Gone to seed
Thanks to the sinister survival skills of weeds, seeds from as far back as 30, 40 or 50 years ago can be hanging out in the soil profile waiting for the right conditions to grow.
When weeds succeed, they are actively building a seed bank in your soil. One mature Palmer amaranth plant in a soybean field can add thousands of seeds to the soil. Other weeds aren’t as prolific, but each weed that goes to seed carries out its biological imperative to survive by depositing seed for the next generation.
In some ways, the weed seed bank in each field is a memory—not just from escapes but from cultural practices and field use history. Farmers of a certain age remember a different weed profile from the days of 30-inch rows or deeper tillage. The same can be said for major changes in herbicide chemistry.
Multiple factors dictate how long a weed seed will remain viable in the soil. Seed coat type is an obvious factor. Seeds with durable coverings tend to stay viable longer wherever they land in the soil profile. But even long-lasting weed seeds are susceptible to microbial decay, being eaten by insects or birds, and germinating in fatal conditions.
Several years ago, Steven Fennimore, a weed scientist at the University of California, Davis, did a meta-study on weed seed banks. The data Fennimore collected showed that most weed seed banks have multiple species but that a few dominant ones make up 70% to 90% of viable seed in the soil. A second group of seeds, those not adapted to survive in the current cropping system, make up 10% to 20%, with the balance being newly introduced weed species and seed from previous crops. These percentages make sense in terms of what we call “weed shifts,” as dominant weed species change with tillage or herbicide practices.
Those figures assume there has been no significant physical movement of weed seed, of course. Growers who had land inundated with floodwaters in 2019 might experience a weed shift as flooded land returns to production.
“When soil moves, we can see more emergence of weeds like cockleburs, a seed that can remain viable in the soil for 30 to 40 years,” said Dr. Jason Weirich, vice president of sales and agronomy for MFA Incorporated. “Where we see soil deposits from riverbanks or scouring of topsoil, we are going to see weeds in some fields that we haven’t seen in a while. That’s the nature of seeds that can remain dormant in the seed bank.”
The good news is that the seed banks are affected by efficient weed control. Iowa State weed scientist Bob Hartzler has done work to show that consistently clean fields reduce the number of weed seeds in the soil, making control easier to maintain. The same research offers a significant caveat: It only takes one management disaster to rebuild weed seed bank deposits.
Whether talking to research weed scientists or agronomy advisors, they all insist the best defense against future weeds is control.
“First and foremost, we’re thinking about yield this season,” said Weirich. “And right now, the overlapping-residual method is proving itself effective. A full rate of a pre-emerge herbicide applied at planting followed 20 to 30 days with a post-emerge application of another residual is effective weed control, and effective weed control is going to keep weed seed out of the bank.”
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