Stubble residue can play both a physical and a chemical role in weed suppression. This means there are opportunities to exploit crop rotation and stubble management choices to further enhance these weed suppression effects.

The research supporting these findings comes from a cross-disciplinary team at the Plant Interactions Research Group at Charles Sturt University (CSU). This team investigates the complex relationships and exchanges that occur in the paddock among plants, weeds, the soil microbiome, insects, stubble residue, soil nutrients and the chemical compounds that mediate allelopathic effects.

Based in NSW at Wagga Wagga, the trial paddocks typically produce wheat, barley, canola, oat and lupin broadacre crops, as well as legume and other cover crops used for biological diversity and weed suppression.

Increasingly, growers are adopting intercropping and mixed cropping systems, with particular interest currently in summer cover crops such as teff and buckwheat. This has resulted in an increasing mix of stubble residue and typically high stubble (six to seven tonnes per hectare in 2022).

Summer cover crop – teff. Photo: CSU

In these circumstances, research is finding that retained stubble plays a more intricate role in paddock ecology than first envisioned.

For example, residue roots continue to live for a significant period after the crop is harvested. They also continue to be physically present even after they stop taking up moisture. Rainfall, therefore, can continue to saturate the root channels in the zones underneath crop stubble, which helps fashion soil characteristics in unique ways.

Above ground, too, the residue is sculpting its environment. The research team collaborators at CSU, John Broster and Michael Walsh, have examined microclimatic effects across different types of stubble architecture that result from draper versus stripper headers. They have detected temperature differences on the ground that clearly affect weed seed distribution and emergence.

Overall, the research team is seeing stubble residue provide unique microclimates above and below the soil surface that can have both beneficial and detrimental impacts.

One striking example relates to decomposing residue. It is broadly understood that soil microbiota can use up nutrients very quickly from the soil profile in order to decompose a large stubble load, which causes a shift in soil nutrient dynamics. However, observations indicate that both the above and below-ground residues can continue to exude or leach interesting, natural plant products – even phytotoxins – that affect the soil microbiome, weed seed germination, soil pathogens and even insects.

Summer cover crop – buckwheat. Photo: CSU

The team’s work on bioactive natural plant products is particularly focused on bioherbicides and phytotoxins, as well as molecules that could have growth-stimulating properties.

This work uses advanced analytical techniques that include separation science, metabolomics, genomics and also population and field ecology to investigate the plant and its soil rhizosphere.

What has emerged is a view of plants as being far less passive than previously thought. These crops deploy chemical signalling agents in complex ways, including:

• to defend against a range of pests, including weeds and grazing herbivores.
• to strategically localise these metabolites in the plant and soil rhizosphere, often for crop protection.
• as signals that affect the function and dynamics of soil microbial communities.

Recent studies have focused on plant toxicity to grazing livestock and the specific mode of action of residue chemicals (both stubble and roots) that function as herbicides, cytotoxins and photosensitisers. For example, we have seen residue from wheat, rye, barley and numerous summer annual cover crops produce leachates in the soil surface that contribute to weed suppression.

Understanding and characterising the chemical nature of these compounds or metabolites now forms a strong research focus into the future. The goal is to optimise crop rotation choices and stubble management practices to better exploit the crop residue’s own ability to suppress weeds.

Weed suppression trials

Summer cover crop – vetch. Photo: CSU

The ability of various dual-purpose grazing or non-grazing grain crops – and their residues – to suppress weeds during the summer fallow have been investigated. This research includes two successive field experiments in the Riverina region of NSW.

Significant weed suppression associated with grazing and non-grazing wheat residues was observed, both pre and post-harvest. Grazing wheat exhibited significant suppression of fleabane and witchgrass up to 130 days post-harvest. Grazing and non-grazing canola provided strong and significant suppression of fleabane and witchgrass for up to 140 days following harvest. Interestingly, these crops did not have as much residue remaining on the soil surface as other, less-weed-suppressive cereal crops in post-harvest measurements.

Grazing cultivars were generally more weed suppressive than the non-grazing cereal cultivars evaluated. Grazing oats also tended to be initially suppressive, but long-term suppression of weeds (until planting the following season) was not always observed.

Soil analyses performed in late March indicated that the observed differences in weed establishment were not likely to be reflecting differences in soil moisture availability.

Results suggest that establishment of certain cultivars could effectively suppress weeds both in-crop and post-harvest. The effect is detectable both in the presence or absence of post-emergent herbicide use. It affects problematic post-harvest weeds such as fleabane and witchgrass.

These findings suggest that cultivar and/or cereal crop choice is an economical form of weed management due to in-crop competition and possibly other factors, such as allelochemicals (weed suppressive chemicals).

Currently the project is evaluating soil samples for the presence of isothiocyanates and glucosinolates associated with weed suppression in brassica species. and hydroxamic acids present in wheat, rye or barley residues. This will determine whether the presence of canola or certain cereal residues are associated with higher levels of these suppressive chemicals via allelopathy.

Differences were also observed in weed numbers when comparing stubble produced by stripper and draper treatments. The stripper treatments had lower weed numbers compared to the draper treatments. The weeds observed were mostly sowthistle, but also fleabane, crumbweed, cudweed and hairy panic.

In addition, six years of research involving numerous long-term rotations with 10 different cereal and pulse crops has shown that both the crop before harvest and the residues can contribute significantly to out-competing weeds over time. This results in reduced inputs into the soil seedbank under typical rainfall and soil moisture profiles.

More information: Leslie Weston, leweston@csu.edu.au; Saliya Gurusinghe, sgurusinghe@csu.edu.au