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Be the Early Bird on Summer Weed Control

17 January 2012

AUSTRALIA - Western Australian growers are urged to control summer weeds as soon as possible, even it means getting out the boomspray while completing harvesting programmes.

Significant rainfall in late spring and summer has triggered widespread germination of weeds and ‘volunteers’ from the 2011 crop.

GRDC western panel member and former farm consultant Ralph Burnett acknowledged it was difficult for some growers to spray weeds while struggling to finish weather-affected harvesting programmes.

But he said research and many years of farmer experience showed early spraying provided the most benefits, and it was a false economy to delay.

“By doing it later, valuable moisture and nutrients are lost from the soil, the weeds will be larger and more expensive to kill, and you will have created a ‘green bridge’ allowing pests and diseases to carry over to the next crop,” Mr Burnett said.

“It is better value to spray now, even if you have to follow up with one or two more applications, than applying a single spray just before seeding – by then you will have missed the boat.

“The best time to spray summer weeds is early in the morning, when they will accept the herbicide, and generally you can’t harvest anyway at that time of the day.”

Mr Burnett said the longer the break between killing summer weeds and seeding the 2012 crop, the better the ‘fallow effect’.

“Fallow is very effective – our fathers used a plough, but now we use a boom to create a superior fallow,” he said.

“It is preferable to create a break of at least three weeks during which there is no green plant matter capable of hosting insects, nematodes, foliar diseases and fungal root diseases.

“The longer the gap between spraying and sowing, the less effect the herbicides may have on the emerging crop


“Read and observe the ‘plantback’ period on the label.”

Mr Burnett said uncontrolled summer weeds and crop volunteers also significantly depleted soil moisture and nutrients that would otherwise be available to following crops.

“Some weeds such as goosefoot (mintweed) produce allelopathic toxins in their seeds which adversely affect most crops at emergence,” he said.

“For this reason, early control of goosefoot is essential.”

Mr Burnett said GRDC supported research in WA and NSW found that removing weeds shortly after they began to emerge could preserve 50 to 75 millimetres of soil moisture.

“Researchers have also identified yield losses of 0.5 to 1.0 tonnes per hectare in cereals where the green bridge remained uncontrolled up to seeding,” he said.

“Nutrients such as nitrogen used by the green bridge are no longer available to crops at seeding, even if the green bridge is later killed by herbicide.

“In the fallow created by herbicides, it is most likely that soil bacteria will increase the nitrate nitrogen levels.

“Pathogenic soil fungi such as Rhizoctonia are reduced in fallow due to losing their host plant, and competition with other soil organisms.”

Mr Burnett said that by preserving moisture and nutrients, growers could expect to start seeding their 2012 cropping programme on the lower rainfall ‘breaks’ that WA had experienced in recent years.

TheCropSite News Desk

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