Anyone can buy the herbicide 2,4-D from a shop, but its misuse may have caused millions of dollars worth of damage to cotton crops, prompting calls for more controls.

The large-scale spray drift event hit up to 30,000 hectares of cotton on the eastern Darling Downs late last year.

But Crop Consultants Australia confirmed there had been spray drift detected across every cotton valley planted in Australia so far this season.

Spray drift is the airborne movement of agricultural chemicals outside a target area.

In this instance the chemical was 2,4-D, which kills plants by changing the way certain cells grow and is commonly used to control weeds.

Agronomist Matthew Holding said it was a shock to discover the affected cotton and he had never seen anything like it.

"These would range from low to moderate, and the moderate would be ones where we're starting to actually get concerned."

He said the true extent of the damage was still to be determined.

"It's happened just prior to Christmas I would say, so it's … still starting to rear its ugly head," he said.

Mr Holding said a lot of people were growing cotton at the moment due to strong prices, but sunflowers and small crops were also at risk.

Stuart Armitage, a former president of the Queensland Farmers Federation and former board member of Cotton Australia, said on top of high input costs, such as diesel, fertiliser and labour, the losses would be a huge blow.

"You spend a lot of money getting a crop going, and then someone who we can't name or find or anything … through inconsideration, damages people's crops."

Agronomist Matthew Holding said $500 worth of chemical could have caused the damage, which may total up to $100 million.

Both Mr Holding and Mr Armitage want more controls put in place around the chemical's use to create transparency.

They want 2,4-D listed as a controlled chemical that people would require certification to use.

"I would have thought that if you buy it from a shop, you've actually got to go onto a registered list and I think from that registered list, you've got to have the ability to keep good records and to be audited at any time," Mr Holding said.

Mr Armitage said the situation would not require a government review.

"All that does is need a tick from the government," he said.

In a statement, Biosecurity Queensland told the ABC it was investigating the use of herbicides associated with a reported spray drift incident on the Darling Downs affecting a 160-hectare site.

A spokesperson said further public comment was not possible while a complaint was being assessed.

Weeds become resistant due to wet season

Weeds have exploded across much of Australia due to wet weather last year.

Senior agronomist at Nutrien Dalby Ross Pomeroy said a huge amount of weed control in the areas concerned was ongoing, due to wet paddocks last winter and spring.

"We just couldn't work the paddock, it was too wet. So the situation was that we use a lot of herbicides," he said.

"Fleabane has been our biggest issue I'd say this season, and to a point where we've got fleabane varieties that are very tolerant or very, very resistant of our normal herbicides.

"Queensland, New South Wales, Northern Victoria [are affected and] I've seen it in my travels all the way up through the Queensland coast, around the Cape and it's just about everywhere, all the way through inland."

He said 30 years ago very light rates of glyphosate were used, but now he was having to go across paddocks three times.

"This might be the end of the herbicide era when it comes down to knockdown," he said.