Harvest Weed Seed Control

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 California Small Grain Production

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.

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).

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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.

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.

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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.

Harvest Weed Seed Control

Concerns about a growing resistance to herbicides

In Mediterranean or arid climates, particularly in areas with marginal soils, crop rotations are often limited to a narrow range of hay, pasture, a handful of winter legumes, or rainy-season grasses. Arid conditions and weathered soils drove Australia’s rainfed grain growers to adopt no-till strategies earlier than their counterparts in California. While beneficial from a water use perspective, successful no-till systems depend on herbicides to control weeds that were traditionally kept in check with tillage.

Dependence on herbicides alone in these systems has resulted in weeds with resistance to multiple modes of action. In Australia, there is one documented population of rigid ryegrass (Lolium rigidum) that is resistant to 15 different herbicides, covering seven different modes of action. Italian ryegrass (Lolium multiflorum), an annual winter species commonly found in small grain production systems in California is also notorious for its ability to develop resistance to entire groups of herbicides. One population collected in California orchards has resistance to four modes of action[1], but this population is not yet widespread.

One advantage is that California is still a long way from Australia in terms of herbicide resistance in our small grain cropping systems. We can look to Australia for 20 years of methods and data that have arisen in an effort to combat herbicide-resistant weeds. And, if we can find a way to slow the spread of herbicide resistance, we may be able to continue relying on some of the herbicides that are available in our area.

What is Harvest Weed Seed Control?

When growers need to manage herbicide-resistant populations of weeds without tillage, one strategy is to reduce the amount of seed returned to the seedbank by destroying, or removing weed seed caught during harvest operations. Collectively these strategies are referred to as Harvest Weed Seed Control (HWSC). These methods have long been studied in Australia. Researchers at Washington State University[2], Virginia, Texas, and other parts of the US have been looking into these methods for several years. With increasing herbicide resistance developing in Italian ryegrass populations in California, similar approaches may be worth exploring in the Sacramento Valley.

HWSC methods include several different strategies. Each have their own constraints and challenges, and all are designed to physically destroy seeds during or after small grain harvest. This prevents new seeds from entering the seedbank, including those that are from plants partially or fully resistant to herbicides used in the field. This reduces the rate of resistance development and the spread of resistant genes.

Narrow-windrowing – Sorting straw and chaff into narrow windrows, in an effort to contain weed seed in a small area. Windrows can be burned, grazed, or if narrow enough, allowed to compost/ rot in place. If the windrows are left alone, the high rates of carbon will drive soil microbes to scavenge for nitrogen, which subsequently immobilizes plant available nitrogen. Low available nitrogen adds to a crowded, competitive, and less-than-ideal environment for a large number weed individuals that otherwise thrive in high-nitrogen environments. Although this reduces the overall area for the crop, the tradeoff is reduced weed pressure in the rest of the field. This method is improved by the use of controlled-traffic farming, which ensures that the chaff lines end up in the same area after every harvest (maintaining the competitive environment and containing the weed seed). The material in the windrows can also be destroyed with burning or by grazing. Burning in windrows has been shown to greatly reduce the survival rate of Italian ryegrass seeds, while additional studies have shown that other species of ryegrass have a roughly 10% survival rate in ruminant stomachs. However, grazing and burning, particularly in the context of California’s wildfire risks, have their own understandable challenges associated with them.

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Chaff lining/ Chaff tramlining – This technique is similar to narrow windrowing, but in this case only the chaff is funneled into a narrow area and straw is spread throughout the field as normal. Chaff can also be moved to the sides of the combine (using a lateral conveyor belt), being deposited under the path of the wheels (this process is referred to as “tramlining”). Controlled traffic farming ensures that the seed is concentrated in the same place every year. In the case of tramlining, tractor wheels run over the weedy areas with every field operation, ensuring that any weed seeds that germinate end up growing in compacted, competitive, and highly-trafficked soil.

Direct Baling – Collecting straw and chaff immediately into bales, usually using a tow-along baler behind the combine. After harvest, bales can be moved off the field, thereby removing a large proportion of weed seed before it can disperse into the soil. However, removal of this much biomass can be problematic for growers with low organic matter soils that otherwise benefit from maintaining residues on the ground.

One version of an impact mill attachment added to the back of a combine behind the cleaning sieves (photo: Redekop Mfg)

Integrated impact mills – In these systems, chaff is still ejected back into the field, but only after initially going through a hammer mill that shatters and destroys the seed. High initial equipment investments can cool grower enthusiasm. However, one 2017 report from Australia indicated that 29% of growers considered impact mills to be part of their future operations, indicating that at least some growers consider the benefits to outweigh the costs. Combine compatibility can also be a problem as most of the mills are designed to accommodate the larger combines of the midwest, whereas most California combines are typically optimized for relatively smaller acreage.

Limitations and project work

The caveat to all of this is that in order for HWSC to be effective, fields need to be harvested before weed seeds shatter (falling from the plant to the ground). As of yet the shatter patterns of Italian ryegrass in California are not well known. Additionally, previous research has indicated that Italian ryegrass seed retention at small grains harvest can be highly variable across different locations. For example, researchers from the inland Pacific Northwest have reported Italian ryegrass seed retention rates at harvest of 27-50% whereas a 58% of seed retention has been found in Australia. Furthermore, grains in California are planted at different times of the year and our harvest season occurs at different times of the year relative to Australia (not to mention the Pacific Northwest).

Shatter patterns might well be the same as those in other areas of the world but calling for a full scale HWSC revolution isn’t very useful unless we know that a good amount of the weed seed has not yet shattered in the field when small grain crops are harvested.

In an effort to address this, UC researchers will begin collecting data on the shatter status of Italian ryegrass populations in several areas of the Sacramento Valley leading up to harvest. If we consistently see that a significant portion of the Italian ryegrass seed remains attached to the plant at harvest, then HWSC strategies may offer viable control options for California. Conversely, if ryegrass seed has largely shattered by the time our grain crops are harvested, then California growers may need to consider other options for control.

Of course, the best control strategies for herbicide-resistant weeds still include a mixture of different tools such as: the use of herbicides with different modes of action (including pre-emergent and post-emergent types where possible), the use of diversified crop rotations, and well-timed mechanical control. However, a better understanding of the potential for HWSC in California may give growers an edge in reducing the spread of herbicide resistance.

If growers in Sacramento, Solano, or Yolo County are interested in collaborating with UC Cooperative Extension Agronomists in trials focusing on control of Italian ryegrass in small grain crops or have related questions, please reach out to Konrad Mathesius ([email protected]).

[1] : ACCase inhibitors, ALS inhibitors (imazamox, mesosulfuron), PS1 inhibitors (paraquat), and EPSP synthase inhibitors (glyphosate)

[2] My thanks to Dr. Ian Burke and Prof. Drew Lyon of WSU for sharing their research and insight regarding HWSC of Italian Ryegrass and other troublesome weed species.