Brown manuring reduces weed resistance risk
Incorporating a brown manure crop into the rotation and employing the double-knock herbicide technique have bolstered the armoury of Lockhart , NSW farmers Jason, Adam, David and Heidi Gooden in their battle against herbicide resistant weeds.
The Goodens were among the first farmers in the district to adopt the practice of brown manuring, which is planting a cover crop with the intention of spraying it out prior to weed seed set.
The Goodens first investigated the practice in 1996, after Group A herbicide resistance was discovered in ryegrass populations on their property. The first manure crop was added to their rotation in 1997, having made a decision to proactively seek out and use new weed control tools which also provide other soil and financial benefits.
Adoption of this practice has assisted the Goodens in rotating chemical groups, maintaining ground cover, preventing weed seed set and adding valuable nitrogen from pulse N fixation, as well as providing agronomic benefits of improved soil health and water holding capacity to the farm’s cropping program.
“The detection of resistance really changed the way we thought about things,” David said.
“We decided to get on the front foot before herbicide resistance spread – we didn’t want it to get to a point where it was controlling our farming system.
“We wanted to use as many tools as possible such as making silage, using the full suite of herbicides as well as crop competition and we saw this method as another way of reducing viable seed set to reduce seed numbers.
“As we phased out of our livestock enterprise and moved into a 100% cropping system, we were conscious that taking the pasture phase (lucerne and clover based) out of our rotation would put even more pressure on our in-crop herbicides.
“We are also worried about the risk herbicide resistance poses to the use of a minimum tillage farming system. We don’t want to return to tillage, so we have to use all the tools that are available to make sure that risk doesn’t become a reality.”
The Goodens farm on five properties across 3600 hectares at Lockhart in the NSW Riverina, along with David’s parents and brothers. The Gooden family primarily produces dryland cereal crops on the gently undulating to flat country, featuring clay loam soils and receiving a GSR (Growing Season Rainfall) of between 100-300mm annually.
Wheat, barley and canola are the major “profit crops” – these are sown in in April/ May and harvested in December with target yields for cereal at 4t/ha and canola at 1.8t/ha – while manure crops are grown as break crops in a rotation of wheat/wheat/barley/manure/canola.
The family has embraced modern farming techniques, using controlled traffic on three-metre wheel centres, minimum tillage, and inter-row seeding to maintain stubble residue.
Brown manuring has the added advantages of fitting in with a minimum tillage system, has lower costs in terms of tillage, fertiliser and in-crop herbicides, and provides ground cover over the summer months which helps reduce weed emergence and maintains soil moisture.
Their decision to grow a brown manure crop is determined first by identifying paddocks where weed pressure is an issue. Developing a plan to treat the paddock which fits around the business’s “priority crops” of wheat and canola is then implemented.
Using this method to stay on top of the weed populations as they arise, the Gooden’s are able to sow at least 85-90% of the farm each year to their “cash crops” without impacting on overall farm profitability.
The manure crop chosen, as well as sowing rate and time, varies according to seasonal and soil conditions – field peas, faba beans, vetch and even cereals have been used.
“Our preference is to use a legume because of the nitrogen it provides, but it’s not always essential to use a legume crop if the purpose of the manure crop is primarily to control resistant weeds,” David said.
The Goodens believe the broader agronomic benefits of the weed control method have also contributed to higher yields and protein levels in their cereals, as well as an overall decrease in problem weeds.
“We’ve also noticed advantages when dry years follow a manure crop due to access to the carried over soil moisture,” Heidi said. “Overall I estimate that we’re achieving an extra 200-400kg/ha in our canola yields and 1% higher protein in wheat due to this practice.”
David, who completed in 2010 a GRDC-sponsored Nuffield Scholarship into spray application methods, has also reduced chemical use by 10-20% by changing droplet sizes, better matching his water and adjuvant rates, and adapting his rig to suit the weather conditions.