17 May 1996

Maize weeds thrive in dry

WEED control is poor in young maize crops where pre-emergence atrazine has not worked well in the recent dry conditions.

Failure to tackle these weeds before they affect the growth rate of the maize plants could reduce crop yields by 30%, according to Maize Growers Association agronomist Simon Draper.

"Atrazine applied pre-emergence relies on moisture to be activated and where conditions have been driest weeds are germinating through the atrazine layer," says Mr Draper.

The maximum total rate of atrazine that can be applied in one year is 3 litres/ha (1.2 litres/acre). In dry conditions and where the maximum rate has been applied Mr Draper advises the use of a post-emergence contact herbicide such as Bromoxynil at a maximum rate of 1.5 litres/ha (0.6 litres/acre) or pyridate (Lentagran) at 2kg/ha (0.8kg/ha) to control orache, fat hen and black nightshade.

"This should be applied when maize is at the second to fourth leaf stage and weeds are at the cotyledon stage, with a second application made when maize reaches the sixth leaf stage to catch the second flush of weeds.

"If the weather turns wetter, then the atrazine will provide some control and may make the second application unnecessary," he says.

"In regions where there has been some rainfall atrazine has generally worked well but resistance to atrazine could occur where successive maize crops had been grown," says Mr Draper.

"In this situation most producers apply atrazine at 2 litres/ha (0.8 litres/acre) pre-emergence. This should be followed by a post-emergence application of atrazine at 1 litre/ha (0.4 litres/acre) mixed with either bromoxynil or pyridate at 1 litres/ha (0.4 litres/acre)."

says Mr Draper.

"Where there are known problems with resistance in continuous maize crops no atrazine will have been applied pre-emergence. In these circumstances atrazine should be applied post-emergence at 2 litres/ha (0.8 litres/ha) mixed with bromoxynil at 1 litres/ha (0.4 litres/acre) before weeds progress to the first true leaf stage. A second application with 1 litres/ha (0.4 litres/acre) of atrazine combined with 1-1.5 litres/ha (0.4-0.6 litres/acre) of bromoxynil should then follow.