Knowledge Hub Article

Black-grass - Economic Impact

Economic Impact

Damage

Black-grass can seriously reduce arable crop yields through competition for nutrients, especially nitrogen.  Effects on crop yield vary greatly due to the relative competitiveness of crop and weed - a competitive crop can effectively suppress black-grass.   Typical losses are shown in the table – but much higher losses are likely in uncompetitive crops.

Typical yield losses from black-grass in winter wheat crops

Black-grass plants/m2

Typical % crop yield loss

12

5

25

10

50

15

100

20

250

35

500

50

 

A weed density that causes a 5% yield loss is often used as a threshold value at which herbicidal control is justified to prevent unacceptable loss.  For black-grass in winter wheat this is, on average, 12 plants/m2.

However, because of the risk of higher yield losses, the potential for rapid population increase and the difficulty of assessing weed densities on a field scale, applying herbicides to control black-grass at much lower densities is advised – even at less than 1 plants/m2 in high risk situations.

 

Why is black-grass a problem in the UK?

There are three main reasons:

  1.        Increase in autumn sown crops, especially cereals and oilseed rape, which greatly favour black-grass due to its early autumn emergence pattern.
  2.        Earlier autumn sowing – in 1975 only 5% of winter wheat was sown in September; in recent years this proportion has increased to 50%.
  3.        Herbicide resistance – some degree of resistance now occurs on virtually all farms.

 

Herbicide resistance

First found in the UK in 1982, herbicide-resistant populations now occur on virtually all of the 20,000 farms where black-grass occurs.  Populations show variable degrees of resistance to a wide range of different modes of action, with both ACCase and ALS target site resistance (TSR) and non-target site resistance (NTSR), especially enhanced metabolism, now widespread.

 

Type of resistance

Mechanism

 

Non-target site – principally enhanced metabolism

Results in herbicide detoxification and is the commonest resistance mechanism in black-grass It affects most herbicides to varying degrees, but only in severe cases results in complete loss of control.  Tends to increase slowly.

 

ACCase target site

Blocks the site of activity specific to ‘fop’, ‘dim’ and ‘den’ herbicides.  It only affects these groups of herbicides, but often results in very poor control and can increase rapidly.

 

ALS target site

Blocks the site of activity of sulfonylurea and related herbicides.  It only affects these groups of herbicides, but often results in poor control and can increase rapidly.