Herbicide future under threat

Speaking at a recent herbicide resistance forum at Karoonda, organised by the Mallee and Coorong NRM group and the Karoonda Ag Bureau, the Australian Herbicide Resistance Initiative's Peter Newman, Geraldton, WA, said resistance was a growing problem.

"Herbicide resistance is a big issue worldwide, and in Australia WA and parts of SA have been the leaders," Mr Newman said.

While herbicide resistance in the SA Mallee is not as great as other parts of the country, issues are starting to crop up.

"Our big concern for this part of the world is that brome grass will very quickly evolve resistance to Imi (imidazolinone) herbicides and once they fall over these growers are going to have a lot of trouble on their hands, so we're trying to intervene before that happens and make those herbicides last a lot longer.

"The reason more farmers don't have Imi resistance is because they haven't used enough of it, but it will happen - it's not one of those low-risk groups," he said.

"If you're just in the stage of getting a few resistant populations, that's the warning sign that it's going to happen."

He said herbicides were not the answer to herbicide resistance.

"Herbicides are fantastic - but has anyone actually completely eradicated ryegrass on their property? We've had 30 to 40 years of new ryegrass herbicides coming out, and we've still got ryegrass. Herbicides are brilliant, but they're not the complete answer," he said.

"With a lot of farmers, when herbicide resistance bites, they just start rotating herbicides. They're still living year-to-year, just focusing on killing this year's weeds, then it's all the same the next year. As soon as farmers make it about the seed bank, they start to have wins.

"A low seed bank is all about the farmer calling the shots, not the weeds. If you've got a high seed bank, you might have paddocks that can't go into crop, or you might have to delay sowing until you can get a knockdown. If you've got a low seed bank, you can do whatever you like."

Mr Newman said non-herbicide weed control options - like harvest weed seed control - could help get the seed bank in decline.

He outlined six main methods of harvest weed seed control.

Narrow windrow burning is the most popular, and has been embraced by WA farmers for use on lupin and canola stubbles in sandy soils.

He advised graingrowers to harvest low - a difficult task in undulating, stony country - and try and wait as close to seeding as possible.

"Also, a light cross wind is the way to go - you've got to have some wind to fuel the fire all the way to the ground but too much and the fire can move too quickly," he said.

One problem with narrow windrow burning was that it removed half of the crop residue, leading some farmers to look to other options such as chaff carts.

"If you're a mixed farmer and you want to get into harvest weed seed control, the best thing you can do is have a chaff cart. Sheep love to eat the dumps, and they don't need burning. Weeds don't germinate as they don't like being in a big pile of chaff, and only 1-5pc of ryegrass seeds go through sheep."

Other options included the Harrington Seed Destructor, windrow rotting - using a chute of plastic attached to the back of the header to make a windrow of chaff, which is left to rot - and the bale direct method.

"With bale direct, everything is baled out the back of the header," he said. "If you've got a straw market that will pay the right amount, you're doing it for profit and the weed removal is a bonus."

Controlled traffic farmers have also had success diverting chaff onto permanent tramlines.

"Farmers are putting the chaff on the permanent wheel track and just leaving it there, and there's not that many weeds growing surprisingly," he said. "Weeds don't really like growing in chaff, and if they do grow, it doesn't matter so much as it's just growing on the track."

He said farmers should keep profitability in mind when selecting weed control options.

"There's always the temptation with resistant weeds to change your farming system to fit in with these resistant weeds. It's better to get aggressive with non-herbicide weed control and driving the seed bank down so you can keep your existing farming system going to make a profit."