For many growers burning crop residue to kill weed seeds collected at harvest goes against the grain. Along with the loss of nutrients and ground cover there is significant risk, stress and discomfort associated with burning, even in autumn.
Andrew and Marjorie Boultbee run a predominantly cropping operation near York, Western Australia, with land they own and lease spread across their district. As annual ryegrass became increasingly difficult to control with herbicides the Boultbees adopted narrow windrow burning as a harvest weed seed control method. They soon saw how effectively this technique drove down weed seed numbers on their farm and decided to invest in chaff carts.
Andrew has seen many growers in his district adopt the use of chaff carts only to stop using them because of the costs and dangers associated with burning the heaps. “We soon found that burning the chaff heaps consumed all our attention and the smoke was unpopular with our neighbours,” he says. “Having properties spread out also caused logistic difficulties and with the heaps smouldering over several days we had a few close calls and sleepless nights.”
To keep the weed control benefits without all the problems Andrew and Marjorie decided to stop burning and to start using the chaff heaps as a feed resource for their sheep over summer.
Andrew has found canola and wheat chaff heaps to be very effective for weed control, even if the heaps are not burned. He allows sheep to graze on the heaps first which makes use of the feed resource while also knocking down the heaps to allow him to seed through them the following autumn. This works very well in canola however Andrew has found that running the scarifier lightly along the cereal heaps prior to seeding helps to spread the heaps more and minimises the chance of blockages at planting.
“The sheep eat down and flatten the canola heaps to the point where we can pass through with the seeding equipment and the crop grows through the remaining residue,” says Andrew.
The cereal chaff heaps are also well grazed, however Andrew always runs a scarifier along the row of heaps, knocking them down to about 30 cm in height. Doing this at right angles to the sowing direction means the seeder is able to seed through the chaff zone without blocking up.
“It is important to seed across the line of chaff heaps, not along them,” he says. “We make a habit of creating the heaps in a line across the paddocks at harvest.”
It takes a couple of years for the heaps to disappear altogether and return to full production but the heaps cover only about 1% of the total cropping area. Once the heaps have disappeared there is a noticeable increase in the crop production in those patches, more than compensating for the small loss of production in the first year.
“The remaining residue in the 30 cm deep cereal chaff layer slowly composts during the winter rains,” Andrew says. “Two years after grazing a chaff dump we can notice the difference in that part of the paddock, with stronger crop growth and few weed issues.”
The Boultbees also choose crops and cultivars that they are confident will perform well within their weed management program. Andrew says they look for cultivars that are high yielding, very competitive in the early growth stages and have hard grain that is not damaged by the harvest settings that remove the most weed seeds.
“It pays to set everything up well before attempting to use harvest weed seed control tactics like chaff carts,” he says. “The paddocks must be free of rocks so that there is no impediment to cutting low to the ground and the header must have sufficient power and the correct settings so that weed seeds are taken in the front and end up on the sieve.”
Cutting low is particularly important for soft-seeded weeds like annual ryegrass that do not stay dormant in the soil for many years. Andrew says that leaving low growing or lodged weeds in the paddock in the first year effectively selects for the trait that exposes the weakness of this weed control method.
“It is important to harvest as low as possible right from the start and to have other strategies to deal with weeds that ‘survive’ collection,” says Andrew. “The next step is to make sure the header is set up for optimal performance and to collect as much seed as possible in the chaff.”
It is important to run at high rotor speed and to open the back of the concave up so that seed and straw is efficiently separated. The harvester needs to have the capacity to handle the increased amount of straw and must be set up so weed seeds end up on the sieve and not out the rotor. Andrew also avoids harvesting on cold damp nights where separating the harvested material efficiently is more difficult.
Annual ryegrass and wild radish no longer dictate the Boultbee’s cropping rotation as they once did. The chaff carts keep weed pressure low and allow the Boultbees to take advantage of seeding or marketing opportunities for their crops, with spin-off benefits for their sheep enterprise.
Australian Herbicide Resistance Initiative leader of communications, Peter Newman, says chaff carts capture about 75 to 85% of annual ryegrass seeds and 85 to 95% of wild radish seeds that are present in a crop, without slowing harvest operations.
In a cost comparison of harvest weed seed control methods, WeedSmart estimated that running a chaff cart, including the cost of nutrient removal, to be $14/ha (assuming 2000 ha wheat at 2 t/ha), or even less is a second-hand chaff cart is used.
In Andrew’s situation there is less nutrient removal costs and less costs associated with burning. He identifies rock-picking the paddocks as one of the major costs in his operation but estimates the cost of running the chaff carts is only $8/ha.
Better results in barley
Barley crops play an important part in the Boultbee’s weed management program. Andrew chooses the most competitive barley varieties available to suppress weed germination and growth in-crop. When sown on 260 mm row spacing the tall dense stand lessens lodging in the annual ryegrass, keeping it erect and protecting the seed heads from shedding in the wind.
By swathing the barley they introduce more diversity into the rotation so that every few years each paddock will be cut early rather than later. Barley windrows maintain their shape well and are easy to pick up with the header. There is less shedding of barley grain and weed seed due to the early swathe timing.
“Swathing barley means there is a greater proportion of the cropping area that is cut early,” says Andrew. “With harvest potentially extending through to the end of December the weeds have quite a long time available to mature and shed their seed and so evading capture through any harvest weed seed control measure.”
Then graze and burn in-crop
There are some challenges that arise when barley chaff heaps are not burned. Barley chaff heaps are prone to thatching, which helps protect seeds on the soil surface from getting wet and composting during winter.
“Even after grazing, the soil under the chaff heaps stays dry enough to preserve both barley seed and weed seeds,” says Andrew. “Volunteer barley growing in our wheat crops became a problem that we had to solve and so we have tried in-crop burning of barley chaff heaps in winter.”
From their previous experiences with narrow windrow burning and burning chaff heaps, the Boultbees knew that burning was an effective way to drive down weed numbers quickly but they did not want to go back to the traditional autumn burning method.
“We are seeing good results from in-crop burning of barley chaff heaps when the winter crop has reached the mid-tillering growth stage,” Andrew says. “Unlike burning chaff heaps before sowing, these in-crop burns are very safe, with virtually no risk of escape. Because of the minimal risk involved, one person can easily set fire to heaps across 1000 ha in one day.”
The small fires are well contained and burn out within a day or two rather than continuing to smoulder for several days. Burning in winter makes it easier to predict the wind and Andrew takes the wind direction and location of their neighbours into consideration when burning. “There is no stress or urgency associated with burning in winter and there is much less smoke,” he says.
The Boultbees are using the same idea in high weed density wheat paddocks and Andrew thinks it could also work well in paddocks with a high burden of wild radish in canola.
Extracting the feed value from chaff heaps
Grazing the chaff heaps over summer fills a feed gap for the Boultbee’s 3000 sheep, and has lifted the lambing percentage of the flock to over 100%—quite an achievement for Merino ewes.
The ewes are put in to graze the canola heaps first and on mating they are moved onto the barley heaps. After mating the ewes are moved onto the wheat heaps where they will stay until planting. Once the lambs are weaned they remain on the cereal paddocks with access to barley in a lick feeder to finish them.
The grazing value of the chaff heaps enables the Boultbees to run more sheep over summer and the sheep do better than those that don’t have access to this resource. Andrew says that there is an opportunity to use the chaff in a lot-feeding situation but he has not done this as yet.
Sheep selectively graze the most digestible portion of the chaff heaps including fine leaf material, whole and broken grain and weed seeds, chasing the seeds to the bottom of the heaps. Annual ryegrass, wild radish and wild oats seeds along with some broken cereal grains constitute about 9% of the material in the chaff heaps. The sheep seek these seeds and fragments out and spread the remaining plant material as they feed and trample the heaps. The nutrients from the heaps are then redistributed in the paddock via the manure, particularly when the chaff heaps are located some distance from watering points.
To gain maximum nutritional benefit, the Boultbees put the sheep in to graze the chaff heaps soon after harvest, and move them to new paddocks when they have extracted all the feed value from the chaff heaps and stubble. Providing a protein-rich feed such as barley seed in a lick feeder is a great way to finish the weaners very cost-effectively.
The chaff heaps provide an additional feed resource and allow the Boultbees to increase the number of animals they can run over summer, especially in difficult years when there is more small seed left in the paddock at harvest.
There are risks associated with feeding chaff to livestock that farmers should be aware of. High levels of toxins such as the bacterium associated with annual ryegrass toxicity, phomopsin in lupins that cause lupinosis and ergot, which can cause illness and even fatalities in sheep and cattle. Monitoring the health of the animals while they are grazing, and testing for toxicity in the chaff will reduce the risk of disease.
Research has shown that less than 3% of ryegrass seeds that the sheep consume from chaff heaps will survive digestion. In contrast, almost 1/3 of ryegrass seeds ingested by cattle remain viable in the faeces.
Want more? You can also watch the recording of the webinar where Andrew and Peter discuss the value and practicalities of grazing chaff heaps and stubble.