8 December 1995

Resistance is not down to reduced herbicide doses

By Allan Wright

REDUCED dose weed control in cereal crops is not leading to herbicide resistance. Instead it is full rate applications which leave survivors that is the problem.

That was the message Dr Ken Davies, weed specialist at the Scottish Agricultural College, had for a Home-Grown Cereals Authority agronomy update meeting in Edinburgh last week.

He pointed to evidence from America which suggested full doses were causing some cases of resistance. "Unless the full dose wipes out every single weed, and that is virtually impossible, the few that remain will be the ones capable of producing resistant offspring," he said.

"My advice is to tank mix control chemicals and maintain a sensible crop rotation.

"Blackgrass resistance is becoming a major problem in England, and the main reason is continuous growing of wheat. In Scotland we still tend to follow an arable rotation that includes a break, and that reduces the problem of weeds building up resistance to chemicals.

"If you have any evidence of wild oat resistance, please let me know," he added.

However, reduced herbicide doses was not an option for control of brome grasses – an increasing problem in Scotland. The best answer was good inversion ploughing. If non-plough tillage was adopted, it was best to cultivate as soon as possible after harvest to encourage germination and seed loss where straw was removed, but to delay cultivation where straw had been chopped and covered the ground, providing brome control through germination and seed-eating insects.