The Soil Seedbank
“Soil seedbank” refers to weed seeds present in the soil. The soil seedbank serves as the source of next season weed problems. The majority of seeds in the soil were deposited by plants that escaped control on the last year. Due to seed dormancy, weed seeds can remain viable in the soil for many years and often emerge over many weeks during the growing season.
Seed dormancy, prolonged viability, and extended emergence periods make it easier for weeds to survive across a wide range of environments and management tactics. Current management recommendations target weed seed production to prevent additions to the soil seedbank. Recommendations include controlling weeds while they are small and susceptible to weed control tactics, removing weeds that escaped control, and managing weeds that produced viable seeds to minimize their impact on the soil seedbank.
These late-season practices are not new, but due to wide-spread herbicide-resistance they are being revisited with new technology. In this video Dr. Mandy Bish explains the importance of understanding the weed seedbank and why preventing weeds from producing seeds is so important.
How to Manage the Soil Seedbank?
Currently there are no economical methods to kill weed seeds once they enter the soil. Weed management practices focus on controlling weeds early in the growing season with herbicides and cultivation, then following up to manage those weeds that escape control and prevent seeds from entering the soil.
Studies have demonstrated that preventing seeds from returning to the soil can rapidly reduce the seedbank and resulting weed densities. Allowing weeds to produce viable seeds, however, can cause rapid increases in weed densities. Stopping seeds from entering the seedbank can be achieved by
- Preventing weeds from producing seeds (no seeds, no weeds)
- Enhancing weed seed predation
- Increasing seed mortality
- Removing seeds from the field before the seeds are shed
- If weeds do produce viable seeds, place seeds in narrow strips with chaff lining to improve management next year.
There has been a renewed emphasis on increasing seed mortality, mechanically removing seeds from the field, and altering the placement of seeds. These were tactics used before the wide-spread use of herbicides. With the technological advances in machinery these tactics are again being researched and adapted for use in various regions.
How to Prevent Adding to the Soil Weed Seed Bank
As cash crop harvest approaches you may discover that weeds have escaped your control efforts and are setting seeds. These weeds may be very visible, poking their heads above the canopy of the cash crop. When you go to harvest the cash crop, you are at risk of spreading the weed seeds.
These escaped weeds will create an even bigger problem next year if they are allowed to add the seeds they have produced back to the soil weed seed bank, or to spread their seeds to new fields by hitching a ride on harvesting equipment. You still have a chance to prevent this from happening.
Scout your fields prior to harvest.
You can scout using drones or other means such as physically walking the fields.
Know where the problem areas are: map them out.
Plan to harvest weed-free areas first and weedy areas last.
Limit the spread of weed seeds through the field and from weedy fields to clean ones. Clean the combine before leaving a field. A single Palmer amaranth plant can produce hundreds of thousands of seeds – planning your harvest strategy and cleaning equipment diligently is well worth the time. Remember: scout, map, and plan prior to harvesting to Get Rid of Weeds!
When to manage the soil seedbank?
Prior to planting
The most common tactics to reduce the soil seedbank include crop rotation and tillage. Diverse crop rotations allow for a diversification of other supporting IWM tools, such as herbicides, tillage, and crop competition. Diversified crop rotations are rarely used to manage the soil weed seedbank. The use of only one (or a few) cash crops ultimately allows weeds with similar traits to thrive, replenishing the weed seedbank annually. Tillage can both increase the soil seedbank (by burying seeds) and decrease it (by unearthing buried seeds that may germinate early and die).
The effect of deep tillage on the soil seedbank can be gauged by knowing the main weed species in a field and understanding how long the seeds of the species stay viable: deep tillage is a better choice for short-lived seeds. Grass weeds tend to have less persistent seeds than broadleaf weeds. Strategic deep tillage affects seedbanks depending on weed traits such as germination and seed size.
A specific tillage tactic to manage the seedbank is the stale seedbed method. Stale seedbed management uses tillage to promote weed germination so that another management tactic can then be used to kill weed seedlings prior to crop planting, reducing the size of the soil seedbank. Crop rotations and tillage are fundamental for weed management, but management decisions tend to prioritize economic and logistic factors over effects on weeds.
Prior and or during harvest
Harvest Weed Seed Control (HWSC) is an approach to managing the soil seedbank. These methods focus on cultural and mechanical options to reduce the impact of seeds from escaped weeds at harvest time. These are only effective for weed species that do not shatter and allow their seeds to shed prior to mechanical harvest.
HWSC tactics include:
- Narrow windrow burning
- Chaff lining
- Chaff tramlines
- Impact mills
- Chaff carts
- Bale direct system
Narrow windrow burning is relatively easy to adopt and can provide good results. Both small and large size weed seeds are controlled. This tactic involves a chute that is attached to the rear of the combine that concentrates the chaff and straw residues into a narrow windrow, which is later burned. At around $250 in materials it is also cost effective.
However, this method is time consuming, removes most of the field ground cover, has a risk of fire escape, may not be legal in all jurisdictions, is not an option for all crops (e.g. corn), and it is difficult to achieve a good burn in long windrows. Narrow windrow burning requires some trial and error. Harvesting low is key to obtaining more crop residue to burn.
Choosing the right time and conditions to burn is critical. A light wind is ideal, but be careful of a gusty day. Current research suggests that narrow windrow burning works best in wheat and soybean cropping systems, but other systems are being tested.
Chaff lining is a cost effective HWSC method. Chaff lining takes a chute and diverts only the chaff fraction into a narrow row in the center of the harvester, while the rest of the crop residue is spreadly evenly behind the combine. While weed seeds are returned to the soil, they are in narrow lines instead of being spread across the entire field.
The chaff material is allowed to rot and decay. These lines could be treated differently, using targeted herbicides sprays, or managed with different tools at a site specific level.
Chaff tramlining forms the chaff material into narrow rows on dedicated wheel tracks during harvest and relies on a mulch effect to prevent weed seed germination and emergence, as in chaff lining (above). Chaff tramlining equipment runs around $15,000 to $18,000 depending on your harvester brand and model.
Impact mills, such as the integrated Harrington Seed Destructor or the Seed Terminator, are integrated into the rear of the combine where an impact mill physically destroys the weed seeds in the chaff fraction.
Weed seed contained in the chaff fraction will depend on the weed species and in how many of the seeds are retained on the weed before harvest Impact mills can be highly effective with over 95% destruction of the weed seeds that enter the mill. The current impact mills are expensive and not available in most countries.
Chaff carts are the simplest HWSC system, consisting of a chaff collection and transfer mechanism that is attached to a combine that delivers the chaff fraction to a bulk collection bin, usually a trailing cart, that can be physically removed from the field so the chaff can be burned or grazed.
Major downsides to this method are that it adds extra length to the combine so it can be difficult for small fields or fields without much room to navigate and it requires extra time to empty the carts.
The bale direct system consists of a large square baler that is attached directly to the harvester which constructs bales from the chaff and straw residue. There is a limited market for the bales and a large risk of immediately spreading weed seeds to other fields.
All of the HWSC methods are similarly effective, but come with different initial costs, operating costs, and residual costs. Additional pros and cons related to feasibility, and level of nutrient removal or redistribution should be considered. The best HWSC method for your farm comes down to cost and other management concerns.
Table 1: Pros and cons of the six different HWSC methods.
|Total Cost||Objective||Soil Nutrient Removal||Pros||Cons|
|Chaff lining||low||Concentrate weed seeds in a thin row for future management||low||Easy to use and low cost. Low residue removal.||Limited research on managing weeds in the chaff lines.|
|Chaff tramlining||low/medium||Concentrate weed seeds in a thin row for future management||low||Chaff goes into tramlines. Low residue removal.||Soil compaction.|
|Windrow burning||low/medium||Build a chaff + straw row for future burning||high||Up to 95% weed seed destruction in certain residues.||Risk of fire escapes, may be restricted.|
|Impact mills||high||Destroy weed seeds during harvest||low||Highly effective on a wide range of weed seed sizes.||High initial start up costs.|
|Bale direct||medium||Concentrate weed seeds in bales||high||Adds value to diversified rotation farms. High value bales.||Small market.
High capital cost
|Chaff cart||medium||Concentrate weed seeds coming from chaff||low||Easy to use.||Slows harvest operation towing the cart and chaff disposal. High capital cost|
While the cost of managing the soil seedbank ranges dramatically by tactic, the end goal is ultimately the same. In the future, knowledge about the links between seedbank density and weed emergence patterns may inform additional soil seedbank management practices.
Our current research in HWSC
Our research started four years ago with three ongoing large plot field trials in Urbana-Champaign, IL, Beltsville, MD, and Fayetteville, AR. Those three locations are examining how the use of different IWM tactics can reduce the soil seedbank and combat herbicide resistant weeds. These tactics include the use of crop rotations, cover crops, and two different HWSC tactics, the use of impact mills and narrow windrow burning.
During the same period of time several field trials were conducted testing narrow windrow burning in soybean, grain sorghum, and wheat cropping systems at various locations. More recently, research is being conducted in small plots and on farm to adapt chaff lining to soybean, being this a well-proven technology in Australian wheat.
Our efforts are spread all over the country with the hope that region or state specific trials will help inform growers on the practices best suited for their farms. As our research progresses, our goals are to help growers improve their IWM tactics and to utilize HWSC as an additional method.