Residual Glyphosate Can Lurk in Soil

By: Bill Strautman, Saskatoon Newsroom

Producers need to be aware of the side effects that can occur in their soil when they use glyphosate, says a professor emeritus in botany and plant diseases at Purdue University in West Lafayette, Indiana,

"We need to recognize that any time we have a major change in agricultural practice, we're going to change a lot of things," Don Huber told a recent Agri-Trend Farm Forum in Saskatoon. 

"We may focus on one thing, but we don't just change one thing. There are a lot of other things that change, whether we want them to or not," Huber said glyphosate was patented as a strong metal chelator before it became a herbicide. It immobilizes manganese, which is required for the growth of many plants. 

"This compound (glyphosate) im­pacts plants through mineral deprivation," he said.

"So we need to recognize that fact and then do things that can compensate for some of that reduced micro-nutrient availability for our crops" 

Huber said glyphosate can affect many aspects of plant growth. 

"It impacts the plant directly as a herbicide," he said. 

"It can reduce the efficiency of micronutrient utilization, especially the physiological activity of manganese. Those plants would require more manganese for optimum physiological efficiency. It affects the abiotic environment through chelating nutrients and it's also toxic to rhizobia manganese reducing organisms and certain bio­logical control organisms in the soil. 

"It changes the rhizosphere microbiology. We need to recognize what those changes are because it changes the natural biological control or sup-pressiveness in the soil, plus the physiology and resistance mechanisms in the plant. In some cases, through that chelation, it also enhances the virulence of some of our pathogens.

Huber said glyphosate immobilizes enzymes by using physiologic pathways that require manganese, zinc, copper or iron for activity and a strong chelator that can immobilize them. 

"Tordon is a strong copper chelator. That's why it is more specific to broadleafs. Copper is more involved in broadleaf plants than grasses," said Huber. 

"If you chelate, you also reduce the efficacy of the glyphosate. That's why most producers add a little ammonium sulfate to their glyphosate mixture. It reduces the chelation with calcium and manganese to keep the glyphosate still active." Huber said 15 years ago he tried mixing glyphosate and manganese to treat manganese deficiencies in soybeans in low manganese soils. 

"We could mix them together and keep them in solution, but we didn't get any benefit. We lost herbicidal activity and we couldn't measure any manganese update." 

He said tank mixing with zinc is even worse, which is why tank mixing with micronutrients is not recommended on the product label, except in specific situations. 

Huber said the longer he's studied glyphosate weed management, the more he's seen manganese deficien­cy and poor nodulation. 
"In some of our soils we're seeing it takes up to four years after moving away from glyphosate before we can re-establish normal nodulation on soybeans and some of our other legume crops. 

"It's an area you need to be aware of in legumes. Watch for nodulation. You may need to inoculate in areas you didn't inoculate before because of the gradual reduction in the population of nitrogen fixing organisms.
He said Nitragen, an inoculant company in Indiana, has spent 10 years looking for strains of rhizobia that are resistant to glyphosate. It has not published any successful accomplishments, he added. 

One of the strengths of glyphosate is that it's systemic, which means it translocates in the plant. Huber said this also means it concentrates in rneristematic tissue, which is new growth tissue such as root tips, growth points above ground and the crop product itself. 

"Late application of glyphosate on Roundup Ready cotton can result in only 20 percent of the bolls staying on the plant because of the concentration of glyphosate in the meristematic tissue" he said. 

"We usually have a smaller root system with Roundup Ready crops because of the concentration of glyphosate that holds in the meristematic root tips. “It acts as a reservoir of glyphosate as that material decomposes." 

Huber said the availability of manganese varies, with different crop se­quences and rotations. 

"Continuous corn leaves a whole lot more manganese available for subsequent soybean crops than any of the crop rotations. And soybean has a high requirement for manganese " he said, adding that it's a good fit. 

"But when we got into Roundup Ready corn, we don't see this advantage. It looks just like all the other rotations. That's because of changes in the soil micro flora affecting manganese availability with glyphosate usage. And it doesn't have to be with the Roundup Ready crop. It can be an application as a burn down. It changes that micro flora." 

Tillage methods can also have an impact. Huber said switching to zero tillage from conventional tillage changes nitrogen relationships and nitrification. 

"Denitrification is much higher in no till than in a tilled plot, so residual nitrogen changes with different tillage systems. An important thing to remember is the need for balance and meeting the sufficiency needs of the plant if we're going to have optimum production and product quality."