Glyphosate damage to Oxalis
Glyphosate – over-reliance leads to nutrient deficiencies
Glyphosate is to weed control what penicillin was to disease control in humans when it was first introduced. But as over-reliance on penicillin led (inevitably, as we now know) to resistance in disease organisms, so over-reliance on glyphosate has led not only to resistance in several weed species, but also to the gradual lessening of nutrient availability to plants. Both trends have worrying implications for crop production worldwide.
Glyphosate resistance in weeds
Professor Stephen Powles, of the University of Western Australia, has identified glyphosate resistance in nearly a dozen species of weeds in North and South America (Powles 2008). Where only 14 years ago farmers began growing Roundup Ready crops, including maize, soybeans, cotton and Canola, which survive the direct application of glyphosate while everything else succumbs, the continued use of glyphosate alone – in direct contravention of Monsanto’s advice to rotate herbicides – has seen the emergence of herbicide-resistant weeds that mean crop failure. Powles attributes this sudden rise in weed resistance to the sole use of glyphosate on every crop by farmers eager to maximise their profits. This short-term gain comes at the cost of long-term viability.
Unlike in the USA, farmers in Canada regularly rotate their herbicides with their crops. The result is that glyphosate resistance is unknown in Canada (Holmes 2010). Powles emphasises that rotation is the only way to secure the continued viability of glyphosate.
Hidden nutrient deficiencies
A more insidious trend, although just as worrying, is the emergence of evidence that continued use of glyphosate can reduce the availability of nutrients in the soil, and that regular use on Roundup Ready crops can interfere with nutrient use by plants. Dr Don Huber, professor emeritus at Purdue University, USA, reports (Huber 2010b) that because glyphosate acts by chelating (tying up) certain metal ions, which are essential in plant enzymes, not only does it kill susceptible plants (by stopping the enzymes from working), but it also ties them up in soil. He states that contrary to popular belief, glyphosate does not break down in soil (nevertheless, see the references in our earlier article that describe the breakdown of glyphosate by microorganisms), and that it can continue to tie up metal ions (some of which, such as Mn, Cu, Fe, Mg and Zn, are essential plant nutrients) in the soil, preventing plant uptake. (Certainly glyphosate bound to soil minerals is free to tie up metal ions.) He points to evidence of a long-term decline in crop growth in trials in Germany (Huber 2000a) as a result of glyphosate build-up in soil.
In addition to its effects in soil and susceptible weeds, glyphosate ties up metal ions within Roundup Ready plants as well. The resistance gene in Roundup Ready crops does not degrade the glyphosate in the plant; it simply bypasses its effects. So every time a Roundup Ready plant is sprayed with glyphosate, the plant takes up that glyphosate, which then adds to the store of locked-up nutrients in the plant. This effect explains the brief yellowing of resistant plants after an application.
The net result is a reduction in crop yields, both within the growing season and over several years. (Research cited by Wikipedia showed that crop yields were reduced by 6.7%.)
What you can do
If you suspect the emergence of resistant weeds, your best course of action is to switch herbicides for a year. Speak to your local district agronomist or district horticulturist for advice on suitable chemicals.
If you are worried about the locking up of nutrients, have your soil and plants tested. We can test for the presence of glyphosate residues in both and advise you of a suitable course of action. If nutrient availability is low, we can advise you of how much of which nutrients to apply.
Further reading
Holmes R. 2010. Weed resistance could mean herbicide is futile. New Scientist 2760 (15 May): 12.
Huber DM. 2010a. What’s new in ag chemical and crop nutrient interactions. Fluid Journal 18(3).
Huber DM. 2010b. Ag chemical and crop nutrient interactions – current update. Botany and Plant Pathology, Purdue University, Indiana, USA.
Powles SB. 2008. Evolved glyphosate-resistant weeds around the world: lessons to be learnt. Pest Management Science 64(4): 360–365.
SESL. 2009. The Loam Ranger – Glyphosate.
Wikipedia: Roundup.