Seedbed: What we know and don't know about manganese deficiency

Wednesday, May 5, 2010

A review of the research suggests that glyphosate does have an impact on manganese nutrition in Roundup-Ready crops, but that there's little evidence of a looming crisis

by KEITH REID

Soon after the introduction of Roundup-Ready (RR) soybeans, there were reports of "yellow flash" following glyphosate application, which looked a lot like manganese deficiency. This resulted in widespread recommendations to include manganese with the glyphosate, leading in turn to poor weed control from inactivation of the herbicide in the spray tank.

Since then, there has been considerable research effort but not a lot of clarity in the reporting of the results. The clearest trend appears to be widespread dissemination of any results showing negative effects of glyphosate by the groups opposing genetically modified crops or any large agri-chemical company, while positive or neutral results are virtually ignored.

This leads to many questions from farmers about whether they are losing yield from manganese deficiency, and whether they should be applying additional manganese.

With the questions reaching a fever pitch this winter, as a result of a presentation at a high profile meeting, I scoured the scientific literature to see if I could sort out fact from fiction.

To summarize many hours of reading, my interpretation is that glyphosate does have an impact on manganese nutrition in RR crops, but that most of the research results have failed to make a distinction between effects that are real, and effects that are important.

I could not find any evidence of a looming crisis in manganese nutrition and, while some fields may need supplemental manganese, the number is relatively few.

What do we know about glyphosate?

Chemically, glyphosate falls into the category of compounds that chelate metallic ions from solution. In this case, it has a very strong affinity for ions carrying two or more positive charges (like calcium, magnesium, iron and manganese), and binds these elements very tightly.

Since this binding is part of the herbicidal activity of glyphosate, the chemical is inactivated if it binds with any ions outside of the plant.

This means that tank mixing glyphosate with manganese will likely harm either the herbicide effectiveness, or the manganese effectiveness, or both.

Once in the plant, glyphosate translocates to the growing points and accumulates there. It blocks a particular enzyme pathway, but RR crops sidestep this by providing an alternate pathway to do the same thing. Excess glyphosate in the plant tissue will tie up manganese and actually create a manganese deficiency within the plant.

One field study in Illinois found that the amount of leaf yellowing was proportional to the amount of over-application of glyphosate, and that there was a yield loss proportional to the amount of leaf yellowing. Adding manganese, either before or after the glyphosate application, made no difference to the amount of leaf yellowing or to yield.

It is important to remember that the impact of glyphosate on the plant occurs because it accumulates at the growing points. Otherwise, the potential amount of chelation is not large enough to have much impact at all.

At normal application rates, if all that glyphosate bound was manganese, the maximum amount that was immobilized would be about 300 grams per hectare.

In reality, we can expect manganese to be tied up in proportion to all the other cations in solution, which are a thousand times more abundant, so it is unlikely that binding 0.3 grams of manganese in the soil will make a noticeable difference in most cases.

Questions yet to be answered about glyphosate and manganese. Recently, there has been a focus on the effect of glyphosate on the rhizosphere, the zone immediately around the roots.

While there doesn't appear to be any harmful effect on nodulation at correct application rates, there is some suggestion that the microbial populations on the root surface are changed by glyphosate "leaking" out of the roots. It is too early to tell if this actually makes a measurable difference to the plant, since this microbial layer is very thin.

It is also too early to tell if RR crops are more susceptible to manganese deficiency in those borderline situations where there would have been just enough for a conventional crop.

We know that adding more manganese where there is already lots from the soil will not make any difference, and that the way to manage the deficiency symptoms is by keeping within label rates of glyphosate.

We are less sure of the impact at the low end of manganese supply. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, based in Stratford. Email: keith.reid@ontario.ca