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CS 414 - Weed Science Section 5 Herbicide Behavior in Plants I.
Terminology: A. Absorption - entry of herbicide into the plant. Often called “uptake”. B. Translocation - movement of herbicide from site of entry to other locations within plant. C. Metabolism - changes in chemical structure of the herbicide molecule within the plant. Also referred to as “degradation”. Most changes reduce phytotoxicity of the herbicide. In a few cases, an initial step in the degradation pathway may increase phytotoxicity.
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II. Fate of herbicides applied to plants Once a herbicide has contacted the plant surface, one or more things can happen to it: A. The herbicide may volatilize and be lost to the atmosphere. Extent of losses depends upon vapor pressure of herbicide, environmental conditions (mainly temperature), and how rapidly the herbicide is absorbed into the plant. B. The herbicide may be washed off the leaf surface (rainfall or irrigation) before it has had time to be absorbed into the plant. Extent of washoff depends upon speed of absorption, time of rainfall or irrigation after application, and chemical properties of the herbicide (such as water solubility). The required rain-free periods vary greatly among postemergence herbicides. Labels for some products list a rain-free period (for example, the label may say “rainfall or irrigation within 4 hours of application may reduce control”), but this is often not on labels. More typically, the label will have some nonspecific (and non-helpful) language such as “rainfall shortly after application may reduce effectiveness of this herbicide”. C. The herbicide may be photodegraded (broken down by sunlight) before it has had time to be absorbed into the plant. Extent of theses losses depends upon the herbicide’s susceptibility to photodegradation, the speed of absorption, and the light intensity. D. The herbicide may remain on the outer surface in a viscous liquid or crystalline form (ie., not absorbed). E. The herbicide may penetrate the cuticle, but remain absorbed in the lipid components of the cuticle. In this case, it will have no activity (“kill”) on the plant as it has not reached living tissue. F. It may penetrate the cuticle and then enter the apoplast or symplast, where it may be subject to translocation and metabolism. III. How do weed scientists study absorption, translocation, and metabolism of herbicides? This is typically done using radiolabeled herbicides (herbicides with a 14C inserted somewhere within the structure). Minute amounts of the radiolabeled herbicide, either applied alone or mixed with non-labeled herbicide, are applied to specified parts of the plant (such as a leaf or in the growth media). Radioautographs can be used to visualize the general location of the radiolabel within the plant. The plant can be sectioned (divided into various parts), and those parts combusted in a biological sample oxidizer. The released 14C is trapped and quantified via liquid scintillation spectrometry. Alternatively, the plant tissue
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can be homogenized and the radiolabel extracted using various solvents, and then quantified using liquid scintillation spectrometry. To determine metabolism of the herbicide, the radiolabel is extracted as described above, and the metabolites identified by various chromatographic techniques. IV. Herbicide translocation A. Apoplast - The non-living, continuous network of cell walls, intercellular spaces, and xylem tissue that functions as the conduit for water and mineral nutrients from roots to shoots. B. Apoplastic translocation - translocation via the apoplast. 1.
Apoplastically mobile herbicides follow the same pathway and move in the same direction as the net movement of water within the plant. Transpiration is the driving force.
2.
Soil-applied herbicides, if mobile within plants, are primarily apoplastically mobile. Some soil-applied herbicides, such as dinitroanilines, are essentially non-mobile in plants.
3.
Some soil-applied herbicides, such as triazines and ureas, can also be applied postemergence. They are apoplastically mobile when applied postemergence.
4.
An apoplastically mobile herbicide applied to the soil will enter the roots, move up the stem, and accumulate in greater quantities in leaves with the highest transpiration rate (i.e., older, fully expanded leaves). See Figure 1. Injury symptoms, if present, will be noted first in the areas of highest herbicide concentration (in this case, between veins and around leaf margins of more fully expanded leaves).
5.
An apoplastically mobile herbicide, applied as a single droplet to a leaf, will move outward toward the tip and margin of the leaf and will accumulate around the leaf margin. It will not translocate to other leaves; remember it follows the same direction as water movement. See Figure 2. In actual practice, an apoplastically mobile herbicide applied postemergence overtop would contact essentially all of the foliage. Hence, the herbicide would be found in essentially all of the above-ground portions of the plant although there would still be accumulation around leaf margins.
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If an apoplastically mobile herbicide is applied postemergence-directed to the bottom portion of a plant and contacts only leaves, the herbicide will remain only in the leaves contacted; it will not move to non-contacted leaves higher in the canopy. However, if the directed herbicide comes into contact with the stem, some of the herbicide may be absorbed by the stem and enter the xylem. In that case, the herbicide may be found higher in the plant in leaves not contacted by the spray. Cotton growers, who post-direct a lot of apoplastically mobile herbicides, call that phenomenon “climbing the stalk”.
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C. Symplast - The continuous network of living cells, intercellular protoplasmic connections (plasmodesmata), and phloem tissue that transports assimilates (food) from the site of production (leaves) to the site of use (root and shoot meristematic areas, developing fruit, and storage organs such as rhizomes). D. Symplastic translocation - Translocation via the symplast. 1.
Symplastically mobile herbicides (also called phloem-mobile herbicides) follow the same pathway in the plant as movement of assimilates (or “food”). This is often referred to as a "source-to-sink" pattern, with the leaves being the "source" of food production and apical growing points, roots, and underground storage organs, such as rhizomes, being the "sinks" or areas utilizing the food produced by photosynthesis.
2.
Translocation of herbicides in the symplast is bidirectional. It can be acropetal (meaning upward from point of application) or basipetal (meaning downward from point of application). The direction of net movement depends on the location of areas of greatest assimilate supply (the source) and demand (the sink) within the plant.
3.
Symplastically mobile herbicides are almost always applied postemergence, or postemergence-directed.
4.
A symplastically mobile herbicide applied to a single fully expanded leaf will move out of that leaf and move both up and down the stem via the phloem, accumulating in those areas where assimilate demand is greatest, such as apical growing points, expanding young leaves, developing seed or fruit, and root tips. See Figure 3.
5.
Source-sink relationships ultimately determine the rate, direction, and extent of phloem-mobile herbicide translocation, and this relationship can vary among weeds according to life cycles. Maximum herbicidal activity on annual weeds with phloem-mobile herbicides generally is attained on smaller plants not under stress (such as drought) with the herbicide applied during a period of rapid growth. During this period, the root and shoot apical meristems are primary sinks into which phloem-mobile herbicides are readily translocated along with the flow of photosynthate from mature leaves, the primary sources. Perennial weed source-sink relationships are more complex than that of annuals, and response to phloem-mobile, symplastically translocated herbicides is highly dependent on stage of development. Since a key objective in perennial weed management is to destroy over-wintering vegetative propagules (rhizomes, stolons, tubers, etc.) as well as the foliage, phloem-mobile herbicides should be applied
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during a period in which photosynthate is being accumulated in the propagules. This usually occurs after the plant is relatively large (plenty of leaf development, plenty of photosynthesis occurring). A symplastically mobile herbicide may, therefore, be more effective on perennial plants when applied to older, larger plants as opposed to younger plants. Successful perennial weed management with foliarapplied, phloem-mobile herbicides, such as glyphosate, dicamba, and 2,4-D, is based on application timing relative to source-sink relationships. See Figure 4.
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V. Herbicide selectivity A. Terminology 1.
Susceptibility - positive response to the presence of a given herbicide, resulting in some growth process being altered. A susceptible plant is injured or killed by the herbicide.
2.
Tolerance - lack of response of the plant to a given herbicide. A tolerant plant is not injured or only slightly injured by the herbicide.
3.
Selectivity - differences in response of plant species to a given herbicide. Selective herbicides are much more phytotoxic to some species than others (ie., weeds are killed and crop injured little to none). Non-selective herbicides kill most plant species. Herbicide selectivity is not absolute. Virtually all herbicides can be non-selective if applied improperly, at excessive rates, or under environmental conditions that adversely affect biological tolerance mechanisms. Herbicide selectivity is the basis for successful chemical weed management in crops. Selectivity is the result of physical and/or biological factors that produce differential herbicide toxicity in crops and weeds.
VI. Factors affecting herbicide selectivity A. Physical Factors: influence contact between herbicides and plant surfaces. 1.
Herbicide Application Rate: Selectivity for most herbicides (probably all herbicides) is rate dependent. Selectivity may be good at recommended application rates but poor at excessive rates. An example would be diuron. At low rates (0.8 to 1.6 lb ai/A), diuron can be used for selective weed control in cotton. Diuron at high rates (4 to 12 lb ai/A) can be used for total vegetation control in non-cropland situations. a.
Margin of selectivity: A wide margin of selectivity means the herbicide remains selective over a fairly wide range of application rates. An example be atrazine in corn. Corn is very tolerant of atrazine and can withstand rates much greater than labeled rates.
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A narrow margin of selectivity means there is only a small difference in rates that will give selective weed control and rates that will injure the crop. An example would be metribuzin in soybeans. Herbicides with a narrow margin of selectivity require more precise application, rate adjustments for soil types, etc. 2.
Herbicide formulation: a. Granule vs Sprayable Formulations: Selectivity may be obtained in emerged crops through use of granular formulations as opposed to spray applications. Granules tend bounce off dry foliage and drop through the canopy of emerged plants. With a spray application of the same active ingredient at the same rate per acre, more herbicide will be retained on the foliage which may cause injury. Example: Devrinol granules vs spray for preemergence weed control in established woody ornamentals. Another example would be Broadstar (granular formulation of flumioxazin, used in ornamentals) compared with Valor (sprayable formulation of flumioxazin) used in row crops. b. Adjuvants and Solvents in Postemergence Herbicides: Adjuvants are nonpesticidal chemicals added to a pesticide by the manufacturer to improve the pesticide’s physical or chemical characteristics (ie., to enhance handling characteristics, such as stability of the formulation and mixing). Adjuvants may also be added by the end user to increase weed control, but at the same time they may increase crop response (ie., reduce selectivity). Solvents and other inert ingredients in a herbicide formulation may also impact selectivity.
3. Herbicide Placement: Herbicide selectivity by physical placement refers to any factor(s) that results in a physical (spatial) separation between sensitive crop tissues and/or absorption sites and a toxic dose of herbicide. To be effective in controlling weeds or to cause crop injury, a herbicide must gain entry into the plant. Selectivity may be achieved by placing the herbicide where it is not in contact with the crop or the site of uptake by the crop, or at least the contact with the crop is limited. Physical placement can explain selectivity for certain preemergence and postemergence-directed herbicides. Profile selectivity (or profile tolerance, defined in Section 4) is one example (see Figure 5). Another example would be use of shielded or hooded sprayers and ropewick applicators.
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Figure 5. Profile selectivity vs biological selectivity.
4. Protective Barriers: In special cases, a protective barrier may be used to physically separate the crop from the herbicide. An example would be activated charcoal as a seed coating, root dip, a layer placed between seed and herbicide treated zone, or in transplanter water. Activated charcoal will adsorb the herbicide before it reaches the seed or is taken up by the crop roots. B. Biological Factors: related to differences among plants 1. Morphological Factors: due to differences in structural features of plants a. Leaf Characteristics: may affect interception of spray droplets or retention of spray droplets. Leaf characteristics of concern include leaf pubescence, leaf waxiness, leaf angle, leaf shape. The role of differential spray retention as a factor in determining selectivity between crops and weeds is probably minor for most postemergence herbicides. Species with heavy leaf pubescence may be less affected by a postemergence herbicide than species with smooth leaves. Leaf hairs hold spray droplets off leaf surface, thus reducing absorption into the leaf. In the 1970's, dinoseb (a herbicide no longer on the market) was used for postemergence control of broadleaf weeds in soybeans. Dinoseb, applied with normal application procedures, typically caused severe injury on soybeans. However, injury could greatly be reduced by applying dinoseb at high pressure and low spray volume (basically very fine droplets). Soybeans have hairy leaves, and the very fine droplets would be held off the leaf surface by the hairs. Weed species without hairy leaves could be controlled with an acceptable level of soybean injury.
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Spray droplets will “bead up” on a waxy leaf but may spread out on a less waxy leaf. If the droplet beads up, there is less contact with the leaf surface. Leaves in a horizontal position tend to intercept and retain more spray droplets than leaves in a vertical position. Plants with large, broad leaves may intercept and retain more spray droplets than plants with smaller or narrow leaves. Wild garlic is an example of a weed where good spray coverage is difficult because of waxy, upright, very narrow leaves. Time of day when herbicides are applied can affect control of weeds with nyctinastic responses. Leaves on plants like sicklepod and velvetleaf fold up and/or droop down at night. This would result in less interception of spray droplets. b. Location of growing points Exposure, or lack of exposure, of growing points may be a factor in selectivity of postemergence contact-type herbicides in some situations. Dicot weeds have exposed growing points, whereas the growing point of a grass in protected inside the sheath. c. Rooting depth Shallow-rooted plants may be killed by a preemergence herbicide while deeprooted plants escape injury because the roots are below the herbicide-treated zone. See the previous graphic for profile selectivity. d. Underground Reproductive Organs For perennial weeds, one may be able to kill (burn off) aerial portions of plant, but a new plant will develop from rhizome or tuber. This is particularly a problem with contact-type herbicides.
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2. Physiological factors a. Differential absorption Differential selectivity between crops and weeds is sometimes due to differential absorption of the herbicide (i.e., the weed absorbs more herbicide than the crop). Herbicides on plant foliage or in contact with subterranean plant organs must cross several natural barriers before ultimately reaching their biochemical site of action. Herbicides deposited on a plant surface must first penetrate non-living barriers before ultimately reaching their site of action in living plant tissue. The outermost layer covering all aerial plant parts is the cuticle, which represents the most significant barrier to foliar penetration by herbicides. The cuticle is a complex layer consisting of waxes embedded in a matrix of polymeric cutins with an outermost coating of soluble waxes collectively called the epicuticular wax. The cuticle is a barrier to water loss from the plant. The impervious nature of the cuticle also makes it quite effective at preventing or impeding entry of many foliar-applied herbicides. Epicuticular waxes are hydrophobic (non-polar, oil-soluble, water-hating) and decrease leaf wetability by repelling water and other hydrophilic (polar, oilinsoluble, water-loving) substances. Herbicides that are highly polar do not easily penetrate the nonpolar epicuticular wax layer. In such cases, adjuvants are often used to facilitate herbicide penetration. Conversely, herbicides that are more nonpolar or hydrophobic are more soluble in the epicuticular waxes and thus tend to penetrate plant cuticles more readily. Herbicides that are formulated as salts are relatively polar, whereas those formulated as esters are relatively nonpolar. Cuticle thickness can affect herbicide penetration. Cuticle thickness varies from species to species and plant to plant within species. It may vary among leaves on the same plant. Cuticle composition and thickness depend on the age of the tissue and the environmental conditions that exist during deposition of cuticular components, including the epicuticular wax layer. Leaves that develop under conditions of high light, low soil moisture, and low relative humidity tend to have a thicker epicuticular wax layer than those which develop under low light, adequate soil moisture, and high relative humidity. Weeds that develop under hot, dry conditions are usually more difficult to control with postemergence herbicides. Subsequent environmental conditions (after leaf development) also can affect cuticle thickness and permeability. Adequate soil moisture and high humidity
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tend to keep cuticles hydrated, and these cuticles are more conducive to penetration by herbicides. Hot, dry weather can result in an increase in epicuticular wax, cuticle dehydration, and plant stress, and foliarly applied herbicides are less effective during these periods due to reduced herbicide absorption. b. Differential translocation Differential selectivity between crops and weeds is sometimes due to differential translocation of the herbicide to the site of action (i.e., the weed translocates more herbicide to the site of action than the crop). c. Differential metabolism Differential selectivity between crops and weeds may be due to differential metabolism of the herbicide (i.e., the crop metabolizes, or inactivates, more of the herbicide than does the weed, or the crop metobolizes the herbicide more rapidly). Both rate and extent of metabolism are important. Differential metabolism is the most common mechanism that contributes to herbicide selectivity. Herbicides may be classified according to their ability to undergo biotransformations (to be metabolized) in plants. Stable herbicides are those which do not undergo metabolic deactivation in plants. Glyphosate and paraquat are examples of stable herbicides that are not metabolized to any extent by most plants. This characteristic is not surprising since glyphosate and paraquat are two of the most nonselective herbicides currently available. Metabolically deactivated herbicides are those that may undergo various breakdown reactions (such as oxidation, reduction, hydrolysis, and conjugation) that render them nontoxic in tolerant species. See previous example of atrazine and 2-hydroxy-atrazine. Metabolically activated herbicides, once absorbed by sensitive plants, undergo metabolic transformation that results in an enhancement of herbicide phytotoxicity rather than detoxification. A well-known example is 2,4-DB (structures shown previously). Plants susceptible to 2,4-DB, such as morningglory species, enzymatically convert the relatively nontoxic 2,4-DB to the phytotoxic 2,4-D via a cellular process called beta-oxidation. Legumes, such as peanuts, convert 2,4-DB to 2,4-D much slower than non-legumes such as morningglory, giving the peanut time to deactivate the 2,4-D before lethal
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concentrations accumulate. Examples of differential absorption, translocation, and metabolism are given below. Peanut is tolerant of tetrafluron (experimental herbicide, never commercialized) while jimsonweed is susceptible. Data in Table 1 indicate selectivity is due to both differential absorp tion (jimsonweed absorbed more) and differential translocation (jimsonweed translocated more to the shoot). Table 1. Absorption and translocation of root-applied tetrafluron by peanut and jimsonweed. Pinto and Corbin, 1980. absorbed from nutrient solution (% of applied)
translocated to shoot (% of absorbed)
Peanut (tolerant)
33
52
Jimsonweed (susceptible)
93
96
Species
Peanut absorbed approx. 1/3 as much as jimsonweed. Peanut translocated approx. 1/2 as much of absorbed herbicide to shoot. Approximately 1/6 (1/3 x 1/2) as much herbicide reached shoot of peanut compared with jimsonweed.
Soybean is very tolerant of imazaquin while cocklebur is very susceptible. Results in Table 2 show that selectivity is not due to either differential absorption or differential translocation. Cocklebur absorbed less herbicide than soybean, and cocklebur translocated less absorbed herbicide out of the treated leaf than did soybean. Selectivity in this case is due to differential metabolism. Tolerant soybean metabolized much, much more of the herbicide than the susceptible cocklebur.
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Table 2. Absorption and translocation of root-applied tetrafluron by peanut and jimsonweed. Pinto and Corbin, 1980. absorbed from nutrient solution (% of applied)
translocated to shoot (% of absorbed)
Peanut (tolerant)
33
52
Jimsonweed (susceptible)
93
96
Species
Peanut absorbed approx. 1/3 as much as jimsonweed. Peanut translocated approx. 1/2 as much of absorbed herbicide to shoot. Approximately 1/6 (1/3 x 1/2) as much herbicide reached shoot of peanut compared with jimsonweed.
Corn is tolerant of nicosulfuron while grassy weeds, such as giant foxtail, are susceptible. Results in Table 3 indicate differential selectivity is due to differential absorption, translocation, and metabolism. Table 3. Absorption, translocation, and metabolism of foliar-applied nicosulfuron by corn and giant foxtail. Carey, Penner, and Kells, 1997. Translocated Absorbed
out of treated leaf % of applied
Corn
Metabolized
14C
12
7
87
18
18
41
(tolerant) Gt. Foxtail (susceptible)
Mesosulfuron is applied postemergence in wheat to control Italian ryegrass. Selectivity in wheat is due to differential absorption and differential metabolism, but differential translocation does not play a role in selectivity (Table 4).
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Table 4. Absorption, translocation, and metabolism of foliar-applied mesosulfuron by wheat and Italian ryegrass. Bailey, Hatzios, and Wilson, 2003. Translocation out Absorption % applied
of treated leaf
Metabolized
% of absorbed
% recovered
Wheat (tolerant)
23
5
51
Italian ryegrass (susceptible)
80
3
14
d. Safeners (protectants or antidotes) Safeners or antidotes are chemicals formulated with the herbicide or applied to crop seed which have no effect alone but can protect the crop plant from herbicide injury. There are presently commercial antidotes that protect corn from thiocarbamate herbicides, or protect corn and wheat from sulfonylurea herbicides, or protect corn from chloroacetamide herbicides. The primary mode of action involves stimulation of herbicide metabolism in the protected plants. Data in Table 5 show the effects of an antidote formulated with the herbicides butylate and EPTC on corn injury from the herbicides. Table 6 shows the effects of an antidote or safener formulated with the premixed combination of mesosulfuron plus iodosulfuron on wheat response.
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Table 5. Corn injury with butylate and EPTC as affected by antidote. Martin and Burnside, 1982. Herbicide and rate (lb ai/A)
% corn injury - antidote
+ antidote
Butylate, 4.5
10
0
Butylate, 9.0
40
0
EPTC, 3.4
43
0
EPTC, 6.7
80
0
Table 6. Wheat injury with mesosulfuron + iodosulfuron as affected safener. Crooks and York, 2004. Herbicide:safener ratio
% wheat injury 3 wks
6 wks
No safener
26
17
1:1
8
3
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e. Pesticide interactions It is a common practice to apply combinations of pesticides to crops. These may be applied as tank mixes or as sequential applications (such as an insecticide infurrow at planting followed by a preemergence postemergence herbicide, or a preemergence herbicide followed by a postemergence herbicide). Certain pesticide combinations may affect selectivity. Examples: Organophosphate insecticides may increase soybean injury from metribuzin but decrease cotton injury from clomazone. i. Synergistic interaction Total response from the pesticide combination is greater than the expected sum of responses of each pesticide alone. Example:
Herbicide A gives 30% control Herbicide B gives 40% control Combination of A + B gives 90% control
ii. Antagonistic Interaction Total response from the pesticide combination is less than the expected sum of responses of each pesticide alone. Example:
Herbicide A gives 30% control Herbicide B gives 40% control Combination A + B gives 50% control
iii. Additive effect Total response from herbicide mixture is equal to the sum of responses of each herbicide alone. Example:
Herbicide A gives 30% control Herbicide B gives 40% control Combination of A + B gives 70% control.
Pesticide interactions can be beneficial or detrimental. Table 7 shows an example of a detrimental interaction on weed control. Fluazifop and quizalofop are applied postemergence to control grassy weeds in various crops. Bromoxynil was applied to BXN cotton (cotton resistant to the herbicide bromoxynil; it was commercial for several years, no longer on the
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market) to control various broadleaf weeds. If both grasses and broadleaf weeds were present in BXN cotton, an obvious solution would appear to be to tank mix bromoxynil with a grass-control herbicide such as fluazifop or quizalofop. However, as the data show, these tank mixes are very antagonistic on grasses. Table 8 shows an example of a deterimental interaction on crop response. Thifensulfuron is a postemergence herbicide used in soybean to control certain broadleaf weeds. Chlorpyrifos, malathion, carbaryl, and methomyl are insecticides. The data show a synergistic interaction for soybean injury when thifensulfuron is tank mixed with any of those insecticides. Table 9 shows an example of a beneficial interaction on weed control. A combination of the herbicides trifluralin and alachlor was synergistic on ivyleaf morningglory. The combination controlled the weed much more than expected based on the sum of the individual responses. And, Table 10 shows an example of a beneficial interaction on crop response. The herbicide clomazone was once widely used either preemergence or preplant incorporated on cotton. However, cotton tolerance of clomazone was marginal at best. The insecticide disulfoton could be applied in the seed furrow during planting to safen cotton from clomazone. f. Herbicide-tolerant crops i. Non-transgenic crops Through traditional selection and breeding procedures, cultivars of crops have been developed that have enhanced tolerance of herbicides that are either nonselective or inadequately selective on other cultivars. Examples include Poast Protected corn (tolerant of sethoxydim; no longer commercial), STS soybeans (tolerant of sulfonylurea herbicides that are normally too injurious to use), and Clearfield corn and wheat (tolerant of imidazolinone herbicides). ii. Transgenic crops In recent years, transgenic crops with resistance to herbicides that are normally nonselective have been developed and commercialized. A gene, usually from a bacteria, has been inserted into these crops that codes for tolerance of the herbicide in question. Examples include Roundup Ready corn, cotton, soybeans, canola, and sugar beets (resistant to glyphosate); Liberty Link corn, cotton, and soybeans (resistant to glufosinate); and BXN
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cotton (resistant to bromoxynil). Example: Glyphosate kills plants by inhibiting the activity of the enzyme EPSPS. Activity of this enzyme is necessary in the synthesis of certain amino acids in plants. Roundup Ready crops contain a bacterial gene that codes for a version of EPSPS that has greatly reduced affinity for glyphosate. Table 7. Annual grass control with tank mixtures of graminicides and bromoxynil. Culpepper, York, and Brownie, 1999. Graminicide None
% control - bromoxynil + bromoxynil 0
9
Fluazifop
83
39
Quizalofop
84
25
Weed Sci. 47:123-128.
Table 8. Soybean response to combinations of insecticides and thifensulfuron. Ahrens, 1990. % soybean injury Insecticide none chlorpyrifos malathion carbaryl methomyl Weed Technol. 4:525-528.
- thifensulfuron
+ thifensulfuron
0
21
21 1 5 1
78 40 59 56
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Table 9.
Ivyleaf morningglory control with trifluralin plus alachlor combinations. Prosch & Kapusta, 1983. Treatment
% control
Trifluralin
30
Alachlor
17
Trifluralin + alachlor
85
Weed Sci. 31:23-27.
Table 10. Cotton response to combinations of clomazone and disulfoton. York, Jordan, and Frans, 1991. Clomazone (kg/ha)
% cotton injury - disulfoton
0
+ disulfoton
0
0
0.28
8
1
0.56 0.84 1.12
18 28 39
1 2 3
Weed Technol. 5:729-735.
