No-till's next R&D hurdle: weeds control
GroundCover™ Issue: 111 | Author: John Cameron
Northern region Grains Research Updates coordinator John Cameron reflects on the vast changes that researchers and growers have implemented over the past two decades, and the one problem that just will not go away – weeds
In 1981 many things were different: soil erosion was a primary driver for farming system change, Roundup® was ‘beer coloured’ and cost $22 a litre. There were only four retail agronomists covering the entire Darling Downs and the accepted paradigm was “you must aerate the soil – no-till can’t work!”
As a Grains Update coordinator, I am constantly seeking out the latest research. Today much of this relates to crop nutrition, weeds, pest and disease management, and of course general agronomic issues such as marrying varieties with sowing date, nutrition, seeding rate, row spacing and crop sequence.
However, it is also very instructive if you ever get the chance to read an old agriculture journal. When recently reading a journal from 1904 it was sobering to see how little has changed in terms of the key production issues, but fascinating to see the extraordinary progress of knowledge.
The main change to farming systems throughout my career is undoubtedly the advent of no-till. Replacing tillage with herbicides, in particular glyphosate, has enabled major reductions in soil erosion and capital operating costs, and dramatic increases in crop yield and productivity. One manager can now farm two or more times the hectares that could be farmed with tillage.
In the early days of no-till, yield differences were often hard because of the dominance of cereal monoculture resulting in higher levels of foliar disease in stubble-retained systems. Now, successful growers combine crop sequencing, no-till and increased use of fertiliser to capture the benefits of the higher fallow efficiency of no-till systems.
Large tracts of northern New South Wales and Queensland are now farmed using a combination of no-till and controlled traffic to optimise infiltration and capture of fallow and in-crop rainfall. Much of this is facilitated using GPS self-steer technology.
The location of crop rows is often aligned to fit in a precise configuration with the rows of previous crops to better manage crop disease, nutrition and stubble clearance during sowing.
Through much of the northern region it was not until the mid-1980s that replacement rates of nitrogen became established practice. Until then, fertiliser use was often limited to starter fertiliser at sowing. Back then, the soils were younger and had higher organic matter to buffer nitrogen response.
Like a good many of us, our soils are now 30 years older and, also like us, need a good feed every time they are asked to perform.
Not only is it now commonplace to apply replacement rates for nitrogen, many soils also require replacement rates of phosphorus. Other nutrients such as potassium are increasingly found isolated near the soil surface. In our northern no-till system, potassium does not readily leach and has been taken up by successive crop plants to become increasingly concentrated near the soil surface where crop residue has decayed.
It is now far more common for advisers and growers to regularly test paddock soils for plant-available nitrogen and phosphorus.
Today, we also understand the contribution of pulse crops, the potential contribution from soil microbial activity and the need to have crop nutrient sources co-located deep in the soil profile where the crop can access them later in the season.
Progressive growers are now using stratified sampling tactics for nitrogen to test for nitrogen levels at a range of soil depths. Most growers are using no-till practices and are integrating increased rates of nitrogen to balance the higher water reserves often found in this system.
Planned crop sequences for cereal, pulse, oilseed, cotton and other summer crops provide a farming system that helps manage soil diseases such as crown rot and nematodes, as well as managing bottlenecks in field operations, labour and machinery requirements.
Used in association with no-till and state-of-the-art moisture-seeking planting technology for both summer and winter crops, such systems help manage climate risk and increase the probability of implementing a planned rotation.
Where to from here?
Twenty years of no-till has created enormous selection pressure on weeds for herbicide resistance. In the northern grains region we use rotations between winter and summer crops as a means of cleaning up weedy paddocks, instead of relying on in-crop selective herbicides. This means we have effectively selected for weeds that are resistant to those products used during the fallow, in particular glyphosate.
Since Glean® (a Group B herbicide) was released in 1982, there has only been one other new mode of action group released (the HPPD inhibitors in 2001). With no new post-emergent herbicide mode of action groups likely for at least 10 or more years, the cavalry is not coming. There is no imminent replacement product for glyphosate.
In the short term, the no-till system will be defended by tactics such as pre-emergent herbicides, double knock using paraquat, and the use of tillage when weeds cannot be or are uneconomical to control using herbicides. Technologies such as camera-guided sprayers are helping growers to contain costs.
The long-term solution is clearly going to require a mix of non-herbicide tactics to drive down weed seedbanks. Such tactics have been successful in southern and western grains regions where herbicide-resistant weeds have largely developed in-crop weed populations as opposed to the fallow weed populations.
In the fallow we lack the ability to deploy tactics such as crop competition and we have not yet devised ways to economically harvest weed seed from fallow weeds, the way it is done in other regions using weed-seed capture at harvest.
Apart from the reintroduction of tillage, which will damage profitability and farming’s resource base, there are some weed-management tactics that can be used. These include increasing crop competition, where this is possible. It has been demonstrated that narrower crop rows will lead to both yield benefits and substantial decreases in weed biomass and seed set. Similarly, varieties have been identified that produce higher levels of early leaf area index, a factor known to correlate with a crop’s ability to out-compete weeds.
Crop row orientation research has also shown that crops sown on an east–west orientation intercept more light and compete better with weeds growing in the inter-row.
Each of these tactics could be combined to reduce the weed seed set of a weed that has survived a herbicide application.
However, our battle will not be won through crop competition alone.
Future options could involve robotic technology, where robots armed with microwaves or micro-tillage units and guided by unmanned drone aircraft scour paddocks and crops to seek out and destroy weeds that have survived herbicide treatment.
This is the type of innovation that would maintain the no-till farming system and its inherent benefits and reduce the reliance on herbicides.
All this will come at a cost and that cost must be able to be accommodated by the returns growers receive for the commodities they produce. That is one reality that has not changed and is not going to change.
John Cameron, Independent Consultants Australia Network,
02 9482 4930,
19th Australasian Weeds Conference,
GRDC Project Code CN00009, ICN00013, ICN00016