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Chaff-fed sheep help break weed cycle

Weed seed collection at harvest, grazing chaff dumps instead of burning, and swathing barley crops are key weapons growers Andrew and Marjorie Boultbee have in their arsenal for their war on weeds on their property at York, Western Australia.

The couple’s priority is to implement a farming system where crop rotation choices are uninhibited by planting date, weed density or herbicide resistance pressures.

“To optimise profits, we want to be able to change our rotation quickly – if necessary – in reaction to shifts in commodity prices or seasonal conditions,” Andrew says.

“This is made possible by having a low weed seedbank.”

A man standing in a shed next to equipment

York, Western Australia, grain grower Andrew Boultbee is using chaff cart weed-seed collection at harvest and swathing to reduce the weed seedbank on his property.

Historically the Boultbees used herbicide, rotation and some narrow windrow burning after harvest to control weeds.

However, Andrew says he found burning windrows was costly (he estimates $4 per hectare), time consuming, hard on machinery, unhealthy – keeping him awake at night worrying about fires up to 70 kilometres away – and not conducive to stubble retention.

In 2002, he decided to use chaff carts as an alternative way to capture weed seeds at harvest, lower the property’s overall weed seedbank and facilitate ongoing continuous cropping.

Instead of burning the chaff heaps, the Boultbees started grazing this resource during summer. The sheep flatten the chaff heaps to a very small pile and help to redistribute some of the nutrients in these heaps back out to the paddock.

It is a win-win for achieving good weed control and filling a sheep feedgap that, in turn, has contributed to boosting lambing percentages.

Andrew says the sheep eat down canola heaps and the minimal amount of residue remaining is sown through with no extra treatment.

“The cereal chaff heaps are well grazed, but there is a small pile left,” he says. “So we run a scarifier through this residue, straight down the row of heaps to knock the tops off, levelling it to a height of about 30 centimetres.

“This effectively joins the heaps together into a continuous strip at 90 degrees to the seeding direction, which the seeder can easily go over.”

Research has shown that only three to six per cent of ryegrass seeds that sheep pick up from chaff heaps survive digestion.

“What we are finding is that the weed seeds that get through the chaff carts and sheep digestion have little protection against the composting effect of the 30cm layer of chaff on the ground during the winter rains and they rot,” Andrew says.

“Even though we don’t burn the chaff dumps, we don’t see many weeds in them at all. Two years after leaving a dump, it is the best part of the paddock – big crop growth and very few weeds.”

Andrew says he now uses two chaff carts and the system is a cost-effective tool that is destroying enough weed seeds to lower the seedbank.

However, he says there are costs involved in clearing paddocks of rocks to drop the front of the harvester to cut stubble as low as eight to 10cm and in reducing harvest speeds to about 8km/hour.

Andrew estimates the cost to run the chaff carts is about $5 to $6/ha – similar to the cost of narrow windrow burning.

“Using such a low cut height at harvest and with an increased volume of straw, we have had to upgrade our header size and run two headers, at considerable expense,” he says.

“That has been the biggest downside to moving to weed seed collection at harvest.

“The benefits are that our time of sowing and crop choice is less inhibited by weed pressures, we don’t have to burn chaff heaps and our stock are doing well on this feed resource.”

Using chaff carts, the Boultbees also retain full stubble cover and are noticing increased stored soil moisture at seeding, less waterlogging after big rain events, less wind erosion, increased soil microbial activity and fewer weeds.

Annual ryegrass is their highest-priority weed and another tool to tackle this is to swathe barley crops.

“The idea is to get as many seeds into the front of the harvester as possible and stop any lodging or shedding of ryegrass and brome grass seeds,” Andrew says.

“The same goes for wild radish, which is usually dry and shedding, or green, and therefore it’s hard to get weed seeds down on the sieve.”

Swathed barley crops are left for up to four weeks, allowing standing crops, which have priority, to be harvested.

Andrew says the system depends on swaths being laid “just right”, requiring crops to be cut low and consistently to enable effective weed seed and barley grain collection.

“If you are going to get into the swathing barley game you have to get it right,” he says.

“We use Buloke barley because it is tall and competitive, so it makes a good windrow.

“It also has a hard seed, so we can thresh it hard to make sure that we get most of the weed seeds into the cart.

“We don’t like using Hindmarsh barley for this purpose because it is too short to make good swaths and the seed is soft, so will crack if we thresh it too hard.”

Andrew says in high weed burden areas, two or three consecutive barley crops are grown and swathed in the rotation using this system.

He says canola follows the second barley crop if conditions are right, or is dropped out if prices or the season appear unfavourable.

Andrew now rarely uses post-emergent herbicides for ryegrass control in cereals. Herbicides are rotated with crops, there are long breaks between using the same herbicide in a paddock twice and minimal glyphosate is used prior to seeding.

To hear Andrew Boultbee discuss his chaff cart and integrated weed management system, watch the GRDC Ground Cover video below.

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Scientific studies have demonstrated that resistance can rapidly evolve in weeds subjected to low doses of herbicide. Some weeds can develop resistance within a few generations. Full rates when mixing herbicides too! When mixing herbicides it is important that each product is still applied at the full label rate to ensure high mortality. Applying different chemicals in one mix can provide an additive advantage. It is important to understand the mode of action of each herbicide on the plant when preparing a herbicide mix. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control. ALWAYS READ THE LABEL. Surrounding weed seeds with a combination of pre-emergent herbicides with different modes of action can give a high level of control and help extend the useful life of all the chemicals used. The high level of control must be supported with additional control measures for all survivors. All products with different modes of action must be applied at full label rates for this to be an effective strategy.   Mixing two chemicals with the same mode of action can achieve some additional efficacy, however, the mix should deliver the combined full rate to ensure a lethal dose. The amount of stubble present and crop safety are all important considerations when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice. Many herbicides on the market are a combination of two or more modes of action within the one product. These products must be applied at the full label rate to be effective. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.  

Spray well – correct nozzles, adjuvants and water rates

Spray application is a technical field and growers need to make sure their equipment and application techniques are spot-on. The GRDC Spray Application GrowNote provides detailed information and about 80 videos to demonstrate key skills. Prevent spray-drift The focus of spraying herbicide needs to be on doing the job right so the weeds receive the correct dose and die, and this includes reducing the air borne fraction to a bare minimum. Bill Gordon’s 10 Tips for Reducing Spray Drift Choose all products in the tank mix carefully. Understand the product mode of action and coverage requirements. Select (and check) the coarsest spray quality that will provide effective control. Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. DO NOT SPRAY. Use weather forecasts to inform your spray decisions. Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4–5 km/hr for more than 20–30 minutes, and clearly blowing away from any adjacent sensitive crops or areas. Set the boom height to achieve a double overlap of the spray patterns. Avoid higher spraying speeds. Leave buffers unsprayed if necessary and come back. Continue to monitor conditions, particularly wind speed, at the site during the spray operation High water rates don’t have to slow you down Some growers are concerned that increasing the water rate when applying herbicide will slow down their spray operation and cost them money. However, the biggest financial loss during spraying usually comes from a failed spray job. To keep your spray operation as time efficient as possible when using more effective and reliable application volumes, you can: Use nurse tanks around the farm to reduce the time spent travelling back to a central re-fill point. Use a larger pump, e.g. 2.5 inch, to make re-filling quicker. Pre-mix the batch while the sprayer is operating. Many mixes can be held in the mixing tank for up to 6 hours. However, wettable granules and suspension concentrates will need agitation to keep them in solution. For pre-emergent herbicides in high stubble situations, carrier volume has a large effect on the level of control achieved. Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high Water quality and mixing order Water quality is often overlooked as a possible contributor to herbicide failure and can lead to confusion over the herbicide resistance status of weeds on a property. Water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. Getting the mixing order right is essential for effective spray results. Don’t start mixing until the water quality is right Podcast – Mixing herbicides Adjuvants Sometimes adding an adjuvant is beneficial and sometimes it is detrimental; and there is an art to knowing how to best deploy these additives. When weeds are susceptible to the applied herbicides, the effectiveness of adjuvants generally goes un-noticed. Correctly applied adjuvants can reduce the impact of low level herbicide resistance by helping to maximise the amount of herbicide taken up by the plant.

Clean borders – avoid evolving resistance on the fence line

About one-quarter of glyphosate-resistant populations within broadacre cropping situations across Australia come from fencelines and other non-cropping areas of the farm. Along paddock borders, where there is no crop competition, weeds can flourish and, if not controlled, set lots of seed. The traditional approach has been to treat these weeds with glyphosate to keep borders clean but after 20-odd years this option is now failing and paddock borders are becoming a significant source of glyphosate-resistant weed seed. Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems. In some situations, cultivation can be used to kill the weeds and provide a firebreak, but on light soils this may pose an erosion risk and mowing or slashing may be safer options. Another possible tactic is to continue using herbicides but to ensure that a clean-up operation is carried out before any survivors can set seed. Some growers are choosing to increase the heat on weeds along the borders by planting the crop right to the fence and then baling the outside lap and spraying with a knockdown herbicide to kill any weeds and provide a firebreak. Another good option in some situations is to maintain a healthy border of vegetation using non-invasive grasses. In Queensland, buffel grass is a good example of a grass that can outcompete other weeds while not invading crop lands. If only herbicides are used on fencelines, resistance is inevitable. Surviving weeds on fencelines have no competition and access to plenty of soil moisture, so they set a lot of seed and resistance can easily flow into neighbouring paddocks. Other resources It’s time for a glyphosate intervention Farm hygiene cottons on – Cleave Rogan, St George What’s new in management of herbicide resistant weeds on fencelines? Keeping the farm clean – Graham Clapham, Norwin Don’t jeopardise glyphosate for clean fencelines Keeping fencelines clean Resistance risk to knock-down herbicides on irrigated cotton farms

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