7 November 1997

Nip blackgrass problem in bud – best advice

Herbicide-resistant

blackgrass can add greatly

to cereal growing costs.

Adopting a sustainable but

practical approach to

avoiding the problem in

the first place could make

sound economic sense.

Robert Harris reports

HEAVY land cereal growers who let the most common form of herbicide resistance develop in blackgrass face big losses, especially at low grain prices and yields. A simple but workable approach to prevent the problem could save over £100/ha (£40/acre).

Those are the conclusions drawn by ADASs Jim Orson and David Harris from a study of blackgrass resistance based on two farms at Peldon, Essex. The project was funded by the agrochemical industry through its Herbicide Resistance Action Committee and will be reported at this years Brighton Crop Protection conference.

The problem was first identified at Peldon in 1984. Since then resistance has developed on more than 750 farms. The most common form is enhanced metabolism resistance, as at Peldon. Blackgrass plants develop a partial resistance to a wide range of herbicides by detoxifying them before they reach lethal concentrations.

Resistance at Peldon developed in classic fashion, where heavy infestations of blackgrass were exposed to intensive herbicide use, Mr Orson explains. "Both farms practised continuous winter wheat production for at least 20 years before 1984, using shallow tillage after straw burning.

"In addition, sowing dates were brought forward to September rather than the traditional sowing time of early October."

Neither grower wanted to drop continuous wheat, the most profitable crop on their heavy Essex clays. So management changes were needed to maintain control of the increasingly resistant weed.

Those were split into five stages, and involved increasing reliance on ploughing and later sowing dates to keep weed numbers manageable.

"It is now necessary to plough every acre and to delay the start of drilling until mid-October. In addition, three herbicide applications are necessary," Mr Orson comments.

"Those measures have reduced margins compared to the original system of minimum tillage and a single herbicide application. Based on extra cultivations, herbicide costs and yield losses from later drilling at Peldon, the current system is about £180-200/ha (£72-80/acre) more expensive."

Preventing the development of resistance can halve that cost. In continuous wheat production, ploughing every year and drilling a significant proportion of the crop in early October can do just that, ADAS experience suggests. "Compared to what is needed to contain blackgrass at Peldon, net margins are about £100/ha (£40/acre) higher," Mr Orson reckons.

The ADAS approach will not maximise continuous wheat margins in the short term, co-author David Harris comments. "But if the approach continues to prevent or delay the development of resistance it will pay within a few years."

Recent market trends may tempt farmers back to minimum tillage, he adds. "However, resistance has more impact on margins at lower cereal prices." Those with lower yielding crops will also be hit harder (see graph).

Some growers may have left it too late to adopt a preventative approach since resistance has started to develop on many farms, Mr Orson advises. "The industry must be aware of the dangers and adopt the approaches suggested in the Weed Resistance Action Group guidelines to minimise this profit-sapping problem."

PREVENTION BEST

&#8226 Resistant blackgrass costly to manage.

&#8226 Low yields and/or prices exacerbate effects.

&#8226 Plough, sow early October, and apply one herbicide to avoid or delay resistance.

&#8226 Potential savings up to £100/ha.

PREVENTION BEST

&#8226 Resistant blackgrass costly to manage.

&#8226 Low yields and/or grain prices exacerbate effects.

&#8226 Plough, sow early October, and apply one herbicide to avoid or delay resistance.

&#8226 Possible savings up to £100/ha.

Jim Orson: Keeping resistant blackgrass off the farm can save growers up to £100/ha.