertilizers.)
Otherwise, we encourage you to save time and money by avoiding applications of Mn and other nutrients if your soil test and/or tissue tests do not demonstrate deficient levels.
Editor's note: Moore is University of Idaho Extension Soils Specialist and can be contacted by calling 208 736-3629 or email at firstname.lastname@example.org.