Why herbicides fail

THE IMPORTANCE OF A WEED MANAGEMENT STRATEGY

As glyphosate-resistant weeds become an issue in Ontario, farmers are trying to incorporate more herbicide modes of action into their weed control programs. Unfortunately, some of them are disappointed with the level of weed control they get with conventional chemistry. It is not the chemistry, though; there are plenty of great products on the market. Dr. Peter Sikkema, Professor of Field Crop Weed Management at the University of Guelph, Ridgetown Campus, offers a better understanding of what influences weed control with herbicides.

“I think the correct weed management strategy changes by field; and consequently, I think it takes a knowledgeable individual to put together the appropriate weed management strategy,” says Sikkema. “My guess is that growers have at least 20 different options in terms of weed control for each individual field. Of those 20, one is going to be the ideal one and it takes a knowledgeable person to figure out which one that one is.”

A number of factors contribute to poor weed control, including weather extremes, soil characteristics, weed pressure, application timing and issues, and herbicide choice.

weather extremes
“Most farmers realize the importance of rainfall to dissolve soil-applied herbicides into soil-water solutions so that developing weed seedlings can take them up,” says Sikkema. “But it’s easy to forget if you haven’t used them for five or 10 years.”

Without significant rainfall, it is difficult for post-emergent herbicides to cross the cuticle and enter into the living part of the plant. But too much rain after a pre-emergence application will lead to poorer weed control, as a portion of the herbicide will often leach below the weed seed germination zone. Likewise, too much rain after a post-emergence application can cause herbicides to wash off. While none of these situations are controllable, better planning can certainly help to avoid some of them.

soil conditions
Your soil conditions will also have an affect on weed control. “You need to know the soil type in each individual field – whether it’s soil texture or percentage of sand-silt-clay content, organic matter, pH, cation exchange capacity – all of those things affect which weed management program you’re going to use,” notes Sikkema. “Typically, the higher your cation exchange capacity the more tightly herbicides are bound to the soil and so consequently, you get poorer weed control. In the reverse, if your cation exchange capacity gets lower, herbicides are more available and you get better weed control, but more crop injury.”

Warm, moist soil that sees greater microbial activity will break herbicides down more rapidly. And, for some soil-applied herbicides, soil pH can have an affect on water solubility, ionization properties and the metabolites formed. 

choosing the right herbicide
“You’ve got to know what weed species you have in each individual field,” says Sikkema.” You need to know the life cycle of those weeds, and you need to know the density of the weeds.”

“I think most of the herbicides that are sold in Ontario in 2013 are excellent herbicides,” he continues. “But take Pinnacle, for example. It’s just lousy on common ragweed. And that’s not that Pinnacle is a bad herbicide, that’s just the spectrum of activity that it has. Growers just need to know which weed species they have in each field and match the herbicide to the weed spectrum in each individual field.”

poorly timed applications
“I would think that probably one of the most important aspects of weed management that gets overlooked frequently is the effect of weed size at the time of application,” says Sikkema. “I think that a lot of farmers don’t appreciate that hidden yield loss that they have when they delay the application of an herbicide.”

According to his research, you can lose two bushels of corn a day or one bushel of soybeans a day by delaying the application from Monday to Tuesday morning. “Even though it’s tempting to plan your work for the week on a Sunday evening and then try to fit everything in throughout the week, the reality is that there is a huge potential yield loss waiting to happen if you don’t act at just the right time,” says Sikkema. “And those numbers are not dramatic at all. In our research, it can be even more.”

Glyphosate resistance – a growing problem
In Ontario, there are no less than 71 fields that contain glyphosate-resistant giant ragweed, another 155 with glyphosate-resistant Canada fleabane, and one with glyphosate-resistant common ragweed. While glyphosate-resistant giant ragweed and Canada fleabane can be managed quite well in corn and wheat, the real challenge is with soybeans, IP or Roundup® Ready.

“Our data says that if you have glyphosate-resistant giant ragweed or Canada fleabane, you need to have perfect control the day you feed your crop,” says Sikkema. “If you don’t have perfect control, we have no herbicides, zero – and I can’t believe I’m saying that in 2013 – we have no herbicides that will consistently control those weeds post-emergence in soybeans.”

When it comes to weed management, your number one tool is having a diverse crop rotation with multiple herbicide modes of action in your weed control program. In corn and soybeans, implementing a two-pass weed control program is also advisable.

“Put down a broad spectrum soil-applied herbicide and then clean up with glyphosate or glufosinate or whatever you want post-emergent in crops,” says Sikkema. “If all growers do that we will reduce selection intensity for additional herbicide-resistant weeds going forward.”

“I think every farmer has to plan his weed management strategy, but at the same time they have to be flexible and adjust it on the fly as conditions dictate,” he concludes. •