Need for feed could reduce weed burden

Key points

  • The southern Victorian high-rainfall zone is facing worsening ryegrass problems as herbicide resistance increases
  • There is opportunity to combine livestock production and weed management
  • Fodder crops reduce weed-seed numbers in 10-year rotation

Broadacre cropping in southern Victoria’s high-rainfall zone is facing a ‘ticking time bomb’ in herbicide resistance as recognised weed control techniques battle to overcome the looming problem.

This has been the impetus behind a GRDC-funded project investigating the use of fodder rotations with cropping to manage weeds. One of the researchers involved with the project, David Watson from Agvise Services, says despite southern Victoria being relatively new to cropping, over the past decade or more the region has developed one of the highest rates of herbicide resistance in Australia.

“Some of the reasoning behind that is the propensity of the area to grow both good crops and weeds,” he says.

“Regularly wet and windy weather makes for tough spraying conditions and our cropping rotations have been lousy to the point where almost the only break crop we have – with the exception of a very limited area of faba beans – is canola. We are in a situation where there is a big problem coming at us and the more recognised solutions don’t always work.”

While southern Victoria may struggle with weed control in broadacre cropping, the region has a high capability to produce livestock and, with that, good fodder crops. It also has the advantage of being close to Victoria’s largest dairy region, the south-west, which has a high demand for fodder.

“Nearly every farm in southern Victoria has livestock and they have to eat,” Mr Watson says. “Most mixed farms in our region are maybe 50:50 in their ratio of crops to livestock, so there is a need for feed and we can grow a lot of it – up to eight or nine tonnes a hectare in a fodder crop.

“To me, it makes sense that if you’ve got a weed problem, you don’t have a range of break crops and you have livestock that need feeding then why not grow fodder as the break crop?”

Image of a man

Agvise Services’ David Watson says trials with fodder crops as the break in a cropping rotation have shown a decrease in weed-seed density compared with a ‘typical’ continuous rotation of wheat/barley/canola.

PHOTO: Alistair Lawson, Agcommunicators

Mr Watson and Southern Farming Systems established trials at Inverleigh and Lake Bolac. Both sites have historic weed problems either with wild radish or ryegrass and have been previously cropped.

The three-year project, now in its final year, has trialled several different fodder crops including lucerne, arrowleaf, balansa, Persian clover, subclover, annual ryegrasses, peas and oats, serradella, and wheat and barley for fodder. Using Ryegrass Integrated Management (RIM) modelling, the project compared three different 10-year scenarios:

  • the typical canola/wheat/barley rotation using several commonly used herbicides with varying levels of efficacy and resistance;
  • two years of Persian clover, an ‘autumn tickle’ or shallow cultivation, summer grazing, hay or manuring and then into a typical rotation as per the first scenario; and
  • four years of lucerne cut for silage, summer grazing and winter cleaning followed by a typical rotation as per the first scenario.

Mr Watson says one of the key messages for growers at this stage of the project is that, in theory, it takes at least four years of including a fodder crop in the rotation to significantly reduce the weed-seed population.

The trial used RIM modelling to assess plant populations over 10 years. At the beginning of a 10-year rotation, the number of ryegrass seeds was 20,000 per square metre, with ryegrass plant density at 1000/m2. At the end of the rotation, with four years of lucerne as the break, the predicted number was 140 seeds/m2 with just 7 plants/m2 (Table 1).

Table 1 Results of Ryegrass Integrated Management (RIM) modelling showing predicted annual ryegrass numbers at the beginning and end of three different weed-management scenarios.

Ryegrass density per square metre

Year 0
Year 10
Seeds Plants Seeds Plants
Continuous crop: wheat/barley/canola 10,000 500 16,000 530
Two years Persian clover fodder break crop
20,000 1000 8400 420
Four years lucerne fodder break crop
20,000 1000 140 7

The same modelling showed that in the first scenario, with the commonly used canola/wheat/barley rotation, ryegrass seed and plant numbers increased.

“In a way, the species of fodder doesn’t matter, it’s more about how you manage it,” Mr Watson says. “The bottom line is that you can’t let any plants set seed. Lucerne was good because it has a lot of tools to stop ryegrass setting seed, such as the fact you can graze it hard and cut it for hay. You might use oats, but as long as they don’t set seed it will still do well. With lucerne as the break you get the nitrogen fix. The weeds are also markedly reduced, but it does take time.”

Mr Watson believes there is huge value in using fodder to manage weeds, it is just a case of getting the management right.

“As weed populations increase, we will be able to contribute to a better understanding of the role of fodder against weeds in a cropping system and the other benefits, such as nitrogen production and dry-matter production. Growers will be able to design fodder phases within their cropping program that suit their whole-farm system.

“Our whole-farm system in this region is far more complex than other systems, such as continuous cropping systems. Typically, average annual rainfall is 575 millimetres and there is much less evaporation than other areas. Livestock introduces opportunities rather than limitations in a cropping system in southern Victoria. Livestock have been seen as an imposition on cropping but that view is too simplistic for this area.”

More information:

David Watson, Agvise Services,


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GRDC Project Code SFS00022

Region South