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Soybean seeding rates and herbicide resistance

A weed science extension specialist at the University of Wisconsin-Madison says a high soybean seeding rate does not deter herbicide resistance from developing.  FILE PHOTO.

New research from the University of Wisconsin-Madison refutes the widely held belief that growers should employ a high soybean seeding rate in order to counter herbicide resistance.

Because a higher plant density generally results in a thicker crop canopy, it might seem intuitive that seeding heavily will improve weed control and reduce the likelihood of weeds overcoming herbicides. In fact, this supposition forms the basis of the integrated weed control advice given by many agronomists and crop researchers.

According to Vince Davis, an assistant professor of cropping systems and a weed science extension specialist at the University of Wisconsin-Madison and co-lead on this research, within that logic is a vital flaw. Specifically, herbicide resistance does not develop at a stage in the season when crops are big enough to impact weeds competitively. Therefore, Davis argues, seeding rate cannot positively or negatively impact herbicide resistance development at all.

“We have long known that extra crop plants are helpful for overall weed control throughout the season because they compete with weeds. However, how do extra plants help specifically with resistance management?” he says. “I’m a bit of a lone ranger on this. Ultimately, there are a lot of people who will say we should continue to preach high seeding rates for weed control. I don’t disagree with that thinking, but we have to think of how herbicides work and when resistance develops, and then make seeding decisions based on economics.”

Given that the price of soybean seed has increased by more than 200 per cent in less than two decades, producers need to be very sure of what increased seeding can and cannot achieve, since farm profitability can be hugely impacted by seeding rate. Herbicide resistance develops at the moment that weeds face selection pressure from a herbicide. In other words, herbicide resistance develops at the time of herbicide application. If the moment of resistance development occurs when a pre-emergent herbicide is applied, the crop will be in seed form, so will have no competitive effect on weeds. If, instead, the moment of resistance development occurs when a post-emergent herbicide is applied, the crop will have emerged but is too small to place any significant competition pressure on weeds. Therefore, while a heavy crop canopy later in the season will decrease weed biomass, it will not stop resistance development because that resistance will already have developed many weeks before.

“To reduce selection pressure that could result in weeds overcoming a herbicide, you have to decrease the number of weeds exposed to that herbicide. When someone says growers should use more seed as a resistance management tactic, to me that means those extra plants have to have an effect on the amount and size of weeds exposed to the herbicide, which isn’t possible when the crop plants are tiny,” Davis says.

Davis’ research instead suggests another tactic for battling resistance. The reality is that growers have few options to solve any herbicide failures after a post emergent herbicide application. Therefore, producers should opt to counter resistance as early as possible with a herbicide applied at the pre-emergence timing.

“We sometimes take criticism when we say we need to use more pre-emergence herbicides to protect us from herbicide resistance at post emergence. Some people will say that we’re just advising them to use another chemical to prevent chemical resistance. But, when you use a herbicide at the pre-emergence timing, you have time to react to failures by implementing a post-emergence mop-up. You have a greater window to make better choices to fix failures. If you only opt for a single application at the post-emergence timing, you don’t necessarily have the opportunity to solve resistance issues,” he explains.

 To study the impact of seeding rates when used with and without pre-emergent herbicide, Davis and co-researcher Shawn Conley planted field trials in Wisconsin in 2012 and 2013. The plots were planted at five different seeding rates, half of the plots were treated with a residual pre-emergent herbicide, and then all plots were treated post-emergently with either conventional herbicides or conventional herbicides and glyphosate.

During the course of this research, Davis and Conley used digital analysis technology to be able to quantitatively analyze the amount of cover soybean plants achieved using various herbicide management practices. The finding that surprised the researchers most was that applying herbicide at pre-emergent timing produced an increase in soybean canopy development.

“What we are saying is that using a pre-emergent herbicide is not only the better chemical means to prevent chemical resistance, we also get more canopy development, so we are getting more cultural weed control. This finding was really not expected. Even at lower seeding rates, we saw a better canopy development,” Davis says.   

With proper resistance-management focused weed control in place, producers should make a seeding rate decision based on good agronomics and solid economics.

“At the end of the day, the economics of seeding rate should be determined by those relationships you’d expect under a weed-free environment. Growers should make their rate decisions based on their yield expectations for their geography and not necessarily on the expectation of additional weed control,” Davis says.

“I don’t necessarily disagree with the conventional thinking that encourages using extra seeds for weed control,” he says. “If we go back 15 or 18 years, it was really economical to do that because soybean seeds were so much cheaper. But today, a lot of people with a lot of experience still have the mindset that you can ignore the economics of soybean seeding rate. What has changed over the last decade or so is the cost of soybeans. It’s not an economic decision that should be ignored anymore.”

June 30, 2015  By Madeleine Baerg