General principles of herbicide resistance

Slender winged thistle
Carduus pycnocephalus
developed resistance
to MCPA and 2,4-D in
the 1980s.

K.C. Harrington
Massey University, Private Bag 11-222, Palmerston North, New Zealand

Development of resistance

Herbicide resistance is much less common than fungicide or insecticide resistance. Most weed species complete their life cycle only once each year, whereas fungi and insects often complete many life cycles within one year. This means resistance genes build up much more quickly within fungal or insect populations than with weeds. In addition, following applications of insecticides and fungicides, most susceptible individuals within a sprayed area are killed, leaving only resistant individuals to multiply. In contrast, because herbicide applications do not kill dormant weed seeds, weeds that lack resistance genes are not easily removed from the population and this helps to delay the onset of resistance.

Resistance can eventually build up if herbicides are applied to a weed population many times in succession. Herbicide resistance has developed to triazine herbicides, such as simazine and atrazine, in many countries. This is because the persistence of these residual herbicides allows them to continue killing susceptible individuals, when the dormant seeds germinate, for many months after application. If these herbicides have been applied year after year, such as in maize crops, orchards and nurseries, resistance has finally built up.

Managing herbicide resistance

While resistance to herbicides is still quite rare, it is good sense for farmers and growers to check they are not unwittingly producing populations of herbicide resistant weeds on their properties. It can take many years for herbicide resistance to build up, but some farmers have been using herbicides for a long time and may already have weed populations reaching the point where resistance will soon appear.

In areas sprayed with the same herbicide for many years, it is quite common for populations of weed species that were never susceptible to that herbicide to build up. For example, using simazine every year in orchards results in a build up of paspalum (Paspalum dilatatum), mallow (Malva spp.), summer grass (Digitaria spp.), docks (Rumex spp.) and other naturally resistant weed species. This build up of species that are not controlled by a herbicide is often confused with the development of resistance. However, both problems can be prevented, and overcome once they have occurred, using similar management strategies.

Resistance to agrichemicals has usually occurred around the world in areas where the same crop is grown every year. This results in a build up of disease, insect and weed populations specific to that crop. Successive applications of the same agrichemicals are used every year to control the disease, insect and weed populations, and this practice can result in the development of resistance.

Arable cropping in New Zealand generally differs from that in many other parts of the world. Crop rotation is practised widely in this country, with different crops being grown each year. Pasture is also often grown for several years as part of the arable crop rotation. These practices prevent the build up of pest and weed populations and also result in different agrichemicals being used each year for each paddock. Crop rotation is often advocated overseas to prevent resistance from developing and is probably one reason why so few cases of herbicide resistance have been found here.

Apart from crop rotation, the best way of avoiding problems with herbicide resistance is to alternate the types of herbicides used within any one paddock over time. Herbicides from different group codes should either be used alternately or mixed together to prevent the numbers of resistant individuals building up. These group codes can be found by clicking here.

Specific examples of avoiding and dealing with herbicide resistance problems for situations where resistance has already developed in New Zealand, or is likely to develop, are given on the following pages: