Integrated weed management delivers against herbicide resistance
By using a combination of windrow burning, crop and herbicide rotation, increased crop competition and farm hygiene, Dubbo farmer Maurie Street has halted the previously rapid spread of resistant weeds and significantly reduced their impact on his bottom line.
Mr Street, a former commercial agronomist in Central West NSW and now a researcher with the Grains Research and Development Corporation (GRDC) funded grower solution group Grain Orana Alliance (GOA), has spent much of his time talking to farmers about herbicide resistance, prompting a decision several years ago to practice what he preaches.
“I have spent my entire professional career talking to growers about techniques to manage herbicide resistance but as a small scale cropper I am now trying to put some of that into practice,” Mr Street said.
“I use as many tools as I can to proactively combat herbicide resistant ryegrass on my property.
“To win a war you must win many smaller battles and that is why an integrated approach is our best chance. No one technique is perfect so the key to success is to use as many of them as you can.”
Mr Street only crops 160ha of canola, wheat, barley and lupins on a mixture of sandy loams through to light clay, with an average rainfall of 550mm, but he believes the principles can be applied to farming systems of all sizes.
After trialling a variety of techniques he has found that positive results from integrated weed management appeared almost as quickly as the emergence of the resistance problem.
“This farm has only been cropped for the last six years with at least 14 years of pasture before my wife Kate and I bought it in 2006, which demonstrates just how quickly resistance can develop,” Mr Street said.
“But after a few years of really concentrating on the problem I have gone from having patches of crop suffering about 20-30 per cent yield penalty due to uncontrolled weeds, to almost no impact at all.”
Mr Street is now a WeedSmart Champion, an industry-led initiative managed by the Australian Herbicide Resistance Initiative (AHRI) aimed at enhancing on-farm practices and promoting the long term sustainability of herbicide use in Australian agriculture.
Mr Street said his main problem weed was annual ryegrass.
“But I am also concerned about the development of resistant black oats (wild oats),” he said. “Resistance in summer fallow weeds is the biggest threat, with very few practical alternatives for control.
“I am getting ryegrass tested for resistance this year to confirm my suspicions that the weeds are resistant to at least the FOPs and Group B herbicides, and I strongly suspect they are developing resistance to DIMs, so I may have very few herbicide options left available to me.
“Even if I only have weak resistance of about 20 per cent, and I continue to rely only on herbicides for control it could go to 100 per cent within a few years. I really needed to do something different.”
After hearing of windrow burning at a GRDC integrated weeds management course several years ago, Mr Street trialled the method in one paddock and said the initial success led him to this year expand it across his entire property.
“It has certainly stopped the populations increasing and reduced the population spreading,” he said. “Windrow burning may have slowed my harvest down by about 10pc but only in some crops as much of the time we often harvest quite low anyway.
“It may also have some impact on fallow efficiency and nutrient dynamics but these are costs I am willing to bear.”
Mr Street said the improvements provided by windrow burning had been complemented by other techniques including increasing crop competition.
“Research has shown that crop competition can reduce seed set of weeds by up to 80pc, so this year I increased seeding rates from the traditional 45kg/ha to 75kg/ha.”
It was easy to see how successful it was just by walking around his property and finding areas of thin crop or missed areas.
“In those spots I will find ryegrass with 20 or more tillers, but 20cm either way in thicker crop, the ryegrass plants may have only three tillers, so much less seed is produced and returned to the seedbank.”
Mr Street also believes crop rotations are an important tool.
“Changing crop types allows for easier herbicide rotations but also allows for staggered sowing dates to better accommodate pre-sowing knock downs,” he said.
“To get optimum yields for canola, lupins and long-season cereals the crops are dry sown or sown early, with either no knockdowns applied or knockdowns applied before weeds are fully emerged.
“Shorter season wheat and barley crops allow for effective knockdown of weeds and take the pressure off in-crop herbicides.”
He is also interested in other alternatives such as hay or silage crops where weeds can be cut off before they seed, or summer crops where weeds could be targeted with different herbicides.
“Farm hygiene is important as well – I have enough problems with my own weeds let alone spreading them with seed or machinery,” Mr Street said.
“Grading my own seed to a high standard is the first step because if there are weed seeds in my planting seed there is a very good chance they are resistant, so I am also careful where and who I buy from.”
Mr Street also sprays his firebreaks and fence lines using “double knocks” and residual herbicides, and has minimised the width of his fence line breaks to minimise the potential for fence line resistance.
“I also try to follow the mantra, ‘if I can’t sow it don’t spray it’ particularly for areas such as contour banks and other non-cropped areas.”
Other options he has considered are slashing or cultivating the breaks or planting competitive pasture or plants to try and out-compete the weeds.
“All these techniques do come at a cost, but we have always paid to control weeds so now instead of going to town to buy drums of chemical, we pay for it in a different way,” he said.