21 February 1997

Targeting and timing pays

Broad-leaved weed control can benefit greatly from a more detailed approach. Amanda Dunn reports on recent ADAS research

FARMERS should take a more scientific and economic approach to weed management, argues Derek Wade, research consultant at ADAS Rosemaund.

"Weed problems should be identified and then targeted using mixes and rates that have potential to offer yield benefits," he says.

"Even today, were still seeing a strong element of pride influencing decision making. Farmers are looking for a completely clean crop and apportioning, sometimes inappropriately, a significant amount of their chemical spend to autumn applications in a bid to achieve this."

It may be possible to adjust chemical strategies by identifying the specific weed problem in the autumn and then targeting that weed specifically, he suggests.

Low numbers

"Alternatively, if the weed identified is present in low numbers, poses little threat to the growing crop and is unlikely to cause contamination in the harvested sample, then it may be viable to concentrate control to a spring application."

Adopting an economic approach of identifying the problem and then targeting control is complemented by introducing a scientific angle to the equation.

"New trials work that ADAS is carrying out shows quite clearly how timings of different chemicals can offer the farmer a greater financial benefit," says Dr Wade.

The work, carried out on behalf of AgrEvo at ADAS Rosemaund, looks at cleaver control in winter wheat and the choice between fluroxypyr (Starane) and amidosulfuron (Eagle).

"Cleavers is the number one weed problem that farmers face in the spring," explains Peter Hill, senior scientific officer at ADAS Drayton. "Autumn herbicide applications may well knock the problem, but if poor timings reduce efficacy, there may still be a need for overspraying in the spring."

Research conducted in the early 90s shows how critical it is to get the timing and dose of Starane right, he says. By being more precise about applying the chemical in ideal conditions – a warmer temperature, with the weed in active growth – better control is achieved and economic advantage results.

"The new era of chemicals has seen the introduction of Eagle which offers reliable control of cleavers under different conditions. Eagle works best in lower temperatures before the weed becomes active."

The two chemicals complement each other, so farmers need to consider the conditions, look at the research data and make a decision about which spray to use, he advises.

Break crops such as pulses and oilseed rape offer little scope for spring control of cleavers and other broad-leaved weeds, says Mr Hill. "Although benazolin-ethyl, as in Galtak, is taking us towards a reasonable strategy for cleavers control in oilseed rape, it is imperative farmers focus their energies on good, early establishment of the crop.

"Quick autumn crop growth will achieve good ground cover. A full leaf canopy will restrict light and reduce potential weed establishment."

Significant increase

Over the past two years trade data shows a significant increase in the autumn use of herbicides in cereals, says Dr Wade. "Were probably at the peak of that momentum now. But with the potential introduction of restrictions on IPU-based products, we also have environmental considerations that will affect our present strategies.

"We may start to see a shift away from residual acting autumn chemicals towards more specific, contact acting herbicides, with a knock-on effect changing the emphasis towards spring control of broad-leaved weeds.

"Last year saw the approval of new tank mixes of sulfonylureas. We are looking at the yield benefits of different combinations and different application rates of sulfonylureas. We hope to have results from these trials by next season." &#42

Oilseed rape offers little scope for spring weed control, says ADAS.


Yield summary t/ha

StaraneEagle

1.0l/ha40g/ha

Mid Mar9.239.26

Early April8.909.07

Mid April9.009.21

Early May8.838.81

Mid May8.688.70

Early June8.658.31

Untreated8.348.44

Average8.818.33

Yield potential protected by herbicide application from

mid-March to mid-April

Source: ADAS Rosemaund


CROP EQUIVALENTS

Number of cereal plants equivalent to one weed, in terms of competition for light, water and nutrients.

cleavers7.2

mayweeds0.6

chickweed0.5

field speedwell0.5

ivy-leaved speedwell0.3

shepherds purse0.1

fat hen0.1

fumitory0.2

knotgrass0.2


Cleavers control in winter oilseed rape using Galtak 1.5l/ha

Plottreated/screeningsyield

untreated%adjusted for

screenings t/ha

1T1.132.92

2U9.0262.75

3T0.9032.75

4U12.8342.89

5T0.562.95

6U3.3313.00

Galtak showed useful suppression of cleavers in observation plots when applied in February 1996. Results confirm reduction in cleaver seed contamination of harvested seed.

Source: ADAS Rosemaund


AgrEvo cleavers control in winter wheat trial 1995/96

Weed assessment summary –

% cleaver control 22/07/95

Spray timingStaraneEagle

1.0l/ha40g/ha

Mid Mar98.33100.00

Early April98.3398.33

Mid April100.00100.00

Early May100.0096.67

Mid May90.0056.67

Early June80.0076.67

Average81.6775.48

Control of cleavers by Eagle better at earlier timings and by Starane better at later timings.

Source: ADAS Rosemaund