Keeping one step ahead of black-grass in rotation

Black-grass is an increasing problem, traditionally associated with autumn-sown crops, but growers are urged to find solutions for control throughout the rotation. Dominic Kilburn reports.

Rotational planning should be at the heart of any successful black-grass control strategy, and including non-cereal and spring crops in the rotation allows alternative herbicides to be used, and may reduce black-grass infestations.

Those are the thoughts of Rothamsted weed specialist Stephen Moss, who says growers are currently achieving moderate success against black-grass, but must start thinking about the long-term aspects of control, particularly in light of widespread herbicide resistance issues.

“With no new modes of action on the horizon, growers need to look at the different levels of control over the whole rotation, in addition to non-chemical control methods,” says Dr Moss.

Updated data suggests that 80 per cent of farms sprayed for black-grass have resistance to at least one herbicide, and some have resistance issues with a number of different herbicides, he says.

“While many growers recognise they have resistance on their farms, many don’t seem to be taking the issue seriously enough to prevent it building up further.

“There can’t be total reliance on pre-em product stacking. Pre- and peri-emergence herbicides have their own problems with resistance building up, and if you stack the pre-em herbicide pile too high, it may fall down due to greater resistance and crop damage.”

A Rothamsted experiment on spring cereals demonstrated an 80-90 per cent reduction in black-grass populations in spring wheat and spring barley during 2010. However, there were half the number of black-grass heads in spring barley compared with spring wheat.

“The trouble with spring crops is that we just don’t know the impact of different drilling dates on weed infestations and no one knows as yet if delayed drilling of a spring crop is good for controlling black-grass. In practice, growers will get the crop in when they can.”

Spring-sown crops tend to be less vulnerable to black-grass infestations, as most weed plants will have emerged before sowing and can be destroyed by seedbed cultivations, or use of a total herbicide such as glyphosate.

However, intensive cultivations and early sowing can result in serious infestations in spring crops.

Limited options
There are concerns about limited herbicide options and confusion as to what are the best herbicides to use in spring cereals, says Dr Moss. He cites Avadex (tri-allate) as an example of a grass-weed herbicide that can be used in spring barley, but not in spring wheat. Chlorotoluron, pendi-methalin and prosulfocarb (as in Defy) are all pre-em options.

“Defy + pendimethalin at pre-em, followed by Axial (pinoxaden) as a post-em contact is probably one of the better spring cereal options for black-grass control, but I cannot find any independent data on the likely efficacy on black-grass, or risk of crop damage in spring crops,” says Dr Moss.

“With so few post-em options, pre-em herbicides are probably essential for successful control of black-grass in spring cereals. This is an area where more independent data would be really useful.”

Oilseed rape growers must also look longer-term and where possible they should use lower resistance risk post-em herbicides, such as propyzamide and carbetamide, he adds.