Herbicide resistance in weeds

poppy in oilseed rape

Herbicide resistance in grassweeds and broad-leaved weeds

Herbicide resistance definition:

‘Herbicide resistance is the inherited ability of a weed population to survive a rate of herbicide that would normally kill it.’

Herbicide resistance is already widespread in grasses, with resistant blackgrass in particular being increasingly difficult to control, to the extent that it has come to threaten viable cereal production on some farms, predominantly in eastern England. It is now generally accepted that some degree of resistance is found on virtually all farms on which blackgrass herbicides have been regularly used. Herbicide resistance in broad-leaved weeds is also now found in the UK, with three key broad-leaved weeds – common poppy, chickweed and scentless mayweed, having confirmed resistance to the ALS-inhibiting group of herbicides. Although resistance has only been detected in these three species in the UK, worldwide experience shows that resistance could evolve in many other broad-leaved weeds too, so vigilance is needed. Resistance in broad-leaved weeds needs to be taken seriously, to try to avert a situation as severe as in blackgrass.

There are different mechanisms of herbicide resistance:

Target-site resistance - due to a modification in the herbicide binding site (usually an enzyme) which precludes a herbicide from effectively binding.

Enhanced metabolism resistance – results in herbicide detoxification.

Herbicide resistance was first identified in the UK in blackgrass in 1982. Resistance in Italian rye-grass is also widespread, though is currently less problematic than with blackgrass. It also affects wild-oat, though currently, resistance tends to be more localised than with blackgrass and rye-grass. Three main types of resistance are present in UK grass-weed populations – enhanced metabolism, ACCase target site resistance and ALS target site resistance.

In poppy, chickweed and mayweed, ALS target site resistance has been confirmed in all three species. Enhanced metabolism has not been found in broad-leaved weeds in the UK.

Herbicide resistance in broad-leaved weed populations is probably underestimated, and farmers should ensure they are mixing modes of action when applying herbicides to prevent the situation worsening. Broad-leaved weed resistance was first confirmed in England in 2000, and the latest data show that almost 100 farms were affected by 2011 (see table). Another survey is being conducted this year, and experts predict that the situation will have worsened.

 

Chickweed

Poppy

Mayweed

Resistance first found

2000

2001

2002

Farms

>40

>25

5

Counties

13

9

3

Comment

Mainly Scotlandand N. Ireland

Limited, but increasing

Limited

Source HGCA Managing weeds in arable rotations a guide, updated Spring 2014

Herbicide resistant chickweed is largely confined to Scotland, where a predominantly spring cropping pattern is practised, and ALS-inhibitors have been used for weed control year on year. However, with the increasing blackgrass problem, many growers in England will be switching to spring cropping, particularly spring barley, as an aid to blackgrass control, but this could lead to an increased risk of broad-leaved weed resistance developing. Herbicide resistant poppy populations are also frequently found, especially in eastern England.