Residue issues rear head

Residual chemicals require a certain amount of rain to break them down - some need up to 250 millimetres.

With some chemicals there is a buffer period of nine months between their application and planting the next crop, which is normally not an issue, but dry periods crack open a series of problems.

Large parts of SA, NSW and Vic have received far less than the prescribed 250mm since the herbicides went out last year.

YP AG agronomist Chris Davey said with most of SA exposed to a dry spring last year followed by a dry summer, it may be difficult to grow some crops in 2015.

"The accumulated rainfall has been low, from chemical application in 2014 to seeding this year," he said.

"The amount of rain and time required varies from product to product, however a lot of products fit under a similar requirement of a nine-month time frame, with 250mm of rainfall after application of the chemical.

"The nine months will elapse without a problem but a lot of areas have not fulfilled the rainfall requirement."

Mr Davey said growers would be assessing crop rotations and possibly changing the types and varieties sown this year.

"Growing more tolerant varieties, or changing crops altogether, will take place, which is not ideal just before seeding," he said.

"Another tactic may be for farmers to delay seeding some paddocks until they have received enough rainfall, but this is also not ideal as everybody knows the importance of sowing on time."

Group B chemicals were the biggest cause for concern.

"Clearfield or Imi-tolerant crops are quite prevalent now, so sourcing one of these varieties may be quite easy for the farmer," he said.

"These crops have been bred to tolerate Group B chemicals a lot better than the conventional varieties, so have a shorter plant-back period of the chemical, if any."

Agronomise's Simon Craig, based in north-west Vic, said Clearfield system herbicides needed to be monitored as usual.

"We've had rainfall ranging from 10mm over summer to 150mm, but as it was so dry prior to the rain, even those who had those big falls will have to be careful," he said.

Residual chemicals such as clopyralid (Lontrel) would not be as big an issue given low stubble loads. Other potential problems could be with 2-4-D ester products.

Mr Craig expected farmers to consider another Clearfield crop, not necessarily to utilise in-crop weed control but to ensure there was no plant-back death at emergence.

"Clearfield varieties such as Kord wheat are well known and utilised in Mallee regions," he said.

He expected to see a swing into lower-risk, lower-cost cereal crops this year given cash flow issues after last year's poor yields, which would, in turn, affect chemical rotations.

"There is going to be a lot less canola planted and more cereals going in, which puts pressure on the rotation," Mr Craig said.

He said a lack of rotational options may mean another problematic year for 'warley' - barley infested with wheat volunteers that cannot be controlled in-crop.