Consider for a moment what’s out there in your seedbank, meaning the number and assortment of weed seeds in a given field right now. That seed bank determines, perhaps more than anything else, how you should approach weed control and herbicide strategies now and in the future.
Your seedbank isn’t a static mixture. It changes, depending on the pressure applied to it by herbicide and technology choices, rotation, production practices, the assortment of local weed species and your geography. What you have today could likely be different than it was 5 years ago, especially if you’ve recently been seeing more weeds at harvest.
The more pressure you put on weeds, the more likely you’ll reduce the overall abundance of weed species. Even as some species slip by Roundup, the herbicide continues to suppress still-susceptible species, further reducing the inventory of those seeds in the bank.
At the same time, the process selects for resistance among species that become problems, now or later.
That concept comes out in an ambitious study that looks at shifts in weed diversity in the Corn Belt, the Delta and the Southeast. (A paper from the study, by the way, was named the 2016 Outstanding Paper Award in weed science by the 2016 Weed Science Society of America. The study’s title: Seedbank and Field Emergence of Weeds in Glyphosate Resistant Cropping Systems in the United States.
States included in the study were Mississippi and North Carolina in the South and Illinois, Iowa, Indiana and Nebraska in the Midwest. The selected states already had a high adoption rate of glyphosate-resistant crops, which allowed researchers to make comparisons across corn, soybeans and cotton as they started collecting samples in the last decade.
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But as time went on, the other species became less common in some fields and glyphosate-resistant weeds became the dominate problems.
“Initially, very few of these fields were infested with glyphosate-resistant weeds – Palmer pigweed, waterhemp or horseweed (aka marestail),” says Bryan Young, a Purdue University weed scientist. “When sampling started we did have some glyphosate resistance, mostly horseweed in some continuous no-till soybean fields. At the start, glyphosate was helping to maintain a diverse weed population, especially in the soil seed bank. The seed bank held weed species that were primarily sensitive to glyphosate.”In all, researchers worked in 156 commercial field sites, pulling shallow, uniform samples over a three-year period. Each year they froze the samples to simulate overwintering temperatures. From there, they spread the soil on greenhouse growing trays and waited to see what emerged. They could then ID species, count the number of emerged seedlings and work out percentages to determine the mix. These numbers also were compared to a series of visual counts of weed species that had emerged in the fields.
The research was really just capturing the start of glyphosate resistance in some fields by the conclusion of the study, says Young, who was the corresponding author of the study – meaning the team member who brought all the data together for analysis.
“All of a sudden when there’s a species that you can’t control and it’s resistant, weed diversity drops off and you are mainly dealing with the resistant species going forward,” Young continues. “If anything, glyphosate actually increases the diversity of weed species in a given field early on, something other studies indicate. But when a species in the seedbank does develop resistance, increased applications and rates of glyphosate tend to reduce diversity at the same time.”
That’s not to say other species still aren’t present, but their numbers have been reduced so much in the process that they are a marginal factor, at best.
“If you go back 15 years to when glyphosate-resistant crops were being adopted, many growers said their fields were cleaner than any other time they could remember,” says Young. “We reduced our seed bank quite a bit in that period and still had diversity. Even though you drove populations down in your seed bank, you still had a mixture of species. But constantly pounding with glyphosate reduced the abundance of susceptible species and also selected for resistance, which led to the problems we’re dealing with now.”
Sites in the study had to have a minimum 3-year field history with one of three scenarios:
- A continuous glyphosate-resistant (GR) crop.
- A rotation of 2 GR crops.
- A GR crop rotated with a non-GR crop.
“We would have liked to have included fields that had no history of GR crops but could not find enough in any of the study areas to have a valid sample,” Young says. “We also weren’t able to work LibertyLink into the study because of limited adoption in all the major crops at the inception of the research in 2006.”
Points and conclusions that came out of the study include:
- The mix of weed species just prior to a post-emergence herbicide application tends to closely match the diversity of weeds in the current seed bank. While evaluations were made on other dates, the population mix just ahead of that application “was most similar to the composition of the weed seedbank” as identified in the greenhouse portion of the study.
- It’s hard to make sweeping regional generalizations about weed population trends or resistance management. While regional patterns exist, how seedbanks evolve still has much to do with farm-level decisions. “Sometimes we hear blanket statements about how to approach resistance management, but this work really shows that you have to be cautious about that,” says Young. “Management decisions really get down to conversations with the farmer or how he develops an approach on his own. Resistance starts on individual farms, so we must be careful not to discount differences in regions and farming practices when we suggest the best methods to combat weed resistance to herbicides.”
- How often farmers use the GR trait in a given period isn’t as important as all the other decisions they make, at least when it comes to delaying the development of a dominant resistant species. Much depends on how a grower integrates GR technology with other factors. These can include varying cultural practices and mechanical weed management, along with the diversity of herbicide modes of action. Also figuring into this would be the frequency of glyphosate applications and any rotation of crops or herbicide-related traits. Modes of action weren’t a focus of the paper “but overall diversification of herbicide modes of action is important,” Young points out. “Some of these modes of action are relatively new but others are 30 to 40 years old, such as growth regulators like 2,4-D or dicamba ahead of soy planting for burndown of horseweed.”
(Other authors of the study included: Lauren M. Schwartz, David J. Gibson, Karla L. Gage, Joseph L. Matthews, David L. Jordan, Micheal D. K. Owen, David R. Shaw, Stephen C. Weller, Robert G. Wilson.)