Hormone’ weedkillers major New Zealand herbicides

Press, 20 April 1979, Page 8

Hormone’ weedkillers major New Zealand herbicides

By R. W. MOFFAT. Although a wide range of agricultural chemicals have been used in NewZealand over the past 30 years to control weeds in all types of situations, the major chemical group, both in widespread use and volume, has been the phenoxy or “hormone” weedkillers. The phenoxy herbicides, as they are commonly known, are based on various phenoxy alkanoic acids. These herbicides were discovered almost 40 years ago incidental to research on naturally occurring plant growth hormones. The phenoxy herbicides are weak acids only slightly soluble in water and petroleum oils. AI- • hough the acids are the biologically active form, 'hey are normally converted to water soluble amine salts, alkali metal -.alts or oil soluble esters for convenience in handling and application. These

are mixed with other ingredients such as emulsifiers, w-etting agents and solvents to make commercial formulations for specific uses. Esters generally have greater herbicidal activity than water soluble salts. In New Zealand today the butyl and isooctyl esters are the two major oil soluble formulations available with the potassium and sodium salts as well as the diethanolamine salts being the main water soluble formulations. Although many phenoxy alkanoic acids have some degree of biological activity most are not commercially available due to their limited effectiveness or cost. The ones presently used in New Zealand are 2,4-D, MCPA, 2,4-DB. MCPB, dichlorprop, mecoprop and 2,4,5T. Of these 2,4-D and MCPA are used for control of a wide range of broadleaf weeds in pas-

posed to it in small tures and cereals. The butyric herbicides, namely 2,4-DB and MCPB are used only in those situations where a high degree of legume selectivity is required, as they are less active and more expensive than 2,4-D and MCPA. The choice of herbicide depends on the predominant weeds present as well as the selectivity to the crop. Dichlorprop and mecoprop are primarily used in mixtures with other herbicides for control of broadleaf weeds in cereals and turf to broaden the spectrum of weed control. The herbicide 2,4,5-T is the predominant herbicide used for the control of brush weeds such as gorse, blackberry, broom and sweet brier, which are of considerable significance to the New Zealand economy. The phenoxy herbicides are absorbed by plant foliage, stem tissue and

roots, although the latter pathway is not considered very important in their over-all commercial use. Once absorbed, the phen-' oxys move within the plant (translocate) along the pathways that carry food and water to accumulate in actively growing parts of roots and stems. The degree of translocation within the plant varies considerably. Movement of 2,4-D is quite considerable within annual broadleaf weeds such as thistles but all phenoxys have very restricted movement within gorse and most other woody perennial plants. Leaves and buds twist and curl and new growth of stems and leaves is malformed. There is interference with cell division and enlargement, with food utilisation and a number of other vital processes. The exact mechanism of'these actions is not known, as is exact function of natural plant growth regulators not clearly identified.

Despite the phenoxy herbicides having been widely used for the last quarter century they have not been substantially re-, placed by more recently developed herbicides. A number of these more recently developed herbicides such as picloram, dicamba and bromoxynil are now used in combination with the phenoxy herbicides to improve the final end result but in most instances the original phenoxy herbicides remain the “work horses” for weed control. The phenoxy herbicides have maintained their ’ dominant position due to being the most cost effective for the particular job and usually less hazardous and less injurious to the environment than alternative methods. The use of herbicides is now widely accepted as one of the operations essential to the efficient production of food and fibre. In the U.S. the use of the phenoxy herbicides is estimated to reduce the cost of production of crops on which they are used by 5 per cent and to reduce the over-all production costs the country by one per cent.

As with the use of all chemicals in our daily lives, there are potential hazards in the use of phenoxy herbicides, and a number of clearly defined procedures have to be followed to minimise these. The terms “hazard” and “toxicity” are often confused. Everything, including water, is toxic if one consumes or is exposed to it in large enough amounts. But nothing is hazardous

if one consumes or is exenough amounts. The phenoxy herbicides are predominantly toxic to green plants and are much less toxic to mammals, birds, fish, reptiles, shellfish, insects, worms, fungi and bacteria.

The phenoxy herbicides are.considered moderately toxic to mammals. The acute oral toxicity for mammals varies depending upon the test animal and the particular chemical or formulation. The herbicides are absorbed after ingestion, transported via the plasma, concentrated in the kidneys and rapidly excreted in the urine, largely in the original chemcial form. The derma! toxicity for mammals is low and the toxicity to fish highly variable and primarily associated with the oil soluble formulations. The highly toxic TCDD dioxin, an unavoidable contaminant of 2,4,5T is present in commercial formulations at such low amounts as not to alter the toxicological properties of the formulation or to endanger human health or to affect plants or animals in the environment. All 2,4,5-T used in New Zealand is required by law to contain less than 0.1 ppm of TCDD dioxin and in practice the levels are significantly below this concentration.

Applicators do not have to carry out any special handling procedures when using the phenoxy herbicides except the normal common sense precautions when handling chemicals. These include avoiding contact with the skin and inhalation of spray mist washing exposed • skin after use. Depending on the type of application, the dilution of the product concentrate with water is in the order of 1:20 to 1:300.

It has sometimes been wrongly held that all plants treated with phenoxy herbicides are toxic to livestock. All phenoxy herbicides used on pasture carry the label statement that after spraying, poisonous species may be made more palatable and stock should be kept from the area until plants have died down. Well known poisonous plants in pasture are tutu and ragwort.

When phenoxy herbicides are apnlied, residues occur on the vegetation and soil. In the course of susceptible plants dying down, some residues remain to move onto the soil, although the majority of the residue on the plants decomposes as the plants rot. The herbicide remaining in the surviving vegetation is broken down over a few days to a few weeks. Forage from pastures and range type vegetation treated with 2,4-D, 2,4,5-T and MCPA has initial residues in and on the plant material of approximately 100 ppm per kilogram of chemical applied per hectare. Forage residues decline with a half-life of one to two weeks. Thus, within two to four weeks levels have fallen into the range of one-half to one-sixteenth of the initial value. Domestic and wild grazing

animals tolerate up to 2000 ppm of phenoxy her-, birides continuously in feed and larger amounts for short time so there is no hazard to. animals from their use. Residues of the p h e n o x y herbicides leached into the soil are bound to some extent by soil particles and are held up in the surface organic zone of the soil. They do not leach into ground water nor move significantly into streams or adjoining areas from surface run-off.

The phenoxy herbicides are decomposed by variou s mi c r o-organisms which are universally present in soils. As an cation of the rate of breakdown in the soil, MCPA is essentially destroyed in six weeks while 2,4,5-T takes approximately three months. Most of New Zealand, which has comparatively high organic soils, adequate rainfall and a long growing season, provides ideal conditions for high microbiological activity and thus rapid breakdown. Residues in water are decomposed by micro-organisms and sunlight and decline at a similar rate as in soils, which may be slowed by a deficiency in oxygen. The phenoxy herbicides do not concentrate in food chains.

The principal hazard in the use of the phenoxy herbicides is the spray drift leading to injury of desirable plants outside the treated area. In some

areas of the world some cases of plant damage have occurred due to the volatilising under hot conditions and moving off the target area. No such cases have been recorded in this country and any plant damage has been caused by spray drift off the target at the time of application. The risk of plant damage due to spray drift is inherently greater with aerial application than ground application due to the greater release height.

There are a number of ways in which the hazard from spray drift can -be obviated. Agricultural Chemicals (Vineyards) Regulations set out the time of the year phenoxy herbicides can be applied near vineyards with the distance of the target area from the vineyard varying with the type of formulation used and the season. Dust formulations of 2,4-D were once widely accepted for thistle control in hill country; however, as these formulations were very prone to drift, their use has now been practically eliminated although it was a relatively cost effici'ent method of control for the farmer.

The drift of spray droplets can be greatly reduced by producing larger and heavier droplets that tend to set rapidly in the target area. The correct calibration of spray equipment to generate droplets with a diameter greater than 150 microns is desirable. Thickening agents are available which when added to the spray mix produce larger droplets and fewer fine particles. Specialised foam generating nozzles also produce the same effect. Finally, and probably the most important factor, is the operator’s decision on how to treat a particular area. Phenoxy herbicides should not be applied in a potentially hazardous situation when a temperature inversion or a warm layer of air is overhead. The absence of wind or an inversion ceiling reduce turbulent mixing or ventilation and promote a build-up of small

drops and vapour which can readily drift to sensitive crops. Due to the terrain and climate of New Zealand, the formation of significant inversion layers is very rare. When using phenoxy herbicides close to sensitive crops, the operator should ensure there is a positive wind away from these plants at the time of application. From a practical point of view the characteristic odour, particularly of the oil soluble formulations of 2,4-D and 2,4,5-T is readily identified at very low concentrations. The main cuases of this odour are very small quantities of chlorinated phenols and diesel-type solvents in the product formulation which are not biologically active at these concentrations. In other words, although the “hormone type” smell may be identified, the amount of active phenoxy herbicide present will be well below the threshold level at which susceptible plants show an identifiable response.

This has, of necessity, been a very brief review of the general characteristics of the phenoxy herbicides but also covers the type of information considered before the registration of any agricultural chemical. It is true that in recent years greater and more strict requirements are being and have been made by registration authorities for newer compounds than in the days when the phenoxy herbicides were first developed. However, the phenoxy herbicides and other established agricultural chemicals do have the advantage of wide-spread use involving both human and environmental exposure where no serious health problems have occurred. Due to the controversy in recent years around 2,4,5T, this group of herbicides has been the subject of close scrutiny and many additional studies to answer the questions raised, with the net result being that they continue to be used under the presently accepted practices.

In assessing the value

of any chemical wherever it may be used in daily life, a value judgment has to be made of the benefits of its use compared with any possible adverse environmental effects. The concept of zero risk is not a realisitc

alternative as decisions have to be made on limited human toxicological data and in the case of the phenoxy herbicides, the original decisions on their use. with some minor modifications, still stand.