Narrow windrow burning could be the answer for grain growers looking for a simple, low cost way of destroying weed seeds after harvest.
Narrow Windrow Burning involves adding a chute to the harvester (top) to direct the chaff into a narrow windrow, which is later burnt (bottom) to destroy weed seeds.
Photos: Michael Walsh
The practice reduces the seedbank – which is one of the key components of integrated weed management in tackling the increasing problem of herbicide resistance. Dr Michael Walsh, from the Australian Herbicide Resistance Initiative (AHRI), says the key to success of narrow windrow burning is the intensity of the burn.
“To guarantee the destruction of the weed seeds you need temperatures greater than 400 degrees for 10 seconds for ryegrass, and 500 degrees for 10 seconds for wild radish,” Dr Walsh said.
To achieve this, the method involves concentrating the harvest residue into a windrow that’s roughly 500 millimetres wide using a chute attached to the harvester.
“Most growers make their own chutes, and say it only takes a couple of hours at most,” he said. “There are design drawings available on the AHRI website to help growers get started.”
When harvesting for narrow windrow burning, Dr Walsh says even though there is little difference to standard harvesting, there are two rules.
“Number one is don’t stop. Always reverse up, because there’s still material coming out of the harvester and you don’t want it piling up the chute and blocking up the header. Rule two is cut low. The lower you cut, the more weed seed you’ll collect, so cutting at ‘beer can’ height or lower is essential in reducing seed bank inputs.”
The more complicated half of the strategy is in the burn – balancing the need for an intense burn to destroy the seeds with the risk of losing control of the fire. For this reason, Dr Walsh recommends starting with a canola or legume crop, which have a lower risk of losing control compared to wheat or barley.
“The challenge in burning the windrow is getting the right conditions. If the temperature is too high, the fire will be difficult to control. If there is some rainfall, you have to wait two weeks for the chaff to dry out. But the most important factor is the wind, you need a light breeze, around 10 kilometres per hour or less to fan the fire, but it needs to be slow for the fire to burn all the way down through the chaff, and not run across the top of the windrow, leaving most of the seeds untouched.”
Dr Walsh says that it is not uncommon for growers to struggle with the burn in their first year, so he recommends growers try narrow windrow burning for at least two years, to get comfortable with the process, before deciding whether it has been a success.
However when conditions are right, benefits can be observed immediately, with lower weed numbers in the first year.
“If seed destruction is being used as the last resort, and the weeds are out of control at say 100 plants per m2 at harvest time, then you won’t see an effect in the first year. But in a moderate infestation of 5-10 plants/m2, then it’s very likely there would be noticeably less weeds in the next growing season.”
Narrow Windrow Burning in the Medium and High Rainfall Zones
A GRDC funded Agribusiness Trial Extension Project by Grassroots Agronomy is assessing options to allow narrow windrow burning in regions where high yields currently restrict the potential of the process.
While narrow windrow burning is currently used on some lupin and canola crops in the HRZ and MRZ, the practice has been limited on cereal crops due to the physical barriers of windrowing high stubble loads and the increased risk of fire escape.
Through on-farm demonstration trials, Grassroots Agronomy is investigating harvest heights and farmer-modified chute designs to reduce these barriers.
Results are expected mid-2015.
Greg Condon, 0428 477 348, email@example.com
Dr Michael Walsh, 08 6488 7872, firstname.lastname@example.org
Find chute design drawings at the AHRI website.