[GRDC 01 Sept 2014] AUSTRALIA — Broadacre cropping in Victoria's high-rainfall zone is facing a ‘ticking time bomb’ of herbicide resistance as recognised weed control techniques fail to overcome the looming problem. This has been the impetus behind a 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.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:
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/
The same modelling showed that in the first scenario, with the commonly used canola/
“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.
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