What are the economic benefits of diverse rotations?

Crop rotation choices drive farm profitability. When weeds start to dictate crop choices, input costs inevitably rise.

Chris Minehan, farm business consultant with Rural Management Strategies in Wagga Wagga, NSW, says effective weed control is non-negotiable given the impact of weeds on crop yield.

“Weed competition can have a huge impact on crop yield so, the only profitable way to farm is in a low-weed system,” he says. “Crop choice and rotation planning are very powerful weed control tactics, and central to a biologically-robust and sustainable farming system.”

Longer-term rotation planning has the greatest impact on weeds as growers implement a strategy to drive down weed numbers using as many WeedSmart Big 6 tactics as possible.

“In the Riverina, our acidic soils limit our ability to grow some of the grain legumes, but we have herbicide tolerant canola and legume brown manure options that are very effective in reducing grass weed numbers,” says Chris. “In assessing the value of crops grown specifically for their weed control benefits, it is vital to calculate the gross margin of the whole rotation to see the longer term impact.”

The WeedSmart Big 6 strategy to grow more crop and fewer weeds offers a range of herbicide and non-herbicide tactics that growers can deploy against weeds throughout the crop rotation to minimise the impact of herbicide resistance on their bottom line.

Is a double break with a brown manure a cost-effective weed control tactic?

A double break provides opportunities to hit grass weeds hard with alternative herbicide and non-herbicide tactics. In some areas, there are two break crop options that provide income through grain or hay, but in other areas, like the Riverina, the pulse options might be limited. An effective option from a weed control perspective is a brown manure pulse, such as vetch or field pea – but is this cost-effective?

To find out, it is necessary to look at the cost of the brown manure and the benefits for the following crop. Using a real-world example of five adjoining paddocks of canola where one paddock was previously brown manure vetch and the other four were previously wheat, Chris showed that the net benefit of the brown manure vetch was $210/ha through extra yield and reduced nitrogen fertiliser in the canola.

He says the canola on vetch brown manure yielded 60 per cent higher than the canola on wheat. The brown manure crop cost $630/ha, comprised of a $470/ha opportunity cost from not growing wheat, plus $160/ha to establish and manage the brown manure vetch.   

In the canola phase, after the vetch brown manure, the yield benefit of 1.05 t/ha was worth $680/ha. Along with a reduction in nitrogen fertiliser worth $160/ha, the total benefit to the canola phase was $840/ha.

If you then subtract the cost of growing the brown manure crop ($630/ha), you have a net benefit over the two years of $210/ha from growing the vetch brown manure.

What other benefits come from a brown manure phase?

Soil health benefits and future weed control cost savings represent additional value from implementing a brown manure tactic. It is impossible to put a firm monetary value on these benefits, but they are in addition to the measurable benefits outlined above.

A brown manure phase should provide 100 per cent control of annual weeds such as ryegrass and prevent seed set, thus driving down the weed seed bank. Chris says that having less weed seed in the seed bank means that growers using this system can often skip using a pre-emergent herbicide in the following crop, a substantial financial saving in itself. 

The following crop will have less weed pressure and, therefore, be more vigorous and have a competitive advantage over any weeds that germinate during the season.

Unlike a bare fallow, brown manuring provides the biological benefits of a crop, including nutrient cycling and improved soil tilth.

Including a brown manure phase also reduces the demands on labour and machinery resources, leading to lower-cost farming. This can allow time for soil amelioration works such as lime application or drainage to improve the performance of poorer areas or paddocks.

Where should I start to implement a double break with a brown manure?

Chris has clients using a four-year rotation with a brown manure phase for 15 years or more. These growers produce a similar amount of grain off 75 to 80 per cent of their cropping land, compared to cropping the entire area without brown manure.

Getting started is the most difficult step! Chris recommends beginning with the weediest paddock, which is probably the most costly to manage and will represent the best return on investment through reduced input costs in the brown manure phase.

In other regions, a cereal might be the best option for a brown manure phase, and hay can also provide excellent weed control outcomes.

The second crop in the double break also offers additional weed control options depending on the crop choice. In Chris’ region, canola is commonly the second break crop, and growers have several choices of herbicide-tolerant or more competitive cultivars, depending on their situation.

To be sure that the benefits will flow in your farming system, you can also test the idea using the Weed Integrated Management (WIM) tool for your most costly weed. The WIM tools are a suite of hands-on, user-friendly models that help farmers and agronomists evaluate the long-term cropping profitability of different weed control methods.

GRDC project: ORM1505-002SAX

Other resources

Podcast – Diverse crop rotations to drive profit and sustainability

Is crop rotation an economic option for managing weeds?

Pulses to attack weeds on many fronts

GRDC Ground Cover – Plan to build farm resilience