Can a diverse farming system be profitable and hard on weeds?

Results from over a decade of farming systems research have illustrated that many factors need to be considered and implemented for a cropping enterprise to be profitable, manageable, and to minimise risk. 

Tony Swan, a CSIRO farming systems researcher, says that decisions related to weed control vary depending on the crop rotation implemented. Multiple factors, including stubble management, nitrogen use, grain storage, and machinery cost, influence the profitability of the rotation.

“Field trials often focus on one or two critical factors, aiming to understand the effect between treatments,” he says. “It is important when conducting trials at a small plot scale, that the researcher, agribusiness consultant and the grower are confident the results can be scaled up to commercial ‘whole farm’ level.”

With a large body of research findings on a wide range of weed management strategies, crop rotations, and stubble management options in the southern NSW region, the multidisciplinary team scaled up the results and tested them in real-world scenarios for farms with 3000, 5000 and 10,000 ha of crop.

“For example, when it comes to planting equipment, irrespective of opener type, we found no difference in the profitability or effect on weed numbers where more expensive herbicides were used,” says Tony. “Tyned openers were more effective when lower-cost herbicides, such as trifluralin, were used, mainly because fewer of these herbicides are on-label for a disc opener.”

Growers who implement the WeedSmart Big 6 strategy, which includes both herbicide and non-herbicide tactics, should be able to achieve high profitability while keeping the weed seedbank low.

What are the keys to good stubble management for weed control?

Zero-till, stubble retention and fallow weed management have been widely adopted in Australian cropping systems – and for good reason. In combination, these farming system practices protect the soil from erosion, aid in moisture infiltration and storage, and reduce weed numbers in fallow.

However, it is important not to let heavy stubble compromise the timeliness of operations or hinder weed, disease or pest management. A stubble load of 2–3 t/ha (i.e. 70% ground cover) is enough to achieve erosion control and water infiltration outcomes. 

If the stubble exceeds 3 t/ha, consider using any of these post-harvest management tactics to reduce the load: mulching, incorporation+nutrients, baling, grazing or late strategic burn (especially before a second wheat crop or before sowing canola). Including a legume in a diverse crop sequence will help manage stubble, improve nitrogen efficiency and assist in converting the cereal stubble into organic carbon.

Cereal stubble is a good source of carbon (C) but provides very little nitrogen (N) for subsequent crops. Therefore, when stubble is retained and incorporated, there can be significant N tie-up. To compensate, in a canola-wheat rotation, apply deep band N and add 5 kg N/t of cereal stubble at sowing. Alternatively, sow a pulse legume, such as faba bean, lentil, chickpea or vetch, into the wheat stubble, followed by a canola crop.

From a weed management perspective, be flexible and proactively manage stubble to suit your seeding system. Both tyned and disc systems are equally effective for weed management and crop performance when stubble load is managed appropriately to allow for diversity in pre-emergent herbicide use.

What is an effective rotation to keep annual ryegrass numbers low?

Initial testing of weeds for herbicide resistance is crucial to determine the most appropriate herbicide and the best management options available. A crop rotation that includes a double-break of two non-cereal crops, such as faba bean-canola or vetch-canola, was found to be the most effective tactic for reducing herbicide-resistant annual ryegrass numbers.

A ‘diverse-low-value’ system (low-N, faba bean-canola-wheat) that includes a pulse crop in either a 3- or 4-year sequence matched or improved the profitability and reduced the financial volatility compared to the ‘baseline’ systems (canola-cereal-cereal) at both the small plot and whole-farm scale.

Where there is an existing annual ryegrass weed issue, the ‘diverse mix’ systems (low-N, vetch hay/BM-canola-wheat) where pulse crops can be brown manured or cut for hay may be more profitable than a ‘diverse’ system with a pulse grain crop, depending on how the resistant weed is managed. The combination of experimental and whole-farm data supports the agronomic value of including a pulse crop (e.g. faba bean, chickpea, lentil, vetch) in farming systems in southern NSW.

What other management systems need to be in place?

The profitability and sustainability of this system rely on the farming enterprise having adequate machinery and labour resources in place.

When scaling up to the ‘whole-farm’ for a cropping operation that incorporated legume crops in the rotation, it was apparent that factors such as on-farm storage for pulse grain and adequate machinery and labour resources were critical to the profitability of the farm and the subsequent effectiveness of the weed-suppressive cropping program.

The economics easily stacked up for new grain storage facilities to enable better marketing of pulses such as lentils, chickpeas and faba beans. On the other hand, the rapid increase in machinery costs made it difficult to justify the investment in machinery for more timely operations. The recommendation is to carefully consider purchasing quality second-hand equipment.