Weed Seeds In Soil

Quantitative analysis of emergence of seedlings from buried weed seeds with increasing soil depth – Volume 49 Issue 4 Harvest Weed Seed Control – How It Depletes Soil Seedbank Some Australian farmers collect chaff in pull-behind carts like this one to keep weed seeds from dispersing. Some Australian farmers

Quantitative analysis of emergence of seedlings from buried weed seeds with increasing soil depth

Trials were carried out to investigate the effects of seed burial depth on seedling emergence rate of 20 weed species. Marked depth-mediated variation in emergence ability of the different species was observed, together with a general pattern of decreasing emergence with increasing soil depth. At 10 cm, only johnsongrass, velvetleaf, catchweed bedstraw, and cutleaf geranium emerged, albeit only in limited numbers. Species most severely inhibited by burial depth were buckhorn plantain, large crabgrass, common purslane, chickweed, and corn spurry, none of which emerged from beyond 6 cm. In all species, depth-mediated inhibition was found to be sigmoidal (polynomial regression). In addition, the number of seedlings and rate of seedling emergence decreased when depth of burial increased. The depth at which the number of emerged seedlings was halved varied by species and ranged from 3.6 cm for common purslane and chickweed to 7 cm for velvetleaf and catchweed bedstraw. Excessive burial depth generally induced dormancy (in roughly 85% of cases) rather than suicide germination. A close inverse relation (second-degree equation) between seed unit weight and depth-mediated inhibition was observed. The physiological involvement of depth inhibition in seed bank ecology is discussed.

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Harvest Weed Seed Control – How It Depletes Soil Seedbank

Some Australian farmers collect chaff in pull-behind carts like this one to keep weed seeds from dispersing.

Some Australian farmers collect chaff in pull-behind carts like this one to keep weed seeds from dispersing.

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

  1. Preventing weeds from producing seeds (no seeds, no weeds)
  2. Enhancing weed seed predation
  3. Increasing seed mortality
  4. Removing seeds from the field before the seeds are shed
  5. 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.

Palmer amaranth that escaped control during the growing season, with viable seeds ready to return to the soil seedbank. Photo: Claudio Rubione

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.

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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:

  1. Narrow windrow burning
  2. Chaff lining
  3. Chaff tramlines
  4. Impact mills
  5. Chaff carts
  6. 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.

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Windrow formation for burning in a wheat field highly infested with Italian ryegrass in Arkansas. Photo: Lauren Lazaro Windrow burning needs to be carefully planned ahead of time. Wind speed and direction, relative humidity, availability of tillage equipment to build preventive fire fences, and access to plenty of fire extinguishers are some of the issues to be considered for safely managing the fire. Photo: Claudio Rubione

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.

A plastic chute fitted to the harvester funnels the chaff containing the majority of weed seeds present into a narrow band in the middle of the CTF run. Photo: Mic Fels

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.

Chaff tramliners are variable and quite expensive. Photo: Peter Newman, WeedSmart

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.

Integrated Harrington Seed Destructor. Collected chaff is sent to the mills, which pulverize weed seeds and small residue into dust, which comes out through the back of the machine. A four year study on the technology is being carried out across the US. Photo: DeBruin Brothers

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.

Chaff cart pulled behind a combine in Australia. Photo: Michael Walsh, Australian Herbicide Resistance Initiative

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.

The bale direct system provides a new system for harvesting corn and baling corn stover in one simple step. Picture credit: http://biomassmagazine.com/articles/10864/john-deere-hillco-introduce-single-pass-round-bale-system

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.

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.