Sugar Producer

June/July 2010 Sugar Producer

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In the Field NUTRIENTS MANGANESE FERTILIZERS ARE THEY WORTH IT? by Amber Moore There has recently been some specu- lation 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. Soybean Study 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 stud- ies illustrated increased incidence of Mn deficiency in some (not all) glyphosate re- sistant varieties in comparison to conven- tional 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 gly- phosate 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. Current Research Needless to say, the current research on Mn deficiencies in glyphosate resistant www.SugarProducer.com 15 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 resis- tance 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 us- ing glyphosate resistant sugarbeet varieties for only a few years, there has been no ex- tensive research completed on sugarbeets. However, university researchers from North Dakota, Michigan, Wyoming and Idaho have reported that Mn deficiencies have not been detected in glyphosate resis- tant sugarbeets in their respective regions. Finally, addressing interactions in the soil, Hartzler mentions that while gly- phosate has the ability to chelate cations in the soil, it is far more likely that glyphosate will chelate cations in the greatest abun- dance, such as calcium and magnesium, as opposed to Mn and other micronutrients. Because calcium and magnesium con- centrations in Idaho and other dry climate soils far exceeds plant requirements, defi- ciencies from glyphosate chelation would be highly unlikely. Recommendation The best recommendation is to manage nutrients for glyphosate resistant sugar- beets 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 gly- phosate 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. n Editor’s note: Moore is University of Idaho Extension Soils Specialist and can be con- tacted by calling 208-736-3629 or e-mail at amberm@uidaho.edu.

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