28 August 1998

Blackgrass disaster now a

manageable problem

Herbicide-tolerant blackgrass

is considered a major

headache capable of

wrecking arable farming

businesses. But life exists

beyond tolerant weeds, as

Edward Long reports

ALMOST 20 years ago the first herbicide-resistant blackgrass was confirmed on an Essex wheat growers farm. But what was once viewed as a disaster in the making is now considered manageable, thanks to a carefully devised control strategy.

The two-pronged approach involves cultural and chemical methods to achieve acceptable control.

"There is life after the discovery of resistant blackgrass on a farm," says John Sawdon of Peldon Hall Farm, Peldon near Colchester. "But there is also a cost involved. We have had to change cultivations policy, switch to later drilling – which reduces yield potential – and still spend money on herbicides which have been less effective than normal."

Target yield for the 600ha (1500 acres) of wheat, the only crop grown on the clayland farm, is 9t/ha (3.6t/acre). A third is Abbot, Hereward and Malacca for milling and the rest Brigadier, Equinox and Maverick for feed.

As wheat has been grown continuously for about 30 years, blackgrass is a major threat to yield. If left uncontrolled it could easily cut yield by a third, Mr Sawdon reckons.

The first hint of trouble came in 1980/81 when more of the weed survived chlortoluron treatment than expected. "This came as a surprise and was very disappointing. For two to three seasons we had taken a lot of trouble to get everything right in an attempt to reduce the infestation.

"We used appropriate nozzles, carefully calibrated our sprayer and ensured the timing was spot on to do the best possible job." But control was below 85%. "Everything else seemed to be right so there was only the chemical left to blame."

Blackgrass samples were sent for analysis and results confirmed resistance, believed to be the first in the UK.

"The first step of my recovery programme was to introduce rotational ploughing every three or four years." One possible explanation for continuing poor control is that it is difficult to kill blackgrass seedlings in the early autumn by ploughing unless they are inverted properly, notes Mr Sawdon. "We can only plough to the depth it will work on the day, which is usually no more than nine inches."

The next step was to delay drilling to encourage seed to germinate ready for spraying off with paraquat, and later glyphosate. Drilling had started on Sept 20, but was moved back three weeks to Oct 10.

"This works well, so long as there is sufficient rain in August and early September to germinate the seed. But blackgrass is reluctant to chit to order. In a dry time there is little advantage from drilling late."

During the 1980s the proportion of the farm ploughed was gradually increased so that by the turn of the decade all the land was turned over each year.

Straw burning was felt to be a useful tool to help get rid of unwanted waste and weed seeds. "Banning it was a loss from the weed control point of view, but it was obvious it was environmentally unsustainable. It is a shame it has been totally banned – limited rotational burning could help with the control of resistant blackgrass and other grass weeds."

In the early 1980s the appearance of resistance to several front line herbicides focused greater attention on chemical control. It was decided to mix and match a range of products to prevent such resistance developing at Peldon Hall Farm.

The first move, which was also needed to grow chlortoluron-susceptible varieties, was a switch to isoproturon, to which the farms blackgrass was more susceptible. The current approach involves tri-allate (Avadex) pre-emergence followed by post-emergence IPU.

"Avadex provides a clean start, killing blackgrass as it emerges and sensitising surviving seedlings to follow-up IPU. I am sticking to old chemistry as it cheaper than the newer products and I am trying not to induce the more complex target site resistance before the arrival of the next generation of blackgrass killers."

This season, with a more rational approach to pesticide use, Mr Sawdon reduced his herbicide bill by 40% – settling for a few more grass and broad-leaved weeds as a result. He doubts whether he will see a difference in yield.

"I would rather live with continuous wheat and put up with a few weeds than have weed-free crops in a rotation badly hit by take-all. The resistance of my blackgrass is a worry and although we can cope with it without suffering major loss I am looking forward to the new chemistry," Mr Sawdon concludes.

SAWDON STRATEGY

&#8226 Annual ploughing.

&#8226 Spray off chitted weed seed.

&#8226 Three-week drilling delay.

&#8226 Avadex + ipu herbicides.

&#8226 No-contact herbicides to

avoid target site resistance.

&#8226 Next generation blackgrass

killers eagerly awaited.

Herbicide-tolerant blackgrass can be held in check – but only by deploying all available weapons, says Essex farmer John Sawdon.