AGRICULTURAL APPLICATIONS OF PETROLEUM PRODUCTS

Herbicidal Uses of Oils A. S. CRAFTS and H. G. REIBER

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 29, 2018 | https://pubs.acs.org Publication Date: June 17, 1952 | doi: 10.1021/ba-1951-0007.ch009

College of Agriculture, University of California, Davis, Calif.

Summarizing the growth of the utilization of petroleum oils in herbicides, an evaluation is made of the present-day chemical weed-control methods in which oils are employed—as toxicants, as solvents, as filming agents, and as carriers. The unique properties of oil in the pest control of the future are also indicated.

Early use of petroleum products i n weed control was generally l i m i t e d to the local area a n d i n v o l v e d mostly the use of waste materials—acid sludge, various aromatic bottoms, low grade oils, and by-products of little commercial value. W i t h increasing emphasis on weed control a n d firebreak maintenance, particularly i n the Southwest where roadside, fences, and d i t c h banks become unsightly and are a fire hazard during the summer, Diesel fuel and smudge-pot oils were used i n increasing quantities. D u r ing the twenties a n d thirties, use of these oils for puncture-vine control b y counties, firebreak maintenance b y highways, a n d roadside a n d fence-fine weed control b y farmers h a d reached a volume of several million gallons annually i n California alone. O i l sprays i n lieu of tillage had been proved beneficial i n citrus orchards. Various oils and emulsions were being used i n increasing amounts i n the control of Johnson a n d B e r m u d a grasses i n many agricultural areas. Throughout this period A v o n weed killer, a highly aromatic residue from the Edeleanu process, was about the only oil sold for purely herbicidal purposes. M e a n while, Diesel o i l was becoming popular as a n automotive fuel a n d attempts were being made to improve its q u a l i t y b y removing its aromatic ingredients, a process that decreased its toxicity to plants. I n 1936 Sinox, a dinitro selective weed killer, was introduced a n d widely used, not only i n cereal crops b u t i n peas, onions, flax, a n d other important crops. I n the early forties a private dealer i n the Salinas V a l l e y of California discovered the selective herbicidal use of stove oil i n carrot crops. A s war conditions made labor scarce and m i l i t a r y demands called for increased production, this proved a great boon to the vegetable growers. O i l sprays i n 1943 a n d 1944 saved m a n y thousands of acres of carrots t h a t would have been lost if hand labor h a d been required to weed them. M e a n w h i l e , guayule production h a d utilized to the m a x i m u m the tolerance of that crop for Diesel o i l emulsions. O i l sprays allowed complete mechanization of this crop, which was considered critical i n our war production. B u t stove o i l , if not carefully used, left a n oily flavor i n carrots. Synthetic rubber reduced the need for guayule. F u e l o i l prices h a d advanced to a level that almost prohibited their use for weed control. T h e future of oil i n the herbicide field was i n doubt. Y e t farmers needed oil sprays for their irrigation systems, which were infesting their fields w i t h weeds, and for their fence lines, which were harboring thrips and other insects a n d diseases. These problems of research were undertaken b y the agricultural experiment stations i n California, Massachusetts, N e w Y o r k , a n d other states. 70

AGRICULTURAL APPLICATIONS OF PETROLEUM PRODUCTS Advances in Chemistry; American Chemical Society: Washington, DC, 1952.

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Toxicity Research One investigation (1) sought more definite information on the effect of molecular structure on toxicity. A l t h o u g h i t had been well established that aromatic and olefin compounds were toxic as compared to alkanes, i t seemed worth while to pursue this study further, w i t h the idea that if some one compound or series of compounds could be identified as the significant factor i n toxicity, i t could either be isolated b y some refining procedure or be synthesized from other substances. I n view of the development i n synthetic toxic agents, described i n this paper, this phase of the project has become somewhat less important. I t was equally essential then, however, that some o i l diluent should be available, so that toxicities of o i l soluble, water-insoluble substances could be studied. Experiments (1) indicated that n-hexane or n-cetane was reasonably satisfactory as a nontoxic diluent for short-time tests. T h i s was confirmed b y the work of Leonard and H a r r i s (2), although the higher alkanes, even decane, i n the 9-week test, showed marked toxicity toward such plants as cotton, soybean, a n d others. T h e y also found that n-tetradecane produced considerable injury below the surface of the soil. T h i s result h a d also been experienced b y the writers w i t h n-cetane a n d h a d been classed as chronic toxicity. Leonard and H a r r i s , however, subjected plants to six sprayings i n a 6-week period, a much more rigorous treatment t h a n the writers had applied. T h e i r conclusions of the increasing toxicity of compounds from hexane to dodecane, w i t h some decrease i n tetradecane, were based on examination 9 weeks after the first application of o i l . T h i s general rule relating toxicity t o molecular weight was evidenced i n results of investigation b y the writers, using appropriate fractions of gasoline a n d kerosene which h a d been subjected t o the usual acid treatment. I n regard to unsaturation, on flax especially, a marked toxicity of cyclohexene, as compared to cyclohexane, was observed (1). Leonard and H a r r i s (2) investigated this further and noted t h a t while hexene differed little from hexane, both being relatively nontoxic, higher members of the series, even decene, produced severe b u r n or death i n two sprayings, while with n-decane such severe damage was not noted unless five treatments or sprayings were made. T h i s result was much more noticeable on cotton t h a n on soybeans. T h e aromatic fractions were recognized as of more herbicidal value. However, the use of pure compounds i n relatively high concentrations w i t h repeated sprayings at definite time intervals was required to prove that i n general the toxicity varied directly with number of substitutions on the nucleus. Similarly, the toxicity was shown to increase with the length of the side chain, at least through C H , the b u t y l group. L i t t l e effect was noted from the amount of branching on the side chain. These results were clearly shown b y the work of Leonard a n d H a r r i s as well as that of the writers. Recently i t has been noted that certain polycyclic aromatic compounds cause marked chronic toxicity, particularly on large grass plants. K i l l i n g such grasses is one of the most difficult weed problems under field conditions a n d i t s importance should justify further research. Possibly the phenolic constituents of oils should be studied further, i n view of the now k n o w n toxicity of substituted phenols. Such compounds h a d been removed b y chemical treatment from the oils used i n most of the original work of the writers. 4

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Refined Selective Spray Oils Refined oils of the Stoddard solvent type, safe a n d effective for handling carrot crops, were developed after a relatively short period of research. T h e i r use spread to celery, parsnips, parsley, and anise. Foresters, finding that conifer seedlings were tolerant to the refined oils, sprayed forest tree nurseries. Research proved t h a t a somewhat less toxic oil could be used on flax and onions. T h e use of refined selective spray oils found a definite place i n the agriculture of our country. AGRICULTURAL APPLICATIONS OF PETROLEUM PRODUCTS Advances in Chemistry; American Chemical Society: Washington, DC, 1952.

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Fortified Oil Sprays Research on activation of d i n i t r o sprays had proved t h a t the substituted phenols were more toxic t h a n their salts. These compounds are oil-soluble a n d therefore could be used to bolster the dwindling toxicity of fuel oils. W h e n d i n i t r o b u t y l and a m y l phenols a n d pentachlorophenol were found effective, the general contact o i l emulsion sprays were developed. Since the removal of aromatics from fuel oils lowered toxicity, attention wae directed to other highly aromatic fractions. A v o n weed killer, a very aromatic material, had proved toxic and i t was soon proved that many other aromatic fractione were effective. Unfortunately, most of the old sources of aromatic fractions were soon exhausted, b u t tests proved that the bottoms from the catalytic cracked stocks were similarly toxic. Shell N o . 20, Standard N o . 2, a n d a host of other toxic weed oils soon came onto the market, a n d the demands on Diesel a n d smudge-pot oils were alleviated. Since a few pounds of a substituted phenol fortifier will make 100 gallons of Diesel oil more toxic, the advantage of heavier fortification was i n doubt. Tests proved that a little fortifier goes a long way as the covering capacity of the oil soon becomes the l i m i t i n g factor. T h e problem of extending the oil itself was studied b y agricultural experiment stations i n California and H a w a i i and b y oil companies and herbicide manufacturers. I t is now known that a fortified o i l emulsion spray consisting of a few pounds of fortifier, 10 to 20 gallons of low grade fuel o i l , h a v i n g a v i s cosity of 50 seconds or less, some oil-soluble wetting agent, a n d 80 to 90 gallons of water will k i l l general weed growth almost as effectively as will a straight weed o i l . T h e most effective wetting agents have been sulfonated naphthenic petroleum fractions. Likewise, the fortifiers m a y be made from phenols or substituted phenols derived from petroleum. A q u a t i c seeds i n irrigation ditches interfere with the flow of water and often result i n serious loss to farmers. Chlorinated benzene w i t h appropriate emulsion stabilizers has proved effective i n the control of such weeds. T h e Bureau of Reclamation through its Denver laboratories tested m e t h y l substituted benzenes and found them equally toxic to aquatic weeds. A s a result, the aromatic solvents, both from coal and petroleum sources, are proving a boon to farmers. A n d then came 2,4-D. E m e r g i n g from the war as a miracle weed killer, this material grasped the imagination of A m e r i c a n farmers. W i t h i n three years i t became the center of a multimillion-dollar business. C o u l d o i l fit into the 2,4-D picture? T h e answer is decidedly " y e s . " W o r k with the dinitro sélectives prepared the stage for 2,4-D. T h e y were highly selective; they could be applied b y airplane; they had been p u t on at volume rates as low as 25 gallons per acre. A hormonelike chemical, 2,4-D moves around i n the plant. W o r k i n California i n the early spring of 1946 proved conclusively t h a t 8 gallons of water were as effective as 160 as a carrier for 2,4-D. B u t below 5 gallons per acre, water sprays were hard to apply. W o r k i n the lake states and i n C a n a d a i n 1946 proved that 2,4-D could be applied i n as little as 0.4 gallon of o i l per acre and give good results. D i s t r i b u t i o n was the major factor and the agricultural engineers soon handled that problem. I n the wheat belt of the U n i t e d States and C a n a d a , many millions of acres of grain have been sprayed w i t h 2,4-D. O i l has been used as a carrier where water is not plentiful or where i t is less practical t h a n o i l . O i l is beneficial not only as a carrier b u t also as a spreading agent. O i l e m u l sions carrying considerable wetting agent as a stabilizer have proved effective as 2,4-D carriers on many hard-to-wet plants. Over 100,000 acres of rice are sprayed annually w i t h 2,4-D i n California alone, and practically a l l of this spray is i n the form of a n emulsion, for experience has shown that oil aids i n the spreading of the herbicide on sedges, cattails, a n d tules. Oil-emulsion carrier has proved essential i n the h i g h dosage application of 2,4-D to cattails and tules i n ditches. I t is proving essential i n brush control, i n the control of hoary cress and Russian knapweed—two deep-rooted perennial weeds that resist control efforts. I n fact, oil as a filming agent is proving AGRICULTURAL APPLICATIONS OF PETROLEUM PRODUCTS Advances in Chemistry; American Chemical Society: Washington, DC, 1952.

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beneficial i n almost a l l 2,4-D sprays designed to control perennial plants. I n a l l such sprays, the o i l content should be around 4 gallons per acre, and the o i l should be low enough i n toxicity so as not t o injure the foliage of the treated plants.

Oil in Present-Day Weed Control


Surveying the field of chemical weed control today, i t is found that oils fit into i t i n many ways: As toxicants, the aromatic and olefinic compounds are most effective. A s solvents, aromatic fractions are most useful with pentachlorophenol; aliphatic fractions are effective as solvents for 2,4-D esters. A s filming agents, cheap, readily emulsifiable oils are useful. A s carriers, oils enter the picture where low-volume airplane applications are made. The cheap oils are used i n higher volumes i n grass killers. Possibly new materials w i l l displace the present wasteful high-volume spraying of grasses. Oils form the base for a number of wetting agents and emulsion stabilizers. They are probably the cheapest source of such reagents. Oils could and actually may supply many of the basic chemicals from which special herbicides are synthesized.

Economy of Oil Resources in Future Weed Control A s we t u r n to the future, we are faced w i t h several serious questions. N o one now doubts the essential role of agricultural production, a n d few would question the importance of weed control. B u t , as important as i t is, how far should we go i n using high-energy fuel such as o i l for k i l l i n g weeds? There are t w o criteria upon which this question must rest. T h e relative advantages of using oil must be considered. If oil can do a better job, if i t can do i t more q u i c k l y , more conveniently, or more effectively, the answer is clear. T h e use of o i l for herbicidal purposes might prove as justifiable as its use as a motor fuel. A second a n d possibly more critical question is " C a n pest control be done w i t h less o i l ? " U n d o u b t e d l y , i n planning future work w i t h oil we must devise more economical a n d efficient methods for t h e use of our o i l r e sources i n pest control. 2,4-D. T h e most widely used herbicide at present is 2,4-D. A s far as the selective use of 2,4-D on cereal crops is concerned, the amine formulations seem perfectly satisfactory. A s a translocated herbicide, results are not too good. I n some localities, 2,4-D has been effective i n treating perennial weeds, b u t i n other places results have been disappointing. T h e differences seem related to the movement of the material after i t gets into the plant, a n d here the physiology of the plant, as well as the nature of the chemical, is concerned. A s 2,4-D is applied to the foliage, i t first must penetrate the cuticle. T h e more oil-like the molecule, the more readily i t may go i n . T h i s is evidenced b y the superiority of the esters over the salts on a n equivalent basis. W h e n the 2,4-D has diffused through the cuticle, i t must then move through several layers of l i v i n g cells, the mesop h y l l , from whence i t migrates into the phloem or food-conducting channels. M u c h evidence now indicates that 2,4-D accompanies foods i n moving out of the leaves, through the stems, and on to points of utilization or storage, m a i n l y the growing shoot tips a n d the roots. Here the molecules are i n water solution a n d , as they move through the cuticle, across the mesophyll, a n d along the phloem, the partition of the chemical between these phases will depend o n the nature of the molecule—oil-like molecules penetrate cuticle readily; more polar forms are compatible w i t h the aqueous phases i n mesophyll a n d phloem. Obviously, i t is difficult to fill both these needs w i t h a single molecular form. A n oil-soluble amine salt of 2,4-D has been s y n thesized that has proved to be translocated effectively; possibly, the free acid m a y fit the requirements most accurately. Since the ester forms do translocate, possibly they hydrolyze readily within the plant, releasing the acid to move along the phloem channels. AGRICULTURAL APPLICATIONS OF PETROLEUM PRODUCTS Advances in Chemistry; American Chemical Society: Washington, DC, 1952.


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Recent research has proved 2,4-D to be effective i n brush control, and oil seems to be essential i n the formulations. F o r spraying, a fairly heavy, low t o x i c i t y o i l has proved best. There are indications t h a t a coarse spray which only spots the leaves w i l l foster translocation of the 2,4-D after i t has been absorbed. Oils are also essential as carriers of 2,4-D for basal treatments. Here penetrat i o n of the bark is essential. Whether k i l l i n g the bark is desirable is yet to be determined. M u c h research is needed to define the role of oil i n the basal spray and to find the particular oil best fitted to the method. D N . T h e dinitro general contact weed killers are usually formulated w i t h some oil. I n these, the oil serves as b o t h solvent and filming agent. T h e emulsifier is usually also incorporated to stabilize the emulsion and give wetting properties. To lower costs and conserve o i l , i t is desirable to use the m i n i m u m effective volume i n these sprays. Possibly some of the penetrating oils may fit into this need. N o research is k n o w n t h a t is designed specifically to find the best oil for this purpose. M a l e i c H y d r a z i d e . Best results w i t h this new chemical to date have followed aqueous sprays of the diethanolamine salt. Treatment of grass w i t h comparable amounts of the chemical i n oil has been less effective. T h i s is contrary to the results w i t h dinitro and 2,4-D sprays; possibly the p a r t i t i o n coefficient of the oil-soluble material is not right. A s w i t h 2,4-D, penetration is only one aspect of the herbicidal action; translocation is involved where grasses h a v i n g underground rhizomes or crowns are i n v o l v e d . F o r k i l l i n g these, migration through the phloem channels m a y be the essential factor, and the more polar salts m a y prove best. Pre-emergence H e r b i c i d e s . P r o b a b l y the first pre-emergence treatments were stove-oil sprays on carrots and onions i n the Salinas V a l l e y . F o l l o w i n g their successful introduction, d i n i t r o sélectives were tried w i t h success i n the T u l e L a k e region of California. T h i s t r i a l was i n early summer. T h e following winter similar trials on onions i n the Sacramento delta were ruinous; the crop had to be plowed up a n d replanted. E v i d e n t l y , weather played a decisive role i n the use of pre-emergence treatments. W i t h the introduction of 2,4-D, pre-emergence methods again became prominent and again weather proved decisive. M o r e recently, toxic oils and fortified oils have been used; some crops have been saved, and other crops have been ruined. There are two distinct pre-emergence methods, one a contact spray that kills weed seedlings emerging before the crop, the other a pre-emergence treatment through the soil. F o r contact pre-emergence work light oils of high t o x i c i t y , such as Stoddard solvent, are safest although possibly relatively expensive. Stove oil is also relatively safe; Diesel oil can be used, b u t if the crop seedlings are pushing through, they w i l l be injured. Crops h a v i n g big, vigorous seedlings are less susceptible to i n j u r y t h a n are small seeded ones. A r o m a t i c oils have proved hazardous, as the oils or the vapors from t h e m may severely injure seedlings of the crop. Fortified oils are even more hazardous, because the fortifying agents form highly toxic residues i n the soil. O n l y under special circumstances, usually i n v o l v i n g d r y weather w i t h no rainfall u n t i l after the crop is well established, m a y fortified oils or their emulsions be safely used for pre-emergence work. M o s t effective 2,4-D pre-emergence treatments are through the soil. P r e emergence sprays by the residual or soil-treatment method should be used w i t h a crop t h a t is relatively tolerant of the herbicide. Selectivity between the crop a n d the weeds depends upon the spread i n susceptibility between t h e m . F o r this reason, pre-emergence treatment w i t h 2,4-D m a y be safely used on certain corn varieties, while on others i t is hazardous. I t is most successful on heavy to m e d i u m soils if soaking rains follow, and i n light sandy soils if rainfall is scanty. T h e opposite p a t tern of rainfall will give less satisfaction. Oils play no part i n this 2,4-D pre-emergence treatment. However, heavy weed oils have been used alone and with fortification i n established crops like cane or alfalfa to k i l l weeds d u r i n g the dormant season. Oils have been pre-eminent i n grass k i l l i n g , but w i t h the introduction of isopropyl ΛΓ-phenyl carbamates, trichloroacetates, and maleic hydrazide, satisfactory


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substitutes m a y be found. T h o u g h this m a y reduce the b u l k use of low-grade oils, it should not discourage the use of oils i n weed control. However, only where oils serve a useful purpose i n better a n d cheaper herbicides should they be withdrawn from use as fuel to be applied i n the control of weed pests.


Summary I n the utilization of petroleum oils i n the field of chemical weed control, oils function as toxicants, as solvents, as filming agents, and as carriers. I n view of the very effective synthetic compounds now used as toxicants (substituted phenols), the toxicity of the oils themselves is somewhat less important t h a n i t once was. Oils m a y serve as adjuvants i n formulations i n v o l v i n g 2,4-D, 2,4,5-T, d i n i t r o compounds, trichloroacetates, a n d others. T h e y have the unique property of aiding in the con tact, spreading, a n d penetration of herbicides. I n addition, synthesis of wetting agents, emulsifiers, and special herbicides m a y be dependent o n petroleum products.

Literature Cited (1) Crafts, A. S., and Reiber, H. G., Hilgardia, 18, 77-156 (1948). (2) Leonard, Ο. Α., and Harris, V. C., Proc. So. Weed Conf., 3, 91-102 (1950). RECEIVED

November 14, 1 9 5 0 .


AGRICULTURAL APPLICATIONS OF PETROLEUM PRODUCTS

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