Chemical Methods of Weed Control

New Zealand Journal of Agriculture, Volume 84, Issue 4, 15 April 1952, Page 273

Chemical Methods of Weed Control

THE effective control of weeds is always a subject of great interest to market gardeners, farmers, and those concerned with pasture improvement. The Extension Division of the Department of Agriculture has conducted extensive trials on weed control by chemical methods, and in this, the first part of a two-part article, L J. Matthews, Weeds Research Officer, Department of Agriculture, Wellington, gives information on the results so far obtained from these trials. The writer gives details of the various chemicals and their effect on different types of weeds, and advice on the best time of application of each preparation. The second part of the article will appear in next month's issue. THE chemical weedkillers consist of two groups, the hormone preparations and the non-hormone, the latter including the grass-weed killers and the phenol preparations. HORMONE WEEDKILLERS Hormone weedkillers, more correctly called growthregulating compounds, form an important group. Most weeds fall into the category of broad-leaved, flowering plants and many are destroyed by this group of chemicals. The advantages of hormone weedkillers over other types of chemicals used for weed control are that they are nonpoisonous to animals, non-corrosive, and relatively easy and pleasant to use. They are derived from three parent materials, M.C.P. A (2 methyl, 4 chlorophenoxyacetic acid), 2,4-D (2,4-dichlorophenoxyacetic acid), and 2,4,5-T (2,4,5trichlorophenoxyacetic acid). Salts, Amines, and Esters From each of the parent acids (M.C.P., 2,4-D, and 2,4,5-T) the following salts, amines, and esters are formulated: — . Sodium salt of M.C.P.: This is the most selective hormone weedkiller; it kills a smaller range of weeds than the 2,4-D preparations, weeds with a waxy leaf being little affected. It is the most suitable preparation for crop spraying. Sodium salt of 2,4-D is slightly more toxic and hence less selective than the sodium salt of M.C.P. As it is only 4 per cent, soluble in water, it is always sold as a powder, but for ease in mixing a 3 per cent, mix is recommended, that is, 31b. of sodium salt to 10 gallons of water. Amine salt of 2,4-D: This material is more toxic than the sodium salt of 2,4-D. It has been employed largely for the control of weeds in crops, but as it is less selective than the sodium salt of M.C.P., there is more likelihood of crop damage. Its use should be further developed for weeds that are difficult to kill such as convolvulus and Californian

thistle, which possess a mass of interlacing roots. With these weeds a slow foliage kill results in fewer regrowths than if the above-ground parts are destroyed quickly by the ester preparations. Esters of 2,4-D: As is shown in Table 1 on page 274 both volatile and non-volatile esters are manufactured. Because volatile esters pass off toxic fumes under high temperatures, they are dangerous to nearby plants if humidity is high and the wind is negligible. In enclosed spaces non-volatile esters should be used. It should be noted, however, that spray drift of both non-volatile and volatile materials is toxic. The esters are largely oil based and, unlike the water-based preparations, are not affected by rain immediately after application.

The esters are the most potent materials. As contact chemicals for the destruction of annuals and seedling perennials the oil-based esters are two or three times as toxic as the salts, but on deep-rooted perennials they usually give inferior results. The water-based salts and amines are repelled by plants such as periwinkle which possess a waxy coating, or cuticle, on the leaves. For such plants an oil-based ester is necessary to secure penetration or absorption of the hormone. The addition of dieselene or kerosene also aids absorption of the hormone into weeds such as periwinkle which are difficult to kill.

In general terms esters give better penetration, but not translocation, that is, movement of weedkiller into the roots. The salts and amines have poorer penetration, but are translocated better in the plant.' If sufficient penetration is secured, the salts and amines give a slow foliage kill, which ensures that the roots are partly or wholly destroyed, so that fewer regrowths occur than if the foliage had been killed quickly. A recent development is the preparation of water-based esters. They have a higher toxicity per unit than the oil-based esters and also possess some of the properties of amines and salts. Consequently, they may prove valuable for the destruction of deep-rooted perennials with leaves sufficiently waxy to repel the salts and amines. As shown in Table 1, M.C.P. and 2,4-D formulations control annual and herbaceous perennials. The 2,4-D esters also control a few species of woody perennials. Formulations of 2,4,5-T: The properties of 2,4,5-T amines and esters closely parallel those of the 2,4-D amines and esters. The 2,4,5-T formulations are used for the control of woody perennials resistant to the 2,4-D preparations. As a rule both 2,4-D and 2,4,5-T formulations are specific in their action. Mixtures of 2,4-D and 2,4,5-T are not recommended for New Zealand conditions. Acid Equivalent The term “acid equivalent” refers to the active principle of the hormone preparations. For liquids the acid equivalent is expressed in pounds of active weedkilling material per gallon, and for dusts the amount .of active

principle per pound of dust. The acid equivalent is the standard for comparison of the potential weedkilling materials in the various formulations. Table 4 shows that one acre of variegated thistle seedlings may be controlled with 0.51 b. of acid equivalent per acre of a water-based salt. This means that if a salt listed in Table 1 contains 31b. of acid equivalent per gallon, 1 gallon of this product would control 6 acres of young variegated thistle. Application Very often weeds requiring treatment are so scattered that an over-all spray is not warranted. In these conditions the rates given in Table 4 may be converted into a spray mixture concentration showing parts per million (p.p.m.) of the hormone in water (see Table 2). The plants should be spot treated with the spray concentration indicated, and a complete foliage and stem cover should be secured, especially for woody perennials. GRASS-WEED KILLERS Unlike hormone weedkillers, which control broad-leaved flowering weeds and are non-toxic to grasses, the grassweed killers T.C.A. and 1.P.P.C., two new forms recently developed, control grasses and are relatively non-toxic to broad-leaved flowering plants at normal rates of application. Table 3 gives details of these preparations.

Much more experimental work is required with T.C.A. and 1.P.P.C., especially on the eradication of grassy weeds in crops. Properties T.C.A.: All the weedkilling preparations of this type are derived from the parent material trichloroacetic acid. The sodium salt has been the most widely used, but the calcium salt may find a place. The sodium salt of T.C.A. is slightly corrosive and irritating to the skin. I.P.P.C. or I.N.P.C. are abbreviations for O-isopropyl-N--phenyl carbamate. I.P.P.C. is available as a wettable powder which can be dispersed in water and sprayed, or mixed with other carriers such as fertilisers; it can also be used as an emulsifiable liquid which readily dissolves in water or oil and is applied as a spray. I.P.P.C. has no disagreeable properties. Mode of Action T.C.A. and the emulsifiable form of I.P.P.C. have a destructive contact action on the foliage of most plants, but

the most important effect is the destruction of grasses by application of the preparations to the soil. The mode of entry is through the roots. For activation both materials require moist soil, and, if possible, applications should be made before rain. Oh heavy soils up to lin. of rain is required for activation. Both materials are more efficient on soils where leaching is not rapid; their efficiency is impaired on heavy clay soils. . The most effective method of employing these materials is in conjunction with cultivation. I.P.P.C. is most effective as an annual-grass killer, but it will also destroy seedlings of perennial grasses. T.C.A. will destroy both annual and perennial grasses. The most effective way of utilising T CA. is to plough the area and spray the upturned sod. Alternatively, the grass may be mown, disced, or burnt before the weedkillers are applied. Among the perennial grasses the twitches are the most susceptible, and fibrousrooted grasses are the most resistant.

PHENOLS

The three preparations of phenol weedkillers are P-C.P. (pentachlorophenol), D.N.O.C. (sodium salt of dinitro-ortho-cresol), and D.N.B.P. (ammonium salt of dmitro-ortho-secondary butyl phenate). Preparations ot P.C.P. are not marketed in New Zealand and formulations of D.N.O.C. have been replaced largely by the D.N.B.P. products, which have been developed more recently. Specific use of the phenols, especially of the D.N.B.P. products, is for the weeding of leguminous crops such as peas and _ clovers. They can be used where hormone weedkillers would cause excessive damage. Preparations of P.C.P. and D.N.O.C. fortified with aromatic oils such as kerosene and dieselene can be used as contact killers. The chief disadvantage of these materials, particularly the new D.N.B.P. products, is that they are acute poisons and the manufacturers recommendations for use should be followed closely. Table 6 summarises the forms of phenol weedkillers.

The phenols are true plant poisons, as they destroy only that part of the plant with which they come in contact.

They are more toxic in warm temperatures. The oil solutions of the phenols are the most effective preparations, and next are the water solutions of the ammonium salts. The sodium salts are inferior and require activation with acid salts such as sulphate of ammonia.

Phenol preparations in oil will destroy annual plants and seedling perennials. Salts of D.N.O.C. activated with sulphate of ammonia kill most annual and perennial seedlings, but plants with a waxy leaf surface and grassy weeds are not killed. The preparations of D.N.B.P. are very selective and kill only the seedling growth of broad-leaved weeds; weeds with a waxy leaf are susceptible only in the seed-leaf stage. TYPES OF WEEDS In many cases the most suitable material to use for weeds growing in waste places (Table d) is the oil-based ester of 2,4-D, as usually these weeds are not growing in association with plants of value. The weeds listed in all the tables dealing with tolerance to hormone weedkillers are grouped into annuals and perennials; the perennial plants are further grouped into:— ; (a) Plants with a taproot and a few subsidiary lateral roots. . . (b) Plants without a main taproot but possessing interlacing lateral roots. , . Annuals and perennials can be divided further into susceptible plants (controlled with water-based materials), moderately tolerant (controlled with esters), and resistant (controlled only with repeat applications of esters m an oil base, for example, dieselene). Annual weeds are most easily destroyed as seedlings and in most cases are highly susceptible and may be killed with one application of weedkiller. The. oil-based esters of hormone weedkillers are the most effective formulations. Perennial weeds possessing a taproot are usually killed with one application. Biennial plants, if susceptible, also fall into this category. The oil-based esters are the most effective formulations when the plants are seedlings. Established plants are best treated with water-based materials. Perennial . weeds with lateral roots are usually difficult to control and frequently require more than one application for complete destruction. The oil-based esters are the most effective formulations if the plants have a waxy coating on the leaves and stems. If there is no waxy coating and the plant is not woody, best control will be obtained with water-based materials. Although the oil-based esters .in many cases are the most effective materials for the control of pasture weeds, their use is not recommended where control can be secured with water-based materials. If control of pasture weeds can be obtained with water-based materials, there is less likelihood of damage to susceptible clover species. Where the use of oil-based materials is required for annuals and taprooted perennials the following formula may be used to convert the rates of application of water-based materials recommended in Table 4 to those required for oil-based esters: 0.51 b. ester —0.751 b. amine; these are both equal to 11b. of the metallic salt (all expressed in lb. acid equivalent). This is so for the 2,4-D and 2,4,5-T preparations. Salts of MCP. are more selective than the salts of 2.4-D.

* Form of weedkiller: Water = water-based; oil = oil-based; talc = in dust form with a talc filler „ O A Not / o a !, la ? r .„ the market - Experimental quantities available or shortly available. New products, for example the ethyl-amine salts °.' r -T, will also be shortly marketed. The straight butyl esters will also b ‘ changed to the isomeric esters of the butvi ector of 2,4,5-T. This type of ester is more easily emulsified. . e L ers 01 tne butyl este’

Type Formulation Trade name Acid equivalent per gallon of liquid or pound of dust Volatile or non-volatile Form of weedkiller* Type of weed M.C.P. Sodium salt oxone” .. “Agroxone 3” .. "Phenoxylene 30” .. "Tuloxone 10” .. "Tuloxone 30” "Agroxone Powder” 1.0 3.0 3.0 1.0 3.0 .001 N.V. . N.V. N.V. N.V. N.V. N.V. Water Water Water . Water Water Talc Annuals, herbaceous perennials and crops tAmine . ■ tButyl ester N.V. V. . Water Oil Resistant crops 2,4-D Sodium salt “Phenoxone” "Frenokone” "SHWK” .. "Economone” “Stantox 90” .82 .75 .75 .75 .75 N.V. N.V. N.V. N.V. N.V. Water 1 soluble powder Annuals, herbaceous perennials and crops Amine salt . "Stantox 20” ... "Stantox 64” .. “Veedar 77” “Ata weed” “Ataweed Concentrate” 2.0 4.0 4.8 1.25 4.0 N.V. N.V. N.V. N.V. N.V. Water Water Water Water Water Annuals, herbaceous perennials, and resistant crops Ethyl ester "Stantox P44” "Weedone 57 V.” .. "Ethone” .. .. “Ataweed” .. .. . "DAO” "Weedust 5” “Weedust 15” “Weedust Concentrate” - .. 4.0 3.6 3.6 3.6 4.0 .04 . .12 .25 V. V. V. ' V. V. V. V. V. Oil Oil Oil Oil Oil Talc Talc Talc Annuals, semi-woody peren, nials, and perennials with waxy cuticles Butoxy ethanol ester “Weedone 57 N.V.” ■ 4.8 N.V. Oil As for ethyl ester tPolyethylene glycol ester Propylene glycol butyl ether ester "A.C.P.-W.30” "Esteron 2,4-D” ... 4.5 4.0 N.V. N.V. Water ' Water For difficult - to - kill semi herbaceous perennials 2,4-D 1 2,4,5-T J 2,4-D . 1 2.4.5- J 2,4-D 2.4.5Butoxy ethanol esters Butoxy ethanol esters Ethyl ester ) Butyl; ester i “Brushkiller 32” “Weedone Standard” “Stantox T.45” .. 1.6 ) 0.79 • ' 1 i 0.6 ) 1 0.3 1 f 1.9 1 > 2.1 f ■ N.V. ■ N.V. V. Oil Oil Oil Not warranted for Nev Zealand weeds 2,4,5-T Salt Triethanol-arnine salt , Not manufactured to date “Relionus 2,4,5-T” .. 3.5 N.V. . Water Treatment of new growth of blackberries Butyl ester . "Butoxone” .. “Stantox T.45S” .. “Summit 2,4,5-T” .'. “r>„u.. n ,, s g 4 5-T” . "Weedone Special V.” 3.6 3.4 3.6 3.6 3.6 V. V. V. V. V. Oil Oil Oil Oil Oil Brush .or scrub plants, foj example, woody perennials and scramblers like blackberries Butoxy ethanol ester "Weedone Special N.V.” .. 3.6 N.V. Oil As for butyl esters . tPolyethylene glycol ester tPropylene glycol butyl ether ester "A.C.P.-W.52” “Esteron 2,4,5-T” .. 5.0 4.0 N.V. N.V. Water Water For difficult-to-kill herbaceous woody perennials, for example blackberry, especially re growths, and first year canes

TABLE 1-HORMONE WEEDKILLERS AVAILABLE IN NEW ZEALAND

Acid equivalent of products Pints of products to be added to 100 gallons of carrier (See Table 1) Table 1) 1000 p.p.m. 1000 p.p.m. 2000 p.p.m. 2000 p.p.m. 3000 p.p.m. 3000 p.p.m. 4000 p.p.m. 4000 p.p.m. 1.0 8 16 24 32 • 2.0 4 8 12 16 3.0 2 2/3 5 1/3 8 10 2/3 4.0 2 4 6 8 • 5.0 1 3/5 3 1/5 4 4/5 6 2/5

TABLE 2—CONVERSION OF ACID EQUIVALENT INTO PARTS PER MILLION (P.P.M.) OF HORMONE IN WATER

Type Formulation Acid equivalent per lb. Form of weedkiller T.C.A. Sodium salt 79.3 per cent. Water soluble powder I.P.P.C. Ester Approx, lib. Oil Approx. lib. Wettable powder Approx. 11b. Oil Wettable powder Dust

TABLE 3—NEW GRASS-WEED KILLERS

Weed Recommended formulation and fate of acid equivalent per acre Most effective stage of growth Time of year for treatment Remarks ANNUALS Susceptible Buttercup (Ranunculus spp.) Cotton thistle , (Onopordon .acanthium) Variegated thistle (Silybum marianum) Winged thistle (Oarduus tenuiflorus) Salts of M.C.P., salts and amines of 2,4-D: For seedlings 0.51b.; older plants lib. As seedlings, but plants susceptible up to flower-head formation Spring and autumn For most of these weeds oversowing and fertilisers are required to arrest further reinfestation by seedlings PERENNIALS (Taprooted) 1. Susceptible Catsear (Hypochaeris radicata) Hawkbit (Leontodon hispidus) Nodding thistle (Oarduus nutans) Pennyroyal (Mentha pulegium) Ribgrass (Plantago lanceolata) Scotch thistle' (Oirsium lanceolatum) Storksbill (Erodium cicutarium) Salts ’ of M.C.P., salts and amines of 2,4-D: For seedlings 0.51b.; older plants lib. As seedlings, but plants susceptible up to flower-head formation u ■ . • Spring and autumn Plants establishing from root suckers should be in full leaf before being treated 2. Moderately tolerant Broad-leaved dock (Rumex obtusifolius) Curled dock (Rumex crispus) Fiddle-leaved dock (Rumex pulcher) Salts of M.C.P., salts and amines of 2,4-D: For seedlings 0.51b.; older plants 11b. As seedlings, but plants susceptible up to flower-head formation Spring and autumn Plants establishing from root suckers should be in full leaf before being treated For seedlings esters of 2,4-D at 1 to 21b.; older plants ' 2 to 31b. As seedlings, but ' plants susceptible up to flower-head formation Spring and autumn Curled dock is more susceptible than broadleaved dock. Fiddle-leaved dock is the most resistant species. The water-based esters show promise of controlling docks without doing excessive damage to clovers Rushes (Juncus spp.) Esters of 2,4-D at 1 to 21b. As seedlings, but rushes susceptible up to preflower formation after mowing Winter and spring mowing; summer treatment Of the tall rush species Juncus effusus is most susceptible. J. pauciflorus and J. vaginatus are more tolerant. J. polyanthemos and J. pallidMS are probably resistant. Tall rush species should be mown before treatment and new growth sprayed in the summer months when 6 to 12in. high. ; The small rush species are probably controlled without prior cutting, for example, toad rush (J. bufonius), etc. PERENNIALS (Spreading roots) 1. Susceptible Buttercup . spp. (Ranunculus acer, Ranunculus re pens) Californian thistle. (Cirsium arvense) Daisy (Bellis perennis) Ragwort . (Senecio jacobaea) Russian knapweed (Centaurea repens) Salts of M.C.P., salts and amines of 2,4-D: For seedlings 0.51b.; older plants 1 to 21b. As seedlings; plants establishing from suckers should be treated when approaching flower-head formation Late spring and early summer Established plants require repeat applications for control. First treatment is made at the flowering stage, followed by autumn or spring applications when the weeds are in the rosette stage. Plants arising from suckers should not be treated before full leaf development. In all cases a slow foliage kill should be aimed at and the acid equivalent should be altered to suit local conditions 2. Moderately tolerant Self-heal (Prunella vulgaris) Salts and amines of 2,4-D: For seedlings 11b.; older plants 1 to 21b. . As seedlings, but plants slightly . susceptible up to flower-head formation Late spring and early summer Established plants require repeat applications for control

TABLE 4-TOLERANCE OF PASTURE WEEDS TO HORMONE WEEDKILLERS

Weed Recommended formulation and rate of acid equivalent per acre Most effective stage of growth Time of year for treatment Remarks . PERENNIALS (Taprooted) 1. Susceptible Burdock (Arctium lappa) Capeweed . (Oryptostema calendulacea) Dandelion (Taraxacum officinale) Mouse-ear (Hieracium pilosella) . Vetch ( Vicia saliva, Vicia angustifolia) Water hyacinth (Eichhornia crassipes) Salts of M.C.P., salts and amines of 2,4-D: For seedlings 0.5 to 11b.; older plants (except water hyacinth) 1 to 21b. As seedlings, but plants susceptible up to flower-head formation Spring and autumn ( except water hyacinth) Water hyacinth is best controlled in lateautumn . and winter months. The most effective material is amine salt of 2,4-D applied at 3 to 61b. per acre. A follow-up spray is essential for complete destruction. If treated in the dormant season, the plants missed in the first application may be controlled before they multiply 2. Moderately tolerant Beaked parsley ( Anthriscus vulgaris) Hemlock (Conium maculatum) Tutsan Hypericum androsaemum) Wild parsnip (Pastinaca saliva) Wireweed (Polygonum, acicular Oil-based esters For seedlings 11b. As seedlings, but plants show slight degree of susceptibility up to flower - head formation when weedkiller applied at rates of 3 to 41b. Spring and autumn Plants show sufficient susceptibility to be controlled at high rates of application. Note: ■ Hemlock shows sufficient reaction to be made edible to stock by spraying. In the case of horses and other non-ruminants poisoning will occur because the hormones do not alter the poisonous nature of the alkaloids. Mature tutsan plants are best treated with 2,4,5-T esters in summer PERENNIALS (Spreading roots) 1. Susceptible Bindweed ( Convolvulus arvensis ) Bell vine (Oalystegia sepium) Stinging nettle . ( Urtica dioica) ' Tansy (Tanacetum vulgar e) Watercress (Nasturtium officinale) Salts and amines of 2,4-D: For seedlings 0.5 to lib. ; older plants 1 to 21b. As seedlings, but plants susceptible up to flower-head formation Summer months for mature plants; spring and autumn for seedlings Established plants require repeat applications for control. First treatment is made at the flowering stage, followed by autumn or spring applications. Plants arising from suckers should be treated , when in full leaf. A slow foliage kill is required to ensure few regrowths. 2. Moderately tolerant Australian tireweed (Erechtites atkinsoniae) Dead nettle (Lamium album) Fennel ~ (Foeniculwn vulgare) Mint (Mentha arvensis) Parsley dropwort (Oenanthe pimpelinoides) Sheep’s sorrel (Ilumex acetosella) St. John’s wort (Hypericum perforatum) Waxweed (Hydrocotyle spp.) Willow weed ■ (Epilobium spp.) Yarrow (Achillea millefolium) For seedlings oilbased esters of 2,4-D at lib.; older plants oilbased esters of 2,4-D at 1 to 21b. ' As seedlings, but plants susceptible up to flower-head formation Spring and autumn for seedlings ; spring and . early summer for mature . plants; summer and autumn for regrowths Repeat applications required for destruction of plants. Mature plants of Australian fireweed are probably most effectively destroyed with repeat applications of esters'of 2,4,5-T. The New Zealand spp. Erechtites prenanthoides is susceptible to oil-based esters of 2,4-D 3. Resistant Periwinkle ( Vinca major) Oil-based esters of 2,4-D and kerosene 1 : 20 . applied at. 3 to 41b. Seedlings probably susceptible to oil - based esters and water Late summer and. early autumn The addition of kerosene or dieselene would help penetration. Repeat applications are necessary for destruction ANNUALS 1. Susceptible Bathurst burr (Xanth spinoswm) Chamomile (Matricaria chamomilla) Cut-leaved geranium (Geranium dissectum) Hedge mustard (Sisymbrium officinale) Willow weed • (P /niobium spp.) ■Salts of M.C.P., salts and amines of 2,4-D : For seedlings 0.51b. ; older plants lib. As seedlings, but plants susceptible up to flower-head formation Spring and . autumn Plants are controlled very effectively with, oil-based esters if growing in association with more tolerant species

TABLE 5-TOLERANCE OF WEEDS IN WASTE PLACES TO HORMONE WEEDKILLERS

Weed Recommended formulation and rate of acid equivalent per acre Most effective Time of year for stage of growth treatment Remarks ANNUALS (Cent.) 2. Moderately tolerant Australian flax (Linum marginale) Cudweed (Gnaphalium luteo-album) Fleabane (Erygeron crispus) Foxglove (Digitalis purpurea) Marshmallow (Althaea officinalis) 3. Resistant Cleavers (Galium aparine) For seedlings salts and amines of 2,4-D applied at lib. ; for older plants esters of 2,4-D at 1 to 21b. As seedlings, but plants susceptible up to flower-head formation Spring and autumn Plants tend to. repel water-based materials in advanced stages of growth . ■ ■ . ‘ ' ’ ' . Y As seedlings 1 to 21b. esters of 2,4-D . As seedlings; old plants resistant • to water and hormone mixtures Spring and early summer Mature plants probably controlled with 2,4-D esters and dieselene mixture

TABLE 5(count.) TOLERANCE OF WEEDS IN WASTE PLACES TO HORMONE WEEDKILLERS

Type Formulation Trade name Active principle (per cent.) Form of weedkiller P.C.P. Not marketed in New Zealand as a weedkiller D.N.O.C. Sodium salt “Sinox” 30 Water soluble "Selectone” 30 Water soluble “Sinox W.” 13 Water soluble D.N.B.P. Ammonium “Sinox W.” “Sevtox” 13 20 Water soluble Water soluble salt “Selectone Selective” 13 Water soluble

TABLE 6—PHENOL WEEDKILLERS