Additional rape and herbicide mixes aid blackgrass control
By Edward Long
BLACKGRASS is such a severe threat and the effectiveness of chemical control so linked to seasonal weather that a Northants heavyland cereal grower has been forced to increase his winter rape area to provide a cleaning crop opportunity for the rotation.
"If I fail to keep on top of the competitive weed a huge infestation can soon build up and the wheat yield is abysmal," says Tony Webb, who farms 160ha (400 acres) at Grange Farm, Lutton, near Oundle.
"Wheat here can do 3.5t/acre, but uncontrolled blackgrass has knocked it to 1t/acre in the past."
Cropping on the farm is 60ha (150 acres) wheat, 28ha (70 acres) winter barley and 48ha (120 acres) rape. Until 10 years ago there was no rape.
A combination of cultural and chemical control kept blackgrass at bay, until a run of tricky seasons allowed infestations to increase sharply.
Control is compromised when a dry autumn is followed by a wet winter, as stale seed-beds and pre-drilling sprays are ineffective, and it is either impossible to get on the land with a sprayer or the chemical will not work in the wet, says Mr Webb.
That happened 10 years ago, so rape was introduced as a cleaning crop. Together with the use of both Avadex (tri-allate) and isoproturon that worked well. For a few years blackgrass and brome seemed to be under control.
But in the mid-1990s it looked as if it was getting away again and it was clear the control strategy had to be changed. As the weather was favourable stale seed-beds were exploited and the area of rape increased from a 1:6 rotation to 1:3.
"I also tried other herbicides including trifluralin, Hoegrass and Cheetah, which did a wonderful job for a couple of years. Then I switched to Topik. When this failed I discovered my blackgrass had developed enhanced metabolism resistance, so I tried Hawk on some wheat and Lexus Class on another bit. Initially both seemed to work well, but the end result was disappointing."
Last year a mix of Lexus (flupyrsulfuron-methyl), Hawk (clodinafop- propargyl + trifluralin) and oil used on Avadex-treated continuous wheat with a heavy blackgrass burden gave over 97% control.
But that result was offset by poor control from the same mixture in another field where the crop had been drilled and sprayed at similar times. There was no sign of trouble until May, when a late flush of blackgrass seedlings appeared.
"This season, in an attempt to know my enemy better, I will be checking plants in a metre square to see whether they are topped by the chemical then regrow, or whether it is new plants emerging."
Another comparison last year was a mix of Treflan (trifluralin) and ipu costing £28/ha (£11/acre) and a Lexus, Stomp, oil combination costing £45/ha (£18/acre). "By February/March both looked wonderfully clean, but by May there was moderate blackgrass where IPU/Treflan had been used.
"I do not know the cost of this seasons Lexus Millennium + Stomp + oil mixture yet, but whatever it costs, if the mixture works, I have to use it. Failure to control blackgrass costs yield and there is a severe knock-on impact on following crops. It could even mean more rape or set-aside," Mr Webb says. *
• Severe headache.
• Resistance confirmed.
• More rape as cleaner.
• Herbicide mixes helping.