Is one-time tillage a weed control option in a no-till farming system?

Many would say that the widespread adoption of no-till and minimum till farming underpinned the expansion of cropping in the northern region and saved many farming families from economic hardship.

While production costs and erosion damage were reduced, the heavy reliance on herbicides led to a significant shift toward weeds that can thrive in this farming system. Some weeds that were previously not considered a problem are now hard to control on some no-till paddocks.

University of Queensland principal research fellow, Dr Yash Dang, says the removal of cultivation has also led to an accumulation of certain immobile nutrients, such as phosphorus, zinc and potassium, in the dry surface layer of the soil where plant roots can not access them.

“Cultivation has a role in distributing nutrients, managing soil and stubble-borne diseases and controlling certain weeds,” he says. “The complete removal of tillage for 15 to 20 years or more on some farms has led some farmers to consider reintroducing strategic tillage operations while maintaining the benefits of a no-till system.”

In 2012, Yash undertook a 4-year project to investigate the effect of tillage on a range of soil properties in no-till paddocks throughout the northern cropping region – from Emerald, in Central Queensland, to Dubbo, NSW.

“We applied tillage using disc and tyne implements and also tested the timing and frequency of tillage operations,” says Yash. “On well-structured soils, there was no detrimental effect as a result of the cultivation. Even on more difficult soils, such as those with sodic subsoil or with hardsetting tendencies, one-off tillage operation at the correct soil moisture content caused only limited damage to the soil and repair was evident within two or three years.”

The research also demonstrated that cultivation is a viable weed control tactic within an otherwise no-till system, to prevent seed set of weeds such as fleabane and feathertop Rhodes grass, which flourish in a chemical-dominant control program.

“The positive effect on weed numbers is usually short-lived and has the potential to have negative effects in the years after cultivation,” says Yash. “Growers considering the reintroduction of occasional cultivation must consider all the pros and cons. Cultivation is just another tool in an integrated weed management system – not a stand-alone solution.”

How often should I use cultivation in my no-till system?

No-till farming systems have provided significant benefits to the soil resource and to farmers’ profitability. This research does not suggest a return to full cultivation. However, the trials showed that one-off, occasional tillage does not have significant detrimental effects on the soil. Care is required regarding the timing, type and frequency of tillage on sodic soils and soils with hardsetting characteristics.

Soil moisture loss is associated with cultivation, so this should be taken into account when deciding when to cultivate.

When should I use tillage?

Cultivation should be a last resort tactic to treat patches or paddocks where weed pressure has reached unacceptable levels and where the species present do not have seed that remains viable for many years once buried.

Weeds such as fleabane and feathertop Rhodes grass are good candidates for occasional tillage to prevent seed set. Numbers of these species can be reduced quickly when a variety of tactics aimed at preventing seed set are implemented for a few consecutive years.

In most situations, a single-pass cultivation before the weeds flower and set seed is sufficient to achieve the desired effect of reducing the target weed population. If the problem is severe, a single-pass operation may be required for a few consecutive years. Timing is critical to achieve effective weed control while not sacrificing a planting opportunity.

What other benefits can I expect from occasional tillage?

Cultivation will accelerate the breakdown of organic matter and mineralisation in the soil. It also distributes immobile nutrients, such as phosphorus, potassium and zinc, deeper into the plant root zone. In a no-till system, these nutrients tend to accumulate in the top few centimetres of soil, which is often too dry for plant roots to access. The release of nutrients may support a short-term crop yield response.

Cultivation will also disrupt insect and disease cycles, potentially improving yield and reducing control costs. Pathogens causing diseases such as crown rot of wheat, yellow spot of wheat, ascochyta blight of chickpea and stalk rot of sorghum can build up in the stubble and soil in a no-till farming system. Likewise, soil insects such as Helicoverpa, armyworms and black field earwigs can proliferate near the soil surface in a no-till system.

What are the risks associated with occasional tillage?

Cultivation is associated with potential nutrient and soil moisture loss and increased erosion risk.

Soil moisture is lost when the topsoil is cultivated. However, at most of the trial sites, stored moisture increased in subsequent years due to improved infiltration. If possible, cultivation should be done when there is a good chance of sufficient rainfall before the next planting opportunity. Mineralised nutrients are more exposed to runoff and leaching losses following cultivation.

In situations where weed seeds have been previously buried, cultivation may bring them back to the surface and initiate fresh germinations. Paddock history is an important consideration in the decision to cultivate.

Other resources

Research paper: Strategic tillage in conservation agricultural systems of north-eastern Australia

Case study: Darren Jensen, Biloela