CSIRO senior experimental scientist Tony Swan says there is “no one-size-fits-all magic crop sequence” for managing weeds, disease or nitrogen. Photos: Nicole Baxter

Crop diversity is helping Australian grain growers tackle two of the most persistent challenges: weeds and crop diseases.

By expanding rotations beyond the once traditional cereal/canola/cereal sequence to include alternative crops, growers are more effectively managing herbicide resistance, reducing disease pressure and building more resilient and profitable farming systems.

Alternative crops can include pulses such as chickpeas, faba beans, field peas, lentils, lupins and vetch, and oilseeds such as safflower.

CSIRO Senior Experimental Scientist Tony Swan prefers to use the phrase “diversifying the crop sequence” and says success relating to weeds, disease, nitrogen and profit will look different for each farm and paddock.

Tailored diversity

The best results are achieved by tailoring diversity to local conditions. This includes testing for soil constraints, disease and weed risks, such as herbicide-resistant weeds, before integrating agronomic, logistical and economic considerations.

“There’s no one-size-fits-all magic crop sequence, whether we’re talking about weeds, disease or nitrogen,” Mr Swan says.

“Successful integration of a diverse range of crops and diverse crop sequences into the farm enterprise depends on a range of factors.”

These include:

• planning with a sound understanding of value propositions and success factors before integrating new crops
• a grower’s management philosophy, risk appetite, operational sustainability and profitability, as well as skills and access to suitable machinery and labour
• the region – rainfall and environment
• soil characteristics and constraints such as acidity, salinity and sodicity, nutrient levels, disease and weed issues, especially the presence of herbicide-resistant weeds.

Chemical rotation

Mr Swan says one of the main benefits of diversifying crop sequences is being able to more effectively mix and rotate chemicals from different mode of action groups across different crop types. This improves weed control while reducing the risk of herbicide resistance.

These are components of the WeedSmart Big 6, which has been broadly adopted by researchers, consultants and growers since its launch in 2016.

The benefits extend beyond chemicals, as demonstrated in an ongoing experiment across four sites in central and southern NSW since 2017.

Adding a pulse, such as faba beans, chickpeas or vetch, to a region’s baseline crop sequences – either canola/wheat/barley or canola/wheat/wheat – reduced synthetic nitrogen requirements for the following crop by an average of $40 to $60/ha.

This pulse/canola/wheat sequence also cut expenditure on herbicides by $13/ha, and improved overall farm logistics and timeliness of operations.

“We have found at all sites there are diverse crop sequences with significantly higher profits, higher water use efficiency, lower risks and better return on investment, while reducing weeds, disease and greenhouse gas emissions compared to the baseline sequence of canola followed by 2 cereals,” he says.

Mr Swan says the research has identified a crop sequence of pulse/canola/wheat/barley, providing 2 double breaks, as among the most sustainable. However, there are barriers to adoption – primarily to do with risk, confidence, skill set and price volatility.

Breaking the cycle

While weed management is a major driver for crop diversity, managing disease is no less important.

Many damaging soil-borne cereal diseases, such as Fusarium crown rot and take-all, survive in cereal roots and stubble and build up with continuous cereal cropping.

NSW Department of Primary Industries and Regional Development Senior Research Scientist Dr Steven Simpfendorfer says introducing non-host crops, such as a pulse or canola, provides a crucial break allowing cereal stubble to decompose and pathogen loads to decline.

“But you need time and moisture for that to happen,” he says.

“That’s where we’ve changed a lot with our stubble retention systems, which are great, but we have slowed the decomposition rate because a lot of our cereal stubble is left above ground and there’s not as much microbial activity for that decomposition process to happen.”

Dr Steven Simpfendorfer says the best break crops for managing yellow spot or Fusarium crown rot produce a denser canopy earlier in the season.

Fungicide resistance management

1. Avoid susceptible crop varieties
2. Rotate crops – use time and distance to reduce disease carryover
3. Use non-chemical control methods to reduce disease pressure
4. Spray only if necessary and apply strategically
5. Rotate and mix fungicides/mode of action groups

Source: Australian Fungicide Resistance Extension Network

Disease breaks

However, not all break crops are equal. Even where a double break can help with problem weeds, it can cause issues when planting a pulse followed by canola. Both are hosts of Sclerotinia stem rot, which can survive in the soil for 4 years or more.

Dr Simpfendorfer says the best break crops for managing yellow spot or Fusarium crown rot produce a denser canopy earlier in the season. This includes canola, faba beans, lupins and field peas, which encourage more prolonged periods of cereal stubble breakdown.

Recent research has found harvesting shorter crops, such as chickpeas and lentils, can inadvertently draw Fusarium crown rot-infected cereal stubble through the header and spread it across a field.

Seed treatments for Fusarium crown rot and fungicide sprays for yellow spot can help reduce the impact of these diseases. But Dr Simpfendorfer says there are no products available that target stubble-borne inoculum loads.

“There is a bit of work being done around biologicals or enzyme digesters to increase the cereal stubble decomposition process, reducing the length of survival and inoculum of these pathogens, but that’s all early days,” he says.

“Management really is identifying risk and then trying to manage that at sowing.”

Test for risk

Testing is key to identifying risk and should be carried out early enough in summer to allow for any management changes before sowing begins.

“We’ve got these really solid rules of thumb when it comes to farming systems and sequencing with pathogens, and they largely hold, but we need to refine it and actually know what levels are there,” he says.

“There’s value in testing, whether it be through PREDICTA® B or stubble testing for cereal pathogens and to track where your levels are at.

“There’s a lot of little things to tweak, but you can’t tweak unless you test.”

The WeedSmart Big 6 tactics

1. Rotate crops and pasture
2. Increase crop competition
3. Optimise spray efficacy
4. Mix and rotate herbicides
5. Stop weed seed-set
6. Implement harvest weed seed control

Source: weedsmart.org.au/big-6

This article appeared in GroundCover