Nufarm shares have lost $1.40 to be trading at $10.70 at noon.

Nufarm says it will not meet its previous profit guidance for the current financial year due to a decline in demand for glyphosate and increased competition in key markets.

The company said that glyphosate sales in May and through the first half of June are tracking significantly below previous forecasts.

Managing director Doug Rathbone said it was not possible to accurately estimate the impact on Nufarm's year-end net profit, but it would be approximately 15pc below its previous profit forecast of $220 million.

"Given the importance of the final six weeks of the financial year, we will not be in a position to assess the final impact on group earnings until the end of July," Mr Rathbone said.

He said a combination of later than normal buying decisions; seasonal impacts; lower application rates and distribution customers not able to hold normal inventory levels; all combined to dramatically impact the glyphosate business in recent weeks.

"We have been saying for some time that the buying patterns this year were much later than normal, but trading over recent weeks has revealed that volumes are also going to be down and will not recover in the current season," he said.

Sales over the past six weeks in Nufarm's two biggest markets - Australia and the United States - have been well below the same time last year.

Price competition is also substantially reducing margins on sales.

"The planting seasons in those markets are now well advanced and the window for achieving additional glyphosate sales before the end of the financial year is closing quickly," Mr Rathbone said.

While seasonal conditions in Australia are reasonable in most cropping regions, Nufarm says sales demand is well below last year as many growers move from an emphasis on pre-plant weed control to post-plant weed control.

"While this will be favourable for our selective herbicide sales, some of those sales may well fall into the new financial year," Mr Rathbone said.