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agriculture weed seeds

Crop weeds: Hygiene – prevent weed seed introduction

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‘Risk-aware’ growers can implement strategies to reduce and avoid unnecessary introduction and spread of weeds. These strategies will reduce not only the likelihood of introducing new weed species but also the risk of importing herbicide resistant weeds.


Impeding weed importation and spread at critical points

Sowing of the seed

Weed seed is regularly spread around and between farms as a contaminant of seed retained for sowing. Seed for sowing is more often than not contaminated with weed seeds and frequently at very high levels. Check seed analysis before buying seed-lots.

Fencelines and non-cropped areas in cropping paddocks (for example, water courses)

Weed infestations often start in non-crop areas (for example, around buildings, along roadsides, along fencelines). Weeds in non-crop areas experience no crop competition and can produce large quantities of seed. Controlling these initial populations will prevent weeds from spreading to other parts of the property.

Machinery and vehicle usage

Strict vehicle hygiene (that is, regular cleaning) can reduce the risk of new infestations and weed spread.

Stock feed and livestock movement

New livestock, or those returning to the property from agistment, can carry weed seeds from other areas. Weeds can be easily imported from different regions or states because livestock can travel significant distances by road within a 24-hour period. Quarantine contaminated fodder in a sacrifice paddock or feedlot so weeds are contained in a small area.

Sow weed-free seed

Determining weed seed contamination levels

To estimate the number of weed seeds being introduced at sowing:

  1. Obtain a random one kilogram (kg) sample of the seed to be sown.
  2. Separate the foreign seed from the crop seed.
  3. Count each type of foreign seed, including weeds and volunteer crop seeds.
  4. Multiply the number of weed seeds of each species by the proposed crop sowing rate kilograms per hectare (kg/ha) This will give the number of weeds potentially sown per hectare of crop.
  5. To calculate the density of weeds per metre squared (m 2) , divide the weeds/ha by 10 000.

While the number may work out to be only small (perhaps 1-2 weeds/m 2 or less), it is important to remember many weed species are prolific seed producers and a single plant growing in ideal conditions can contribute a large number of seeds to the seedbank.

Managing weeds in non-crop areas

Practices to control weeds around the farm

Many weeds are well-adapted to non-arable areas of the farm and thrive in stockyards, laneways, firebreaks around sheds and buildings and in tree plantings.

These non-arable areas are the home to an assortment of annual and perennial weeds and can create problems in many ways:

  • They can be a source of weed seeds for the introduction to other areas of the farm.
  • They can present a fire hazard.
  • They can harbour vermin and insect pests and act as hosts for some plant diseases.
  • They detract from the appearance of the farm.

Weed control around farm buildings

Controlling weeds around farm buildings is required by law to reduce a potential fire hazard. This can be achieved by regular mowing or herbicide application by knapsack sprayer or pressurised hand lead. Knockdown or residual herbicides (or a combination of the two) can be used depending on whether the weeds have emerged, the time of year and the weeds present.

Chemical firebreaks

These can replace or supplement mechanical methods. Advantages include the reduced risks of soil erosion, weed reinfestation and damage to fences by fire as well as providing greater flexibility and speed as show below (Figure 1).

Knockdown or residual herbicides (or a combination of both) can be used.

Weed control on roaded catchments

Weed growth can significantly reduce run-off by breaking up the surface seal and increasing infiltration and by holding a small part of each shower of rain. Weeds also reduce raindrop impact on the soil surface. Raindrop impact causes a crust to form on the soil surface which limits the rate of water infiltration into the soil. Most run-off occurs during winter, so early control of winter-growing annual weeds is important. Regular maintenance and regrading the catchment will control weeds. In between regrading however, apply appropriate (knockdown or residual) herbicides. Residual herbicides can be washed off compacted clay surfaces contaminating run-off water — the choice of herbicide is largely decided by the end-use of the water.

Water weed control in dams

Rising temperatures and falling water levels during summer often lead to the appearance of aquatic weeds. The most commonly-found species are floating pond weed, blunt pond weed, duck weed, red azolla and common milfoil. Before deciding whether to control weeds in dams, remember they may be beneficial and provide food and shelter for fish, other aquatic animals and birds. They might also use nutrients in the dam preventing nutrient enrichment that could lead to algal blooms. Aquatic weeds can be controlled by mechanical and/or chemical means. Mechanical methods such as cutting, mowing, dredging, drying and chaining may be effective but are short-lived. Chemical control must be undertaken with considerable care, considering the identity of the weed, the effect of herbicides on desirable plants, fish and other aquatic life and the eventual use of the water (irrigation, domestic purposes or stock). Take your water weed to your local agronomist or DPIRD office for correct identification and control advice.

Weed control for successful revegetation

Successful establishment of trees from seedlings depends on successful weed control. Weeds around trees compete with young seedlings, harbour pests and vermin (rabbits), cause spindly growth and present a fire hazard when dry.

Targeting Weed Seeds In-Crop: A New Weed Control Paradigm for Global Agriculture

The widespread evolution of multiple herbicide resistance in the most serious annual weeds infesting Australian cropping fields has forced the development of alternative, non-chemical weed control strategies, especially new techniques at grain harvest. Harvest weed seed control (HWSC) systems target weed seed during commercial grain harvest operations and act to minimize fresh seed inputs to the seedbank. These systems exploit two key biological weaknesses of targeted annual weed species: seed retention at maturity and a short-lived seedbank. HWSC systems, including chaff carts, narrow windrow burning, bale direct, and the Harrington Seed Destructor, target the weed seed bearing chaff material during commercial grain harvest. The destruction of these weed seeds at or after grain harvest facilitates weed seedbank decline, and when combined with conventional herbicide use, can drive weed populations to very low levels. Very low weed populations are key to sustainability of weed control practices. Here we introduce HWSC as a new paradigm for global agriculture and discuss how these techniques have aided Australian grain cropping and their potential utility in global agriculture.

La ampliamente diseminada evolución de resistencia a múltiples herbicidas en las malezas anuales más serias infestando los sistemas de cultivos australianos ha forzado el desarrollo de estrategias de control de malezas alternativas, especialmente nuevas técnicas al momento de la cosecha de granos. Los sistemas de control de semillas de malezas en cosecha (HWSC) se enfocan en las semillas de malezas durante las operaciones de cosecha comercial de granos y actúan para minimizar el suministro de semillas frescas al banco de semillas. Estos sistemas explotan dos debilidades biológicas clave de las especies de malezas anuales de interés: retención de semilla al momento de la madurez y un banco de semillas de corta vida. Los sistemas HWSC, incluyendo las carretas de descarga de grano, la quema de líneas angostas de residuos después de la cosecha, el embalado directo, y el Destructor de Semilla Harrington, se enfocan en los residuos de cosecha que contienen semillas de maleza durante la cosecha comercial de grano. La destrucción de estas semillas de malezas durante o después de la cosecha del grano facilitan la reducción del banco de semillas de malezas, y cuando se combinan con el uso convencional de herbicidas, pueden llevar las poblaciones de malezas a niveles muy bajos. Tener poblaciones muy bajas de malezas es clave para la sostenibilidad de las prácticas de control de malezas. Aquí, nosotros introducimos HWSC como un nuevo paradigma para la agricultura global y discutimos como estas técnicas han ayudado a la producción australiana de granos y su utilidad potencial en la agricultura global.

Chaff Lining, Seed Mills Aid in Fight Against Herbicide-Resistant Weeds

FAYETTEVILLE, Ark. — The Arkansas Agricultural Experiment Station is taking part in research to fight the battle against herbicide-resistant weeds.

Catching weed seeds before they start a new generation of herbicide-resistant plants is the tactic behind a relatively new method in the United States that weed scientists in Arkansas, Iowa, Kansas and Missouri have partnered to investigate.

“When you take a look at weed management in general, it’s all really centered around soil seed bank management,” said Jason Norsworthy, Distinguished Professor of weed science with the experiment station, the research arm of the University of Arkansas System Division of Agriculture. “If we can drive those soil seedbanks down it’s going to benefit us in terms of the future populations or densities we have in those fields as well as lessening the risk of herbicide resistance evolution and spread.”

The seeds can be caught in the chaff and crushed by a seed mill or laid down in a “chaff line” to consolidate and create a mulching effect, Norsworthy explained.

Since most of the weed seeds are in the chaff, chaff lining calls for a baffle and chute on the harvester that consolidates the chaff in a narrow row 20 to 24 inches wide on the ground behind the harvester. The chutes can be fabricated by farmers of bought commercially.

Seed crushers do as the name implies, catching the seeds and crushing them. But some seed crushers may work better for different kinds of crops and weed species. That is also part of the experiment station study.

Harvest weed seed control was pioneered in Australia by Michael Walsh, associate professor and director of weed research at the University of Sydney. The method has been widely adopted there to capture weed seeds as they come through a combine during harvest, Norsworthy said. Experiments in Arkansas on chaff lining and seed crushing have been conducted at the Division of Agriculture’s Northeast Research and Extension Center in Keiser.

Prashant Jha, professor of weed science at Iowa State University, showed that more than 95 percent of the weed seeds can be concentrated in a chaff line at the time of soybean harvest during a 2021 study in Iowa funded by the Iowa Soybean Association. Jha, who was a graduate student under Norsworthy at Clemson University, said his studies have shown that chaff lining reduces the spread of herbicide-resistant weed seeds.

The weed seeds that were concentrated into rows emerged four to six weeks later than usual, which allowed the herbicides to be more effective because the weeds were smaller, Jha said. By the time the weeds in the chaff line grew to 2-3 inches, the smaller number of weeds outside the rows had grown to 6-8 inches.

Also taking part in the study is Vipan Kumar, assistant professor of weed science at Kansas State University. Kumar, who was a graduate student under Jha at Montana State University, said his wheat and sorghum chaff lining research in Kansas has shown to significantly reduce kochia and downy brome emergence.

“The mulching effect of chaff lining on weed seed banks further depends on the type of crop being used and the target weed species,” Kumar noted.

Cover crops, various herbicides and seed harvest tactics are part of the multi-level, multi-state experiment on harvest weed seed control, Norsworthy said.

In Arkansas studies on weed seed control, Norsworthy has also experimented with narrow windrow burning. This method collects the seeds in the chaff on the ground, but it is then burned. Norsworthy said his studies showed a 100 percent kill rate of weed seeds using windrow burning, and he has seen some row crop farmers in Arkansas effectively use this method. But he is researching other methods since burning creates issues that include smoke and carbon dioxide release.

After testing narrow windrow burning, Norsworthy tested the Integrated Harrington Seed Destructor. He said it was 99 percent effective in terminating pigweed seeds when the seed mill did not “clog up” in fields with green pigweed plants. Norsworthy is now testing a Redekop Seed Destructor, which he says does not have as much of a tendency to clog on green Palmer amaranth pigweed plants. A desiccate can be used to help dry the Palmer amaranth since the female plants remain green until a winter freeze, he said.

“The end goal is to protect yield potential in fields and reduce the risk of herbicide resistance,” Norsworthy said.