The earliest methods of weed control were hand-weeding or burning. Weeding by hand is adequate for managing small areas of crop or pasture, but larger areas require faster methods. Large areas of hill-country scrub, especially mānuka, were initially cleared by hand, but later, machinery was used to crush scrub before burning it.
Organic farmers manage weeds with steam, burners or mechanical weeding.
The chemical control of weeds has always been seen as an attractive solution because it is relatively cheap and easy.
- Dangerous chemicals such as arsenic were used in the 1920s and 1930s to control many weeds. Today a licence is required to use arsenic compounds.
- Salt was tested for control of gorse and other species.
- Sulfuric acid was used to selectively kill weeds in cereal crops.
- Sodium chlorate was used extensively in New Zealand between the 1930s and 1970s, especially as a spot treatment for ragwort.
- Sinox, the first organic (carbon-based) weed-killing compound, was first used in the 1930s for selective control of weeds in crops, notably peas.
The chemical revolution
The most dramatic changes in herbicide use came after the Second World War, when the phenoxy herbicides 2,4-D and MCPA were developed. They were effective against broadleaf weeds in lawns, pasture and cereal crops like wheat, barley and maize. These chemicals are still used today to kill thistles and ragwort in pastures, and as ingredients in solutions to combat weeds in cereal crops. The herbicide 2,4,5-T was applied to gorse and other scrub until the late 1980s. Its withdrawal followed public concern over its use as an ingredient in Agent Orange, a defoliant used extensively in Vietnam. This was associated with birth deformities, cancer and many other illnesses thought to be caused by dioxin, a toxic and carcinogenic by-product of its manufacture. In the early 2000s, controversy still surrounded the Dow AgroSciences plant in New Plymouth, where 2,4,5-T and other phenoxy herbicides were made. Local residents claimed ongoing health problems resulting from contamination.
The main drawback of using sodium chlorate as a herbicide was its tendency to catch fire when exposed to heat. Overalls or work clothes soaked in the chemical would burst into flames if dried in front of a fire even after being washed. Today, common salt (sodium chloride) is added to reduce inflammability).
Many new products were developed in the 1960s and 1970s. Among them was Roundup, or glyphosate, probably the most widely used of all the herbicides. Although not selective, it kills the whole of a treated plant – the roots as well as the above-ground parts. When organo-silicone was added to Roundup, its uptake by scrub weeds like gorse was improved.
Developing and testing new herbicides is expensive. Few new ones now appear on the market, and many older brands are no longer available. Herbicides are likely to continue being used for many years to come.
Millions of dollars have been spent killing scrub weeds, but unless there is adequate follow-up with fertiliser, pasture seed, fencing and good grazing, the cleared areas will quickly revert to weed. Gorse in particular regrows rapidly from seeds in the soil and from stumps.
Weeds can become resistant to chemical herbicides. This occurs when a small number of individual plants develop a genetic makeup that means they are not killed by the herbicide. When the susceptible plants have been killed, the resistant ones multiply and, if herbicide application continues, come to dominate the population.
The first species in New Zealand known to become herbicide-resistant, in the late 1970s, were the crop weeds fathen and willow weed. Resistance was discovered in the 1980s in two pasture weeds repeatedly treated with 2,4-D or MCPA – nodding thistle and giant buttercup. New Zealand has fewer cases of herbicide resistance than many other countries, but new cases appear every few years.
Weeds in New Zealand known to have developed resistance to herbicides by 2007 were:
- fathen (Chenopodium album)
- willow weed (Persicaria persicaria)
- nodding thistle (Carduus nutans)
- giant buttercup (Ranunculus acris)
- black nightshade (Solanum nigrum)
- slender-winged thistle (Carduus pycnocephalus)
- Chilean needle grass (Nassella neesiana)
- chickweed (Stellaria media).
Biological control (biocontrol) uses the natural enemies of a weed to keep it in check, and is seen by many as the long-term solution to weed problems. This method has been effective in reducing, but not eliminating, ragwort and St John’s wort, which are poisonous to farm stock.
The spread of nodding thistle may also have been deterred by biocontrol agents. Those released include the nodding thistle crown weevil (Trichosirocalus sp.), the nodding thistle gall fly (Urophora solstitialis), and the nodding thistle receptacle weevil (Rhinocyllus conicus). Attempts are being made to find biocontrol agents for many weeds, including gorse, broom, Californian thistle, Chilean needle grass and nassella tussock.