28 August 1998

New thresholds guide economic weed control

By Louise Impey

NEW economic thresholds for blackgrass, cleavers and chickweed control in winter cereals have been produced by IACR-Rothamsted in time for this autumn.

A three-year MAFF-funded study has identified the weed populations which make herbicide control worthwhile. For blackgrass, 18 plants a sq m in the autumn means treatment is warranted. With cleavers the figure is 9, while chickweed spraying should begin when 23 plants are present.

In the spring, these thresholds change to 33 blackgrass, 7 cleavers and 11 chickweed.

"Clean fields are not the way ahead with weed control," says John Cussans of Rothamsted. "Growers must decide on the target weeds which affect yield and economics must drive the decision."

His calculations assume a weed-free yield of 7.5t/ha (3t/acre), a grain price of £70/t and an application cost of £6/ha (£2.40/acre). Current herbicide costs are also included. "Now we have three years of data, it is easy to produce new thresholds as prices change."

The three weed species investigated represent significantly different growth habits. "Chickweed is an early, opportunist weed, black-grass most closely resembles the cereal plant, and cleavers have a totally different pattern of growth and competition to every other weed.

"But they all respond to the environment and to changes in spray timing and nutrient status."

One of the biggest problems facing cereal growers is that they must make weed control decisions early in the season, Mr Cussans believes. "Most of the yield loss caused by these weeds comes after the spring spray timings. The longer you delay a spray decision, the more likely it is to be worthwhile."

The Rothamsted work also looked at the factors which influence and increase the competitiveness of different weeds. "Blackgrass becomes more competitive as residual soil nitrogen increases, which is why growers have had a bigger problem with it this year. High total sunshine and summer temperature also make it more troublesome.

"Chickweed and cleavers are worse on heavier soils – which is probably due to moisture retention, and they are both more competitive if they emerge early."

Cleavers are also encouraged by high sunshine levels, Mr Cussans says.

"But cleavers is a strange weed. It is adapted to shade and grows extremely well under a crop canopy, but at a certain stage in the season is becomes shade averse and grows above the canopy. That is why weeds that are tiny in May and often dismissed become a problem at harvest."

The weather also has a part to play, he says. "In hot summers, for example, chickweed senesces and dies away. But overcast, grey days suit it perfectly and it carries on growing. Different thresholds for wet and dry years could be a logical extension to the work already done. In a wet season the weeds just keep on emerging."

Although Rothamsted has come up with thresholds, Mr Cussans cautions farmers about their use. "Each site is different, and growers must consider the history of the field as well. For some, the cleavers threshold has to be at the level of detection.

"The other consideration is seed production. Managing cleavers populations on yield thresholds alone is difficult because of the efficiency of its seed production." &#42

New weed thresholds for (L-R) cleavers, blackgrass and chickweed can improve herbicide targeting. Delaying application as late as practically possible will improve efficiency further, says IACR Rothamsteds George Cussans.

Economic spray thresholds

Plants a sq m

Autumn Spring % yield

loss

Blackgrass 18 33 4.2/8.4

Cleavers 9 7 6.7

Chickweed 23 11 4.7