16 October 1999

A devil of

a dilemma…

Choosing a weed control strategy is challenging enough, but what happens when you know nothing about the history of the land? Heres how one grower solved the problem in the fourth in our series on tactics to control blackgrass.

JAMES BENNIE had two options. Batten down the hatches or expand and spread the fixed costs. Last year he opted for the latter.

Growing mainly wheat, barley and oilseed rape at Home Farm, Burton Latimer near Kettering in Northants, Mr Bennie took on a further 182ha (450 acres) of land last autumn under a farm business tenancy. Although he knew blackgrass would be a problem, he had no idea of its history, resistance or which chemicals would work best.

With the help of Aubourn Farming agronomist, Roger Davis, he split all 80ha (200 acres) of winter wheat into three equal parcels to try different herbicide strategies on each.

The logic was simple. By pitching the latest and most effective herbicides against each other, with different modes of action and combinations of residual and contact activity, it could be established in one season which approach worked best. The lessons could then form the basis of treatments next year.

Pre-emergence Avadex (tri-allate) was originally planned for some areas but was ruled out on grounds of workload, conditions and cost. Although Avadex can be useful to reduce populations of resistant blackgrass, it was reasoned that if populations were sensitive it could add unnecessary cost to blanket treat.

"My job is to get the best results at the lowest cost to give James top gross margin," points out Mr Davis. "We couldnt go spending £10/acre on Avadex, and another £3/acre applying it, if we didnt know the history of the land."

Instead, the plan was to get on when blackgrass was most susceptible to herbicides, at 1-2 leaves, and find out which post-emergence treatment worked best.

The wheat area was drilled with Hereward and Consort. The seedbeds were of equivalent quality before spraying commenced – a factor that might otherwise bias herbicide performance.

Drilling started in August on the light land using low seed rates of 60 seeds/sq m and increased slowly as autumn progressed, with the majority of the trial area drilled in mid-September.

Three strategies were compared based on:

&#8226 Lexus (flupyrsulfuron)

&#8226 Puma (fenoxaprop + isoproturon)

&#8226 Hawk (clodinafop + trifluralin).

Although primarily a mixer product, straight Lexus at 20g/ha was used to see how it performed against the contacts, explains Mr Davis. To fields where volunteer beans and oilseed rape were expected it was applied in mixture with 0.75 litre of Ardent (trifluralin + DFF).

Puma was applied at 3 litres/ha (equivalent to 0.75 litre/ha of Cheetah plus 1.8 litres/ha IPU). By adding in 2.4 litres/ha IPU 500 as well as 0.75 litre/ha of Grenadier (diflufenican + IPU) a total of 2,500g/ha of IPU was applied along with the fenoxaprop-P-ethyl.

Hawk was applied at 2.5 litres/ha, with a low rate of 1000g/ha of IPU as 1.25 litres/ha of IPU 500 and 0.75 litre/ha of Grenadier. Oil was also added, as recommended by manufacturer Novartis, at 1.0 litre/ha, and two fields with the highest blackgrass populations were also selected for Hawk, to provide a particularly tough test.

The results provided clear pointers for future herbicide use. Lexus gave 75% blackgrass control, which Mr Davis thought "phenomenol" given its price tag. But it also left autumn-germinating wild oats to contend with in spring, as well as annual meadowgrass. "At £6/acre Lexus is an exceptionally cost-effective base with which to build your herbicide strategy," he says.

Puma plus IPU gave poorer control. About half the blackgrass plants survived treatment, with just 50-60% control, but it did control autumn wild oats and annual meadow grass. "It looks, therefore, as if we have blackgrass resistance to fenoxaprop here," explains Mr Davis.

Resistance

"Puma has a definite place in weed control strategies on susceptible blackgrass. This situation illustrates why it is vital to use mixed chemistry. There appears to be fenoxaprop resistance here, but it could equally be clodinafop resistance somewhere else," he adds.

   Hawk control averaged 97%, with autumn wild oats also controlled. Where blackgrass control had been poor, its presence seriously impacted on yield.

  A minimum 96% control of blackgrass is needed, otherwise there is too much seed carryover and serious reductions in yield, adds Mr Davis.

The blackgrass seeds have been sent off for resistance testing, but results will not be back until later this month. Irrespective of the outcome it will be essential to hit blackgrass early and hard because of the sheer size of the population – 1,000 plants/sq m.

"I suspect well probably start off with Prebane (terbutryn) or Avadex Excel as a base treatment, simply to sensitise the blackgrass to later sprays and pull down some of the initial population. This is after weve burned off stale seedbeds with glyphosate.

"Each farm and blackgrass population is different. But whatever the situation, every step must be taken to adopt this early and hard approach."

With this in mind, he says, follow-up sprays will be based on sequences of Lexus and Hawk at varying rates. The former because it is a cost-effective base, and the latter to get control even closer to 100%.

Pendimethalin (Stomp) is useful to vary the chemistry – and it boosts blackgrass control as well as controlling meadowgrass. "On difficult blackgrass sites like this Lexus and Hawk at 1-2 leaf blackgrass stage deliver the goods."

In addition, set-aside will be put on the worst blackgrass areas for the coming season, he says, giving a chance to tidy them up culturally and chemically. Oilseed rape will also form part of the future rotation, giving the opportunity to use a contact graminicide plus propyzamide to provide differing modes of action.

"Well also aim to reduce blackgrass populations by creating false seedbeds and burning off with glyphosate. Attention to detail is key with blackgrass control. Choosing the most effective product is important and then using it along with as many non-chemical control methods of attack as possible," adds Mr Bennie.