1 September 2000



Battling with herbicide-resistant blackgrass is no easy matter.

Here Edward Long talks to a Cambs grower who believes he

has developed a strategy to combat the problem weed

FIELD-SCALE evidence this season and trials in 1999 suggest a Northants heavyland arable farmer has found a strategy for coping with resistant blackgrass, at least for the next two seasons.

"We can never win the battle against this insidious grass weed, but we can buy ourselves a little time by using the appropriate chemical and cultural control methods," says Tony Webb, who farms 161ha (398 acres) of sticky clay at Grange Farm, Lutton, near Oundle.

"But whatever works now is unlikely to still be effective two or three seasons down the line. All we can hope for is a little breathing space to allow infestations to be reduced so some decent yields can be harvested."

Constant threat

Blackgrass is a constant threat at Grange Farm, where a lack of field drains can provide near-ideal conditions for the weed in a wet season and compromise the effectiveness of chemical control.

A severe infestation knocked wheat yield from 8.75t/ha (3.5t/acre) to about 2.5t/ha (1t/acre) in a season when control was difficult to achieve.

Although, infestations have been gradually reduced over recent seasons, Mr Webb stresses that surviving severe blackgrass pressure is an constant struggle. "I cannot afford to ease back, as this would undo all the good achieved over the past decade."

For 15 years he had suspected resistance problems. After Cheetah (fenoxaprop-P-ethyl) and Topik (clodinafop-propargyl) failed to provide worthwhile control in the early 1990s he feared target site resistance might be to blame.

But testing revealed the less serious enhanced metabolic resistance. "That can also neutralise Cheetah and Topik, and, as Laser had worked reasonably well in rape, the diagnosis seemed right."

Even before resistance was confirmed it was decided to reintroduce Avadex (tri-allate) as the first chemical control for all cereals.

Initially it was used in conjunction with a follow-up IPU or chlortoluron spray. The double-barrelled approach worked well and held the line against the aggressive weed for a few years.

When it started to run out of steam other chemicals such as Hoegrass and, later, Cheetah were used.

"When Cheetah started to fail I decided to change the cropping. We had been in continuous cereal production, but needed a break to provide an opportunity to hit blackgrass, which has exploited the straw burning ban, and the increasing brome problem, with another chemical. We chose oilseed rape. The first crop was grown here in the early 1990s on a planned 1:6 rotation."

More chances

But four years ago the interval was reduced to 1 in 3 to provide more chances to hit blackgrass.

In 1999 Avadex was followed by Lexus (flupyrsulfuron-methyl), Hawk (clodinafop-propargyl + trifluralin) and oil on continuous or second wheat where pressure from the difficult-to-control weed was high. The chemical combination cost £45/ha (£18/acre) without the Avadex.

Control levels were compared with a similarly priced mixed of Lexus plus Stomp (pendimethalin) on first wheat where the grass weed pressure was less, and with IPU + Treflan + diflufenican costing £28.50/ha (£11.50/acre).

"Results from the Hawk/Lexus combination after Avadex were variable," says Mr Booth. "It did a good job in one field, but control was barely acceptable in another. In the spring the Lexus/Stomp and IPU treatments looked equally effective, but by harvest more blackgrass had appeared where the IPU was sprayed, so the Lexus/Stomp partnership was the better of the two, and the one chosen for this season."

In this years Regina winter barley IPU plus Stomp costing £37/ha (£15/acre) provided adequate control. Mr Webb realised it was insufficient to prevent an infestation build-up, but early Laser and follow-up Kerb, or equivalent, could provide an effective clean-up in the following crop of rape.

Stale seed-beds

The main plank of the cultural component of blackgrass control strategy is a stale seed-bed. The farm has not been ploughed for over 30 years so the dummy seed-bed is prepared with a disc and press working on Flat-Lifted land.

Weeds are then sprayed off with the glyphosate at least three days before the drill is due to move in.

That provides the start needed to give the crop an advantage and boosts the chances of successful chemical control.

The chemicals chance of doing a good job is also boosted by spraying early. Mr Webb used to spray at the 3 plus-leaf stage. But by then the weed was fairly robust, so control was patchy.

Smaller weeds are far more vulnerable to chemical attack, so the Grange Farm sprayer now gets to work at the 1-2 leaf stage. The use of a glyphosate clean-start reduces the risk of uneven weed growth.

"Both Lexus and Stomp have some residual activity, so this tactic is reasonably safe. Although we have not ploughed here for over 30 years a neighbour did plough a trial strip for us last season. I was unimpressed, as weeds emerged over a longer period and I had to spray my mainline treatment twice.

"My attitude to blackgrass over the past year has also changed. In an era of low cereal prices we must achieve high yields, so I cannot afford to tolerate the level of infestation suffered in the past. I realise that although I have hit on a good control strategy, there is little real chance of the chemicals providing effective control for long," Mr Webb says. &#42


&#8226 Blackgrass resistance confirmed.

&#8226 Avadex reintroduced to help.

&#8226 OSR introduced in early 1990s.

&#8226 Chemical control mixes actives.

&#8226 Minimum tillage 30+ years.

&#8226 Stale seed-bed/glyphosate clean start.

Ha (Acres)

&#8226 Farm size 161 (398)

&#8226 Wheat (Abbot) 59.5 (147)

&#8226 W barley (Regina) 27.5 (68)

&#8226 W OSR

(Pronto/Escort) 50 (124)

Combating herbicide resistant blackgrass has demanded an all-weapons approach at Grange Farm, Lutton, Cambs where Tony Webb believes he is finally getting the troublesome weed under control.

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