Herbicides

Page last updated: Wednesday, 11 November 2020 - 9:16am

Movement of herbicides in the environment

Herbicides have the capacity to move in the environment away from the target area, and to cause damage to non-target plants and animals. There are management options to reduce the possibility of this happening.

Herbicides in plants

A certain proportion of any selective herbicide applied to a crop will be absorbed by crop plants and by tolerant weeds rather than by those weeds which the herbicide is intended to control. These plants are able to tolerate the herbicides because various enzymes within them metabolise the chemical before it can cause permanent damage. Thus this portion of the herbicide is removed from the environment and destroyed.

Susceptible weeds are not able to metabolise the herbicide, at least not quickly enough to avoid being killed. As the plant dies however, its cells rupture and release a range of oxidising enzymes. These enzymes destroy much of the plant tissue, and the herbicide as well.

Thus this fraction of the herbicide also is removed from the environment.

Herbicides in the atmosphere

Most herbicides in the atmosphere get there as a result of fine spray droplets not settling but drifting and finally evaporating. All herbicides have the potential to do this. Some are also sufficiently volatile to enter the atmosphere by evaporating (volatilising) from the surface of sprayed plants or from the soil eg. Trifluralin and the volatile ester formulations of 2,4-D.

The potential danger herbicides in the atmosphere pose to nearby plants is only short term. Ultraviolet light striking a herbicide molecule has sufficient energy to disrupt some of the bonds between its constituent atoms destroying the molecule. Rates of breakdown for any compound will depend upon the intensity of sunlight.

Degradation of herbicides in soil

Many herbicides that are used in one year of a typical cropping rotation have the potential to damage crops grown in other years of that rotation. Fortunately, in an average year these herbicides are broken down (degraded) into harmless compounds by a combination of biological and chemical processes and do not cause any problems for the crops.

From the moment a herbicide enters the soil system, it begins to break down. Most of this degradation is due to the action of microscopic living organisms. These organisms break down the organic material in the soil to provide the energy they need to live (it is their food supply). When a herbicide is added to their environment, it also becomes a potential food supply. Some herbicides are also prone to chemical reactions which alter their structure and render them non-phytotoxic.

These degradation processes all depend upon soil temperature and moisture levels. They increase in direct proportion to soil temperature, and begin when the moisture level rises above the wilting point. Extra moisture increases the rate of degradation but not by as much as higher temperatures.

Triazines (atrazine and simazine), sulfonyl ureas (Chlorsulfuron, Triasulfuron etcetera), imidazolinones (Spinnaker®, Raptor®, OnDuty®, Midas®) are all candidates for carryover. All other things being equal, imidazolinones will be more persistent on acid soils and sulphonyl ureas on alkaline soils. Triazines are very slightly more persistent on alkaline soils.

Any paddock where any of these products were applied in June or later (especially in a year with low rainfall after application) should be regarded as having potentially damaging levels of residues, and so should areas of earlier applications if there were significant periods when the soil surface was dry.

Movement of herbicides by soil or leaching

  • Soil may be left bare as a result of cultivation for cropping, or by destruction of vegetation by herbicides on firebreaks and roaded catchments. If wind or water moves this soil, any herbicides in the soil will be moved also, and may damage desirable vegetation wherever it settles.
  • As rain water moves down the soil profile from the surface, herbicides in the soil will move with it to some extent. This leaching is a dynamic process, whereby the herbicide alternates between being dissolved in the water and therefore moving with it, and being adsorbed onto soil particles and therefore immobile. The rate of movement of any herbicide down the profile therefore depends on its solubility in water and the strength with which it absorbs onto soil particles.
  • Later in the season, when net water movement is back toward the drying surface soil, it is possible to have previously leached herbicide moving up from sub-soil with this reverse water flow.

Residue effects

Triazine residues will vary in the damage they cause according to the seasonal conditions. Residue effects will be much less when the season start is uniform and rainy compared to dry. Root disease will exacerbate the effect of triazine residues as the young seedlings cannot grow away from the residues which are concentrated in the cultivation layer.

Sulfonyl urea and imidazolinone residues are less affected by soil moisture as they are more soluble. The first effect of the sulfonyl ureas is to prune roots. Be careful in duplex soils that have sand over alkaline clay. The sulfonyl ureas can leach down to the clay where they will be more persistent due to the high pH.

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