CHEMICAL METHODS OF WEED CONTROL

New Zealand Journal of Agriculture, Volume 77, Issue 5, 15 November 1948, Page 437

CHEMICAL METHODS OF WEED CONTROL

Recommendations of New Zealand Research Workers

WHEN some 2 years ago it was realised that developments in the control of weeds by chemicals were proceeding at a rate that demanded that a great deal of critical attention should, be given to the subject, the Field Crop Committee, which is the body co-ordinating research work into field crops other than wheat in New Zealand, set up a Sub-committee for Chemical Weed Control to give the problem the detailed study it demanded.’ The following compilation by P. B. Lynch, Field Crop Experimentalist, Department of Agriculture, Welington, of reports from members of the sub-committee is a timely and authoritative statement on the behaviour both of weeds and some economic crop plants toward chemical weedkillers.

ORGANISATIONS represented on the Weeds Committee, as the sub-committee is commonly called, are the Agronomy and Botany Divisions and the Plant Chemistry Laboratory of the Department of Scientific and Industrial Research, Canterbury and Massey Agricultural Colleges, and the Extension Division (including the Soil Fertility Research Station) of the Department of Agriculture, so that practically all research workers in chemical methods of weed control are represented. . . RECOMMENDATIONS OF THE COMMITTEE Recommendations of the committee are as follows: Description of Materials The chemical weedkillers described in this report may be listed as follows: — ■ A. Hormone Weedkillers (more correctly, Growth Regulator Compounds) Hormone weedkillers have, been described previously by the following in “The New Zealand Journal of Agriculture”: —R. K. Ward (July, 1946, issue), J. P. Hudson

(August and September, 1946, issues), and P. B. Lynch (August, 1947, issue). A wide range of materials, which are of two main types, is covered by this description. (i) 2, 4 D acid (2, 4 dichlorophenoxyacetic acid) base, represented by the following compounds:— (a) Esters: Most esters are not soluble in water, but are soluble in oil. Consequently many commercial preparations are solutions of esters in oil. Examples are “Weedone,” “Thompson’s Weedicide,” and “Stantox Ester P 44.” (b) Amine salts: These (and the sodium salts) are mostly water-soluble. Commercial preparations of amine salts of 2, 4 D are “Weedar,” “Dee Cee Pee,” “2, 4 Dow Weedkiller 40,” “Thompson’s Weedicide Liquid Concentrate.” (c) Sodium Salts: Examples are “Chloroxone,” “Stantox Powder 70,” “2, 4 Dow Weedkiller,” “Thompson’s Weedicide Powder 100.” (ii) M.C.P. acid or “Methoxone” (2 methyl, 4 chlorophenoxyacetic acid) base, represented commercially by ’Agroxone.” When rates per Acre of the hormone weedkillers .are referred to in this report they will be given in pounds of acid (2, 4 D or M.C.P.) equivalent. As .many products are sold in solutions of approximately 10 per cent, strength,' these rates will be equivalent to gallons per acre of such materials. However, it is anticipated that in future many, weedkillers will be available in more concentrated forms than 10 per cent, solutions and it is therefore essential that users adjust the gallons per acre to be applied of these concentrated weedkillers on the basis of their acid equivalent content. The dilution of the materials appears to be of secondary importance provided that run-off losses are not excessive. With new low-volume spraying some products may be applied neat as purchased or in dilutions such as 1:2 or 1:4, but spray equipment delivering 100 gallons per

acre would require dilutions of 2:100 (for a 21b. per acre acid equivalent application of a 10 per cent, solution) or 1:100 for a lib. per acre treatment. The advantages of hormone weedkillers may be summarised in that they are non-poisonous to animals, non-corrosive, . relatively easy and pleasant to use, and are effective against many weeds which are resistant to other types of weedkillers and do relatively little damage to pastures or cereal crops. B. D.N.O.C. Compounds (dinitro-ortho-cresol base) D.N.O.C. compounds were formerly used as dyestuffs and insecticides and were developed as plant poisons in France in 1933. The commercial form in which these are distributed in New Zealand is the sodium salt “Sinox;” other commercial D.N.O.C. products are known .as' “Denoc,” “Dinoc,” and “Extar-Sandoz.” The D.N.O.C. compounds are true plant poisons in that they : destroy plant tissue with which they come in contact This is true of the majority, of weedkillers, with the . notable exception .of hormone weedkillers., which act as. growth regulator compounds. Plants mav be stimulated bv hormone-type compounds to excessive growth, and they use their stored food

materials. at such a rate that they literally starve themselves to death. D.N.O.C. weedkillers have certain advantages in comparison with some other forms of weedkillers in that they are relatively non-poisonous and are quickly decomposed in the soil, so that there is no lasting toxic effect. On the other hand they are vivid dyes (yellow or orange), dissolve rather slowly, and are absorbed . rather slowly ,by the plant, so that rain falling shortly after spraying may wash the material off before it has time to take effect. They are contact poisons and are not transported through the plant. Perhaps the most serious disadvantage's the extent to which the effect of these compounds varies with weather conditions. The „ higher the temperature the greater is the effect; also the higher the relative humidity (within the crop) the greater is the effect. The addition of; sulphate of ammonia increases the toxicity i and the rate of absorption of the sodium salt “Sinox” into the plant. An. applications of . 201 b. (that is, 2 gallons of the commercial paste) per acre of “Sinox” is eouivalent to one of 101 b. (1 gallon) of “Sinox” plug 11b.'sulphate of ammonia per acre This is a more or Inss standard application. the concentration of the snrav being of I’ttle importance within the range of highvolume applications. Due to the nature of the material it is doubtful

whether it could be successfully used through the fine nozzles required by low-volume spraying.

The D.N.O.C. group can be regarded as supplementary to the hormone weedkillers, as they are of particular value when used to control weeds in crops which are severely damaged by the hormone weedkillers. The best examples of these crops are onions, leeks, and peas. Both materials may be used with care in linseed and linen flax. Some weeds, particularly some annual weeds, are controlled rather better by D.N.O.C. materials than they are by the hormone-type weedkillers. As a general rule; however, the hormone weedkillers are to be preferred to the D.N.O.C. compounds, as they have considerably greater killing power against a wider range of plants. C. Sodium Chlorate Sodium chlorate weedkiller, or the form known as “Atlacide”, in which calcium chloride is added to sodium chlorate to reduce fire and explosion risks, has. been used for many years in New Zealand, particularly against ragwort and blackberry. It is still considered a very useful material if care is taken in handling it. Clothing and sacks impregnated with sodium chlorate are liable to ignite spontaneously on drying. It is a non-selective weedkiller and will damage pasture grasses, clovers, and desirable crop plants. D. Power Kerosene Power kerosene kills a wide range of weeds and has particular application for weed control in carrot crops when these are sprayed in the one or two fern leaf stage. Applications at a later stage than this are liable to cause tainting. Carrots have proved resistant to most power kerosene sprays, but power kerosene is variable in composition and some lines will kill carrots. It is therefore advisable to try it on a small .scale first. The use of mixtures of hormone weedkillers with power kerosene shows promise against resistant woody plants such as gorse, broom, and blackberry.

No attempt can be made in an article of this nature to deal with the multitude of other chemicals used as weedkillers, some of which are salt, borax, arsenical preparations, thiocyanates, carbon bisulphide, chloropicrin, copper sulphate, sulphuric acid, crude oil, and iron sulphate. The main emphasis of the work in recent years has been on the hormond-type weedkillers and most of the references will be to these materials. ANNUAL CROPS 1. Effect of Hormone Weedkillers on Crops Wheat, oats, barley, and ryecorn: Hormone weedkillers in water solution may be used to control susceptible weeds ,in these cereal crops at quantities up to 21b. per acre acid equivalent. . : Materials in oil solution, such as “Weedone,” should not -be used on arable crops generally, i including cereals, as the risk of damage to the crop is too great. Water solutions such as “Agroxone” and “Weedar” sprays should be used.

Treatment . should be undertaken when the crop is 6 to Bin. high or after the 6-leaf stage and before shooting commences. Linseed and linen flax: Hormone weedkillers in water solution may be used on linseed and linen flax in quantities up to 11b. per acre acid equivalent. Oil-based materials must not be used. With the above limitations it appears that, crops grown for seed purposes may be safely treated. No definite information is. available as yet on the effect of these materials on the yield of fibre of linen flax. Their use on linen flax should be restricted to crops badly infested . with weeds and likely to be seriously reduced in value in . consequence. Linen flax may be safely treated when 4 to ,6in. high; the. plants may droop temporarily, but recover within a few days.

Tomatoes, rape, chou moellier, turnips, - swedes, cabbage, cauliflowers, and all brassica crops should under no circumstances be sprayed with hormone weedkillers. Even spray drift from neighbouring areas is harmful to such crops. . Peas, lupins, lucerne, carrots, leeks, beetroot, onions, pumpkins,. parsnips, beans, sugar beet, and mangolds, although rather less susceptible than brassicas, are liable to. severe damage and should not be sprayed with hormone-type weedkillers. , Potatoes and maize are relatively resistant to hormone weedkillers, but little work has been done in New: Zealand on these crops. Quantities up to lib. per acre acid equivalent of water solutions of hormone weedkillers may be used, but large-scale applications are not advised until more research is conducted. Asparagus appears relatively resistant to hormone weedkillers when in the fern stage, and sprays of water solutions up to lib. per acre acid

equivalent may be used at this stage only. 2. Effect of D.N.O.C. Weedkillers on Crops (A “moderate” application is 201 b. “Sinox” per acre or 101 b. “Sinox” plus lib. sulphate of ammonia per acre.) Wheat, oats, barley, and ryecorn are resistant to moderate applications of D.N.O.C. weedkillers and may safely be sprayed. Linseed and linen flax are susceptible when young and must not be sprayed until 4in. high; after this they are highly resistant to moderate applications of D.N.O.C. sprays. Rape, chou moellier, and all brassica crops are susceptible and must not be sprayed with D.N.O.C. materials.

Onions, leeks, and peas are susceptible as very young plants until the 3 -leaf stage, but may be safely sprayed at a later stage. The D.N.O.C. compounds are consequently useful materials for these crops which cannot be sprayed with hormone-type weedkillers. Lupins, lucerne (when young), carrots, parsnips, beans, sugar beet, mangolds, and potatoes are liable to severe damage and should not be sprayed with D.N.O.C. materials. D.N.O.C. sprays are much more effective on weeds in the seedling stage, but also are liable to damage crop plants if these are sprayed before becoming well established. 3. Effect of Sodium Chlorate on Crops Sodium chlorate is relatively nonselective and cannot be used to control weeds in growing crops, .as severe damage is likely to result from such treatment. 4. Effect of Power Kerosene on Crops The only crops that may be successfully sprayed with power kerosene are carrots and parsnips; all other crops will be severely damaged or killed. PASTURE PLANTS Hormone Weedkillers Grasses past the seedling stage are generally very resistant to hormone weedkillers in moderate applications (up to 21b. per acre acid equivalent), but damage occasionally results, par-

ticularly from the oil-based sprays. Browntop and cocksfoot have definitely been affected temporarily, although permanent injury is very rare. Hormone .weedkillers should be used on pastures or lawns only in conjunction with measures such as. reseeding and topdressing to close up the sward when the weeds , are killed. ‘

7 Seedling grasses are susceptible and should not be treated until at least 6 weeks old. The elimination of susceptible weeds in pastures for seed production with hormone sprays is probably feasible, but little research has been conducted on this method to date. Hormone weedkillers can* be

confidently recommended for the control of many lawn weeds without . permanent damage to lawn grasses. , ■ ' ' Clovers, on the other hand, are liable to damage from moderate applications of hormone weedkillers. The perennial clovers such as white clover will usually recover satisfactorily after a few weeks, but annual clovers such as suckling clover may be killed, The possible effect of the hormone weedkillers on clovers makes it necessary to use these materials with caution, particularly on swards naturally weak in clovers. Heavy applications, say more than 21b. per acre . acid equivalent, should be avoided. The oil-based compounds are generally more severe in their effect on pasture plants. However, as they are frequently more effective per unit of active material than water-based sprays, they may be used in smaller quantities per acre to give satisfactory weed control without severe damage to valuable pasture . plants. L y • x. ‘ - , , Dust preparations of hormone weedkillers may be used satisfactorily against pasture and lawn weeds where the weeds treated will hold appreciable quantities of the powder, However, dusts are considerably less efficient against weeds in crops, which are difficult to cover satisfactorily, and against more upright weeds on which it is difficult to ensure the retaining of sufficient dust on the treated plant. Hormone weedkillers applied as

sprays are generally more efficient per unit of active material than when they are applied as dusts. Uniform distribution is generally easier to obtain with sprays than with dusts. - nN n f Comnonnds ' ’ . .. „ , „ Because the majority of weeds of permanent pasture are perennial weeds against which D.N.O.C. materials are relatively ineffective, the use of D.N.O.C. compounds as sprays for pasture weeds is not recommended, They cannot kill perennial weeds because they cannot penetrate the roots from which fresh shoots arise. Seedlings and shoots may be killed, but underground parts of established plants are not affected. Pasture grasses and established clovers are resistant to moderate applications of D.N.O.C. compounds. . Sodium Chlorate As moderate to heavy applications of sodium chlorate will kill many pasture grasses and clovers, the broadcast application of this substance on pastures cannot be advocated. “Spot” treatment of isolated weeds, such as ragwort, however, gives satisfactory control without general pasture damage. K kerosene Kerosene will kill most grasses and clovers and cannot be used to control

pasture weeds without damage to desired pasture species.

WEEDS Hormone Weedkillers The following lists give present knowledge of the action of hormonetype weedkillers on a number of common weeds of pastures, crops, and waste-land. The lists are incomplete and the information only tentative and open to correction in the light of further research. They should, however, provide a guide to intending users of these forms of weedkillers. The following are the approximate rates of material designated as heavy, moderate, and light: — Type of Pounds per acre 2,4 D Application or M.C.P. Acid Equivalent (as sprays) Heavy More than 21b. (say up to 41b.) Moderate I to 21b. Light '/2 to lib. The weeds have been classified into classes, based on the following reactions to hormone-type weedkillers: — Very resistant: Practically no plants showing a reaction or being permanently affected by heavy applications. Resistant: Practically no plants killed, although some temporary reaction may be shown to heavy applications.

Susceptible: The majority of plants killed or very severely damaged by moderate applications.

Very susceptible: Practically complete kills from light or moderate applications.

The classes cannot be regarded as distinct, because inevitably factors such as weather and soil conditions and stage and vigour of growth of the plant may cause a different reaction from that indicated. Seedling plants are generally more susceptible to treatment. In classes “resistant at all stages of growth” it should be recognised that very young or newly-germinated seedlings of such species may be killed by treatment with hormone weedkillers. (a) ANNUAL AND BIENNIAL WEEDS Class 1: Very resistant or resistant at all stages of growth. Class 2: Resistant as mature plants, susceptible as seedlings. Class 3: Resistant as mature plants, reaction as seedlings unknown. Class 4: Susceptible. Class 5: Very susceptible. (a) ANNUAL AND BIENNIAL WEEDS Class 1: Plants very resistant or resistant at all stages of growth Black bindweed Staggerweed (Stachys (Polygonum con- arvensis) volvulus') Scarlet pimpernel Cleavers (Galium (Anagallis arvensis) aparine) Speedwell (Veronica Chickweed (mouse- spp.) eared) (Cerastium Wireweed (Poly-, glomeratum) . gonum aviculare) Field pansy (Viola ’ . arvensis)

Class 2: Plants resistant when mature, susceptible as seedlings Black nightshade Spurrey (Spergula (Solatium nigrum) arvensis) Chickweed (slender) Redshank (Poly(Stellaria media) gonum persicaria) Catchfly (Silene Tares (Picia spp.) gallica)' ' - ' Class 3: Plants resistant when mature, reaction .as. seedlings ■ unknown *Fleabane (Erigeron L Goatsrue (Galega spp.) . ’ officinalis) ' Flax . (Australian) Willow weed (Epi(Linum marginale) labium' spp.) : - Fumitory (Fumaria .. muralis) ; Class 4: Susceptible plants .. Burdock (Arctium Storksbill (Erodium lappa) ' spp.) Fat-hen (Cheno- Sow thistle podium alba) (Sonchus Foxglove (Digitalis oleraceus) ■ purpurea) ■ Scentless mayweed **Fennel (Foe'ni- . . . (Matricaria in-, culum vulgare) odora) Groundsel (Senecio Stinging nettle (Ur, •vulgaris) : tica .urens). : **Hemlock (Conium **Twin cress . t maculatum) 1 \ (Coronopus didyLand cress (Barbarea mus) spp.) Wild parsnip . . (PasRed dead nettle tinaca sativa) . (Lamium pur- fi pureum) ....

Class 5: Very susceptible plants Hedge mustard Thistle, Scotch (Cir(Sisymbrium sium lanceolatum) officinale') Thistle, Nodding Portulaca (Portulaca (Carduus nutans') oleracea) Thistle, Variegated Shepherd’s purse (Silybum maria(Capsella bursa num') pastoris) _ Thistle, Winged Wild turnip (Bras- (Cardints tenuisica campe'stris) ■■ '■ ’ floris) *. Susceptible in rosette stage . , Resistant when flowering (b) PERENNIAL WEEDS _ A different classification has been adopted for perennial weeds. Class 1: Shoot and root resistant at all stages, of growth. • .. Class 2: Shoot is susceptible, but plant regrows from unaffected underground parts. Class 3: Woody plants temporarily defoliated, or ' leaves and young shoots reacting to treatment, but plant soon recovers. Class 4: Susceptible plants; a proportion of the plants may recover after treatment,' but repeat treatment would control the weed.

Class 5: Very susceptible plants, the majority of which would be eliminated by one treatment. In connection with this classification, the following points should be borne in mind: (a) Oil-based materials are generally more effective per unit of active material than water-based sprays. This is particularly the case with St. John’s wort and ragwort. (b) The classification is on the assumption that plants are treated at the -most favourable season of the year and time of growth. In. the plants in Class 2 the best results can usually be obtained by treating them in the early-flower stage (as mature plants) when there is the best translocation of weedkiller to the underground parts of the plant. Apart from this, plants can be considered as more susceptible as seedlings and when growth is active. It is, however, important to distinguish between true seedlings and new shoots coming up from underground roots or stems. Dormant plants in winter or in dry periods usually show least effect from treatment.

(c) A complete kill of susceptible and very susceptible plants may not be secured because of inefficient distribution of weedkiller. This result is “ most frequently seen with dust applications. - ’■ Class 1: ■ Shoot and root resistant Bidi-bidi (Acaena Moss : sanguisorbae) • ‘ Periwinkle (Vinca *Cudweed (Gnapha- . major) Hum luteo-album) , Poa aquatica . Elderberry (Sam- ' (Glyceria aquatica) bucus nigra) Raupo (Typha Fern (Hard) (Paesia awgustifolia) . . . scaberula) . Rushes (Juncus spp.). Fern (Bracken) Sedge (Carex spp.) ’ (Pteridiwn esculeh- . Sorrel (Rumex turn) acetosclla) ■ - Grasses (weed and Tutsan (Hypericum • twitches) ■ androsaemvm) (Gramine ' .Wild garlic (Allium Marshmallow - vineale) -; (Althaea officinal! . . ' Manuka ' (Leptospermum sco- ' '• par item) ’ ■ ; ' ■'

Class 2: Shoot susceptible, but plant recovers in most cases Bladder campion Horehound (Marru(Silene cucubalis) . bium vulgar e) Californian thistle Oxalis (Oxalis spp.) (Cirsium arvense) Self-heal (Prunella Convolvulus (Field) vulgaris) . (Convolvulus Yarrow (Achillea arvensis) millefolium) Chicory (Cichorium intybus) . Class 3: Woody plants which recover quickly after treatment Blackberry Gorse (Ulex (Rubus spp.) . europaeus) ' ' Broom (Cytisus sco- Sweet brier (Rosa parius) eglanteria) Class 4: Susceptible plants, but complete control unlikely with single treatment Buttercups (Ranun- Convolvulus cuius spp.) (Greater) Capeweed (Calystegia sepium) (Cryptostemma Daisy (Bellis perencalendulacea) »is)

Docks (Rum ex spp.) Pennyroyal (Mentha Geraniums pulegium) {Geranium spp.) Ragwort (Senecio Heath (Spanish) • jacobaea). (Erica lusitanica) St. John’s wort Hoary cress (Car- (Hypericum perdaria draba) foratum) Mint (Field) Waxweed (Hydroco(Mentha arvensis) tyle spp.) Monkey musk (Mimulus guttatus) Class 5: Very susceptible plants Catsear (Hypo- Lupins (Lupinus chaeris radicata) spp.) Dandelion (Taraxa- Plantains (Plantago cum officinale') spp.) Hawkbit (Leontodon Watercress (Nasturhispidus) . tium officinale) Hawkweed (Crepis capillaris) - * Susceptible prior to flowering Of woody plants suckers and young growth of silver poplar (Populus alba var. . nivea) , crack willow (Salix jragilis), and elm (Ulmus campestris) have been killed by sprays. D.N.O.C. Compounds D.N.O.C. compounds are completely effective only against some annual weeds and against seedling plants of some perennial weeds. In perennial weeds it is important to distinguish between true seedlings that are just starting to grow from seeds and new shoots coming up from underground roots or stems. D.N.O.C. materials cannot kill perennial weeds because they cannot penetrate. the J roots . from which fresh shoots arise. In spite of this sufficient control of the aerial parts of the plants may be secured against weeds : such as Californian thistle in crops to enable the crop to get away and compete successfully with any regrowth of weeds. * In view of the foregoing, D.N.O.C. compounds are only of restricted use

against perennial weeds in pastures. Most of the weeds classed as susceptible are resistant when mature. The list is incomplete, as only a relativelysmall amount of work has been done with the D.N.O.C. materials.

Weeds have been divided into four classes, namely: Class 1: Resistant at ' all .’stages of growth. : Class 2: Susceptible only when young. Class 3: Susceptible at most stages of growth. Class 4: Seedlings and shoots killed, ’ but underground parts not affected. Class 1: Resistant weeds " Chickweed (Slender) ‘ Stinging' nettle ■ (St ell aria media) (Urtica urens)' Chickweed (Mouse- Scarlet pimpernel eared) (Cerastium (Anal arvensis) glomeratum) .. Sorrel (Rumex Fumitory (Furnari acctosella)

rumuuiy yj.'wrrbu,f t u> tic CLOSCILa) muralis) . .. , .... Grasses (all) . . (Gramin e a ■ ' Class 2: Weeds susceptible only when young Black nightshade Spurrey (Spergula (Solatium. nigrum') .. arvensis) • ' Clovers (very, early Willow weed (Epi! 0,. ,. stage) (Trifolium bium spp.) ■ spp.) ’ ' : ; - 1 < Wireweed (Poly-' Fat-hen (Cheno- gonum aviculare) podium album) Speedwell (Veronica , spp.) - ..' . - ■ Class 3 s Weeds susceptible at most' '• • stages of growth Black bindweed J : . , Shepherd’s purse (Polygonum ' con- ' (Capsella bursa volvulus) ■ ' ' ■ f pastoris) ■. Pead nettle (Lamium Scotch thistle (Cirpiirpureum) ■.. .:. shim. lanceolatum) Lunins t H . Wild, turnip (Bras(Lupinus spp.) : . sica campestris) Class 4: Shoots killed or damaged . ‘ severely, but roots not affected Californian thistle Catsear (Hypochaeris (Cirsium arvense) radio ata)

Chicory (Cichorium Hawkbit (Leontodon intybus) hispidus) Convolvulus • Hawkweed (Crepis (Greater) (Caly- capillaris) stegia sepium) - Hoary cress • (Cardaria dr aba)

Sodium Chlorate and Power Kerosene It is outside the scope of this article to give detailed information of the effect of sodium chlorate and power kerosene on individual weed species. Generally both materials will destroy the aerial parts of the majority, of plants, including grasses, and they have been used to .clean sites completely of plants. : The usefulness of sodium chlorate in the control of ragwort is widely known. However, the fact that chlorate treatment rarely completely suppresses. regrowths of ragwort illustrates the important point that plants which can . send up fresh shoots from unaffected roots after the

original shoots have been destroyed are rarely completely controlled by such materials. Very heavy dressings of sodium chlorate will, however, penetrate the soil and sterilise it against all plant growth for an appreciable period. : - .?. Power - kerosene has particular application as a spray on carrot crops, which are resistant .to this material. Apart from this use, neither power kerosene nor sodium chlorate is really, selective in action. Consequently they cannot be used on crops or as a widespread application on pastures without causing • damage to economic crop or pasture species. Low-volume Spray Equipment The. application of chemical ’weedkillers in the field is' an. important aspect of weed-control work. The usefulness of dust applications has been discussed, but in general it may be said that dusts have given disappointing results in comparison with sprays.

The recent development of lowvolume spraying has overcome many o f th e difficulties associated with spraying, and in consequence it is unlikely that weedkiller dusts will be widely used in the future.

It has been usual to apply weedcontrol sprays with high-volume, highpressure mechanical sprayers. However, with the realisation that the hormone weedkillers were■ effective r at high concentrations," attention . was given to the adoption of spraying low volumes at low pressure '- using'- high concentrations of the spray solutions. These low rates of application - are possible only- as a result of the introduction of low-pressure nozzles which have been developed in the United States in-the past 2 years. The lowpressure nozzle is designed to give a fine spray of even particle size at pressures as low as 20 to 301 b. per sq. in.

O LAX V7O CIO HJVV CIO *~IV vv_z tJVIU. ' Ov|. XIA. The orifice diameter of such nozzles is in the region of 20 thousandths of an inch for the smaller sizes, which discharge at rates as low as 1 gallon per hour for each nozzle, so that it is possible to spray at rates ,as low as 1 gallon per acre. *.'• ' There are several methods of supplying the spray solution to the nozzle .at . these low- pressures, ’( and because the low-volume sprayer uses small . quantities of water it has been ...possible to design a sprayer;. which may ha- nrifirnlv fro ntnr mnnrforl • T.aTCfQ-

be entirely tractor mounted Largethe weVht and size of ? the Inraver may the weight ana Size or tne. prayer may be considerably reduced. The graving equipment ; required, is . consequently much ; less expensive. ■ J ; - i ’- 1 ,u. / R , ’ Low-volume spraying t may f- be summarised in comparison with highvolume spraying as having the advantages of being less expensive, less water is required, there is “less weight and reduced crop . damage, faster spraying may be achieved, ' and the

possibility exists of using spraying methods in country difficult to cover with a machine or where available water supplies are limited. The use of a knapsack sprayer in hill country is not out of the question when one fill of the sprayer carries sufficient material for 1 or 2 acres.' Certain disadvantages are associated with low-volume spraying, the chief of which is increased spray drift of the fine mist-like spray produced. Spray materials must be completely dissolved or they will not pass through the nozzles. This . means that certain weedkillers applied as suspensions cannot be applied through low-volume nozzles. And, last, with weeds at the base of taller crops, or with bushy plants, where the spray needs to. be applied with considerable force to reach all parts of the plants or to the weeds at ground level, satisfactory penetration is unlikely to be secured with low-pressure spraying. Differences Between Preparations In general, great differences have not been found between the different commercial preparations. The oil-, based sprays may be less affected by rain soon after application and may be more effective in dry conditions. Such sprays have greater penetrating power and tend to be more effective per unit of active material, but they are less selective in action and may damage crop plants and clover species when water-based sprays do not. The M.C.P. compounds persist longer in the soil than the 2,4 D materials; 2, 4 D compounds applied at a very heavy rate in a greenhouse

trial were destroyed in 4. to 6 weeks, while M.C.P., compounds have been known to persist for several months. Seeds are apparently resistant to hormone materials until germination commences. Cleaning Spray Equipment The water-based sprays have been found to be satisfactorily removed by four thorough washings of spray equipment with water followed by soaking overnight with warm dilute ammonia or hot soap solution. On the other hand oil-based hormone weedkillers were practically impossible to remove satisfactorily by a large number of treatments. The only effective method was soaking with 1 per cent, activated charcoal (norite) : suspension

after any free hormone material had been removed by repeated water rinses. However, it was found after such treatment that water left in the pump for a week picked up sufficient 2,4 D to affect tomatoes. However thoroughly spray equipment has been cleaned after using hormones, it should be rinsed through again with clean water before using it again on very susceptible crops. The only safe method is to have separate spraying equipment for use with the oil-based hormone sprays. General Conclusions The reaction of a number of crop and pasture plants and weeds to hormone-type weedkillers, D.N.O.C. materials, sodium chlorate, and power kerosene has been briefly described. Emphasis has been placed on the fact that the information given is based on the results of research work still in progress, and is liable to amendment from time to time. Trials are being continued on an extensive scale, however, and with increasing use of the materials by farmers, knowledge of the practical applications of chemical weed control will rapidly increase.

Weed control with chemical weedkillers should be regarded not as a means of avoiding clean farming practice, but as a means of obtaining clean crops and pastures when circumstances have made these difficult or impossible to attain by normal farming practices. In many cases the cost of using chemical weedkillers makes them uneconomic to apply except to high-value crops. Information is lacking as to the precise benefits in terms of crop and pasture yields from weed spraying in New Zealand. Though the elimination of weeds must in itself result in improved production of pastures and crops, this must be weighed against the cost of such improvement and, with some crops, the possibility of direct damage to the crop plant Itself.

Weed control by chemical means offers considerable promise as an assistance to farmers in their fight against weeds, but it should not be taken for granted that the weed problem is solved nor that chemical means of weed control in any way supersede the well-estab-lished practices associated with clean farming.

Weather information During Haymaking

HAYMAKING and ■ silage-making operations will soon be under way in many districts and one of the most important factors in the success of these operations is the weather: In this connection farmers are reminded that the Meteorological Office provides a special service for farmers. A request telegram to “Weather,” Wellington (the telegraphic address of the Meteorological Office) will receive a brief reply, sent “Collect” to the farmer seeking a special forecast. In cases where the name of the office of . origin of the telegram does not adequately indicate the situation of the ; J —■s

farm a brief reference to location is needed. For example: “Cloudy Valley, 5000 ft.” Farmers should avoid vagueness in their requests— should ask for “Prospects for rain, 15th and 16th, Smith, Tauranga;” not, “Please send weather prospects next few days, Smith, Tauranga.” The period of the validity of most forecasts is normally taken to be 24 to 36 hours, and though under favourable weather conditions this can sometimes be extended up to 3 days, any longer period must be regarded as too uncertain for practical use. — -