SASKATOON – Farmers applying manganese micronutrient treatments should ensure they apply it after a glyphosate application. Manganese applied before glyphosate can be immobilized in the plant.
Don Huber, professor emeritus of plant pathology at Purdue University in Indiana, said there have been reports for 20 years about an increase in the cereal disease take-all in years following glyphosate applications.
“The increase in take-all the next year is very obvious.”
Huber said he has seen more glyphosate interactions in the past 15 years, so improvement is needed.
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He said the herbicide is a simple compound, but it can have profound effects.
Because glyphosate is a chemical chelator, it binds with mineral elements and can immobilize them. Huber said farmers who use ammonium sulfate in tank mixes with glyphosate eliminate some immobilization.
“Otherwise, it reduces the herbicidal activity of the glyphosate, especially if you have certain ones like phosphorus, zinc, calcium and magnesium. You can pretty much destroy its herbicidal activity by that chelation activity with those impurities,” he said.
“It’s systemic in plants and concentrates in the meristematic tissues (growing points), so you’ll get more of it in root tips, shoot tips and reproductive areas. The effect of it is greater in those areas than in the plant tissue as a whole.”
Huber said researchers have found serious problems in cotton, where the Roundup Ready gene offers little protection in the reproductive structures.
When sprayed on soil, glyphosate becomes tied up almost immediately. It won’t move through soil after a broadcast surface spray application, but Huber said it’s different when glyphosate moves through a plant.
“It’s released into the rhizosphere in root exudates and despite what you might read in the literature, it’s not readily degraded or readily bound in many soils. If you’ve got a lot of free calcium in the soil, it’s not going to move very far. It will be chelated – inactivated or immobilized. But if you don’t (have a lot of free calcium), then it’s going to move through the rhizosphere,” he said.
“You’re not putting it in a small area, you’re moving it throughout the rhizosphere. You’re getting distribution through the soil profile that’s quite different than a foliar spray that wouldn’t be systemically moved.”
Huber said nature provides many natural chelators, which form a chemical bond that can increase or decrease an element’s solubility or permeability into a plant.
“We use a number of (chelating) formulations because they increase permeability in the plant. A lot of the dithiocarbamate fungicides, a lot of that activity is increasing the manganese and zinc input into the plant,” he said.
Farmers tend to think of glyphosate as a compound that stops plant processes, but Huber said micronutrients act as the activators, regulators and inhibitors of all the physiological processes involved.
“Why did an enzyme stop functioning in that process? Because you didn’t have the activator or cofactor available for it to move on.”
He said there are many sources of chelators. Root exudates contain products that are released by the plant to increase solubility of nutrients. Organic acids serve that role, not only to increase solubility in the soil through the pH effect, but also to move nutrients across the cell membrane more effectively and efficiently.
Chelated material such as micronutrients affect the activation, inhibition and regulation of many physiological processes.
“The role of manganese becomes critical in a disease aspect because it’s not only involved in photosynthesis, it’s involved in carbohydrate and nitrogen metabolism and other systems,” Huber said.
“With pathogens like take-all and potato scab, the plant’s defence mechanism involves a lignification type reaction. Manganese has a role in regulating some of those metabolism processes.”
Huber said plants can use only the reduced form of manganese. If the manganese becomes oxidized, the plant can’t use it. It ends up in intercellular spaces and is not available for plant processes. As a result, disease pathogens have easier access to the plant.
He said it’s also important for farmers to understand the biology of the rhizosphere, which is the soil area around the roots where nutrients, especially micronutrients, are affected in uptake.
Conditions in the rhizosphere can be as important as the presence of the nutrients.
Those nutrients may not be available or may be sequestered or immobilized.
He said one of the early recognized effects of glyphosate was that it stimulated fusarium root rots on weeds, hastening their death. This increased disease susceptibility was considered part of the herbicide’s mode of action. Increased drought stress and earlier maturity were also observed with glyphosate use.
There are negative side effects. Huber recently attended a symposium at Ohio State on problems with potassium uptake in corn and other crops.
“Even though you apply potassium, if you’ve had four or five years of glyphosate application, all of a sudden we can’t get any potassium up into the plant.”
He said much of the yield lag associated with the Roundup Ready system in soybeans is related to an increase in disease or the reduced efficiency of manganese.
In trials 15 years ago on low manganese soil in Indiana, Huber struggled to achieve decent yields with certain soybean varieties.
“When you went up there with the Roundup Ready version of that variety, it just died. It’s quite specific for the manganese as far as the reduced uptake efficiency goes, both in soybean and corn varieties.”
He said problems of reduced manganese uptake efficiency after glyphosate use are related to the immobilization and translocation of manganese.
“Once it interacts and gets into that chelated form, it just doesn’t move. That’s when we start looking at time of application. When can we put it on? If we put it on at the same time as the glyphosate application, it’s not going to move anywhere and it’s not going to do the plant any good.”
Huber said adding ammonium sulfate to a glyphosate spray with manganese included will help reduce the chelation effect. However, there are still mobility concerns within the plant.
He has compared the isopropyl amine salt of glyphosate with potassium salt and found potassium salt is a much less interactive glyphosate formulation.
While potassium salt might provide better use of manganese, Huber said farmers using manganese should apply it a few days after a glyphosate application.
“If you’re tissue testing 15 days after a glyphosate application, you’re getting the reduced form (of manganese). (Shortly) after the glyphosate application, if you’re testing older leaves rather than the youngest expanded leaves, you could get immobilized glyphosate.
“Once the glyphosate goes on, what’s there is immobilized and it’s not going to be translocated. New manganese is going to have to be taken up,” he said.
The living material in the rhizosphere plays an important role in soil borne diseases and their relationship with glyphosate. Nutrient availability and natural biological control are both issues.
“We know that glyphosate changes the rhizosphere microbiology,” he said. “You have a different balance and biology. In our work and other work, glyphosate is toxic to a lot of the reducing organisms. For iron and manganese, it becomes very important from an availability standpoint. For other elements, it may not have much effect on them because there might not be a direct biological interaction there.”
