Soil-Applied Herbicides

Front 1

Give four traits of soil-applied herbicides.

Back 1

They can have variable activity and residue levels;

They may require incorporation to reduce volatilization and photo degradation;

They are degraded by soil microbes; and

They are degraded more rapidly under warm, moist conditions.

Front 2

List three ways in herbicide interaction with soil is altered.

Back 2

Herbicide characteristics;

The soil type, pH and organic matter; and

Available soil moisture.

The interactions are both variable and unpredictable.

Herbicides differ in the tenacity with which they bind to soil and organic material. For example most herbicides adhere more tightly to organic matter than to clay particles which means as the organic matter content of the soil increases, the application rate of a herbicide needed for weed control also increases.

Some herbicides may bind so closely to soil that they are unavailable for uptake by plants. Some bind so loosely they can be subject to movement in soil water, moving with surface runoff and moving up and down through the soil profile.

Front 3

What does the breakdown of herbicide residue depend on?

Back 3

It depends on soil microbial activity, which can vary with soil temperature, soil moisture and the fertility of the soil. In conditions where the degradation of herbicide residues is low, the crops that follow later in the rotation can be damaged.

Front 4

What are the Group 3 herbicides, what are they commonly used for, and how do they work?

Back 4

Group 3 herbicides are the dinitroanalines (DNAs), and are commonly used for grass and broad-leaved weed control in canola, cereal and legume crops.

They are pre-emergence, soil-applied herbicides which act by inhibiting cell division and elongation.

Front 5

What are four characteristics of Group 3 herbicides (the inhibitors of cell division)?

Back 5

They affect roots more than shoots;

The cause affected plants to have stubby, pruned roots;

They are applied at higher rates on high organic matter soils; and,

They are incorporated to prevent volatilization and photo degradation.

Front 6

If a herbicide is a not a weak-acid herbicide, will it's solubility in soil water and efficacy change with the pH of the soil?

Back 6

No.

Remember: Not a weak-acid herbicide, no change with pH in the soil.

Front 7

Are Group 3s (DNAs) at a high risk for soil leaching?

Back 7

No, these herbicides are strongly absorbed by soil and have a low leaching potential.

Front 8

What is responsible for the degradation of Group 3s (DNAs)?

Back 8

Microbial action in the soil. Herbicide residues can be a concern in following crops during cool or dry conditions.

Front 9

Trifluralin and ethalfluralin are examples of Group 3s (DNAs). Describe how they work.

Back 9

They inhibit cell division in plants by inhibiting the synthesis of the protein 'ropes' that pull the chromosomes apart during cell division.

The DNAs bind to the microtubules in the place of tubulin and stop further fibre synthesis. Degradation, however, continues. Consequently tubules get shorter, not longer, and soon there are no tubules to pull apart the chromosome.

Cell division is interrupted, and 'aberrant' cells are created.

Front 10

What are the small 'ropes' that pull apart and separate the chromosomes during cell divisions?

Back 10

Microtubule fibres. They are made up of the protein tubulin and are constantly being made at one end of the cell and taken apart at the other end.

Front 11

What three things happen when cell division is disrupted?

Back 11

Chromosomes do not line up and separate;

Nuclei replicate extensively; and

A cell wall does not form.

Front 12

Knowing what you know about Group 3s (DNAs), what areas do you expect them to affect most?

Back 12

The regions of the plants that are rapidly growing and dividing. Cell division occurs primarily in the meristematic regions of the plant.

Front 13

How are Group 3s (DNAs) uptaken and translocated by the plant?

Back 13

Because DNAs binds to soil, uptake by roots is limited.

It has been speculated that DNAs may enter the plants in a vapour state. Seedlings intercept DNAs as they grow through the treated soil layer. DNAs are presumed to dissolve readily into the lipid-based membranes of seedlings.

There is thought to be little translocation of DNAs following uptake. Therefore, unless DNAs enter a meristematic region, and enter the nucleus of a cell, they can have little effect.

Front 14

What is the meristematic region of a plant?

Back 14

A meristem is the tissue in most plants consisting of undifferentiated cells, found in zones of the plant where growth can take place.

Front 15

Is selectivity in Group 3s (DNAs) based on metabolism?

Back 15

No

Front 16

The selectivity of Group 3s (DNAs) is unique and based on what factors? (4 of them)

Back 16

The inability of these herbicides to translocate;

Depth of incorporation;

Their highly lipophilic (oil-loving) nature; and

The differing positions of weed and crop meristems.

Front 17

Are Group 3s (DNAs) effective when applied directly to foliage?

Back 17

No, they have little or no effect when applied directly to foliage.

Front 18

Describe how Group 3s (DNAs) select between grass weeds and cereal crops.

Back 18

Primary roots and the coleoptile node (or crown) are important sites of DNA activit in cereal crops and grass weeds. While the position of the primary root is the same, the position of the coleoptile node differs among wheat and barley and some grass weeds (such as wild oat and green foxtail).

Wild oat and green foxtail exhibit mesocotyl elongation, which pushes the coleoptile upward. Wheat and barley do not. Mesocotyl elongation of grass weeds pushes the DNA-sensitive coleoptile node through the DNA-bearing soil layer. The DNAs enter the coleoptile node and inhibit cell division, eventually leading to plant death. Wheat and barley also grow through the DNA-treated layer, but expose only the coleoptile, which is not sensitive to inhibition.

Front 19

What is the coleoptile?

Back 19

The coleoptile is the pointed protective sheath covering the emerging shoot in monocotyledons (monocots) such as oats and grasses.

Front 20

Describe how Group 3s (DNAs) select between broad-leaved weeds and crops.

Back 20

Selectivity among broad-leaved weeds and crops is not well understood. There is a reported correlation between the ability of the plant to tolerate DNAs and the oil content of seeds. The highly hydrophobic DNAs may dissolve in the lipid fraction of these seeds and thus not reach the site of action.

Front 21

What are the symptoms of Group 3s (DNAs) damage?

Back 21

DNAs stop the growth of root tips and cause root tip swelling as cells enlarge without longitudinal cell growth. Shoot elongation is inhibited, the hypocotyl of dicots thickens and stems enlarge. A successful Group 3 herbicide is characterized by little weed emergence. The seedlings that do emerge show root pruning and stubby root tips.

Front 22

What is the hypocotyl region?

Back 22

It is the stem of germinating seedling, found below the cotyledons and above the radicle (root).

Front 23

Are Group 3s (DNAs) toxic?

Back 23

DNAs have no effect on mitosis of most animal cells. They are non-hazardous to birds but some, if added directly to water, is toxic to fish. The strong absorption to soil and the soil application methods minimize potential hazards to fish; however, runoff from fields directly into ponds should be avoided.

Front 24

What are the implications of Group 3s (DNAs) for application? (a list of five)

Back 24

Depth of incorporation and depth of seeding are critical to the success of DNA herbicides;

DNAs are strongly absorbed to soil organic matter. Soil with high organic matter or with heavy crop residues often require higher rates;

DNAs should be incorporated to reduce losses by volatilization and photodecomposition and to correctly position the herbicide for maximum efficacy;

There is little risk of leaching;

and

These herbicides are relatively safe to handle.

Front 25

The Group 8 herbicides are known as what?

Back 25

Thiocarbamates

Front 26

Are the Group 8s soil applied herbicides?

Back 26

Yes

Front 27

What do the Group 8 herbicides do, and what are five characteristics?

Back 27

Group 8 herbicides inhibit Very Long Chain Fatty Acid Biosynthesis and:

Are soil applied;

Must be incorporated;

Placement at the correct soil depth is critical for selective control;

Affected weeds fail to emerge; and

Affects shoots more than roots.

Front 28

Is much known about how Group 8s work?

Back 28

No. While similar to the DNAs (the other major soil-applied herbicides), they may be more appropriately compared to the other inhibitors of fatty acid biosynthesis, the ACCase inhibitors.

Front 29

Name the three common names of Group 8 herbicides.

Back 29

Triallate

EPTC

Butylate

Front 30

What class are Group 8s?

Back 30

Thiocarbamates

Front 31

Name the two common names for Group 3 (DNAs) herbicides.

Back 31

Trifluralin

Ethalfluralin

Front 32

Are Group 8s (Thiocarbamates) weak acids? What are the implications of this statement?

Back 32

No, and as a result their solubility in water and efficacy do not change with the pH of the soil.

Front 33

Do Group 8s (thiocarbamates) have uniform water solubility?

Back 33

No, triallate is the least water soluble, followed by butylate and EPTC. The reverse relationship exists for organic solvents.

Front 34

Do the Group 8s (thiocarbamates) bind with soil molecules uniformly?

Back 34

No, triallate adsorbs to soil colloids, while butylate and EPTC are less titely held.

Front 35

Can the Group 8s (thiocarbamates) be volatilized from soil?

Back 35

Yes, they can be volatilized from soil at high temperatures or from wet soil, unless incorporated.

Formulation can also play a role in the amount of volatilization and. thus the amount of incorporation necessary.

Front 36

How are the Group 8s (thiocarbamates) degraded?

Back 36

Through microbial action. Degradation is rapid under most conditions, but under dry conditions, wheat grown in the following year can be affected.

Front 37

How do the Group 8s (thiocarbamates) work?

Back 37

Thiocarbamates inhibit an enzyme in fatty acid biosynthesis that elongates fatty acids to form waxes and suberin. Both these compounds are important in maintaining a waxy later on the outside of seedlings and in preventing water loss.

While it is known that the herbicide inhibit very long chain fatty acid biosynthesis and that cell elongation and division are inhibited, the physiological link between them is unclear. Even though these herbicides have been used successfully for many years, the way in which they work to control weeds is still unknown.

Front 38

What is suberin?

Back 38

It is a waxy substance found in higher plants. Suberin is a main constituent of cork and is a highly hydrophobic and somewhat 'rubbery' material.

Front 39

How do the Group 8s (thiocarbamates) enter and move through the plant?

Back 39

They enter grass weeds though the coleoptile and coleoptile node as it pushes through the treated soil.

Translocation is not essential to the function of these herbicides, with triallate tending to dissolve in the lipid fraction of seeds and seedlings, while EPTC and butylate move through the soil solution and then into seedlings more readily.

The limited translocation of thiocarbamates occur primarily in xylem and corresponds to their water solubility. Triallate has poor translocation, while EPTC and butylate are translocated more readily.

Front 40

How are the Group 8s (thiocarbamates) selective?

Back 40

Selectivity between grassy weeds and grassy crops depends on the position of the crop relative to the herbicide-treated soil layer. Cereal crops must be placed below the herbicide layer, lest they absorb the herbicide and be affected.

Selectivity in crops and broad-leaved weeds varies. While butylate and EPTC control some broad-leaved weeds, triallate has no activity on broad-leaved weeds. the reasons for this lack of activity are unknown.

Front 41

How are the Group 8s (thiocarbamates) metabolized?

Back 41

While thiocarbamates are metabolized in the plants, it is unclear what role metabolism lays in conferring selectivity.

Front 42

Are the Group 8s (thiocarbamates) toxic?

Back 42

Thiocarbamates have low oral and dermal mammalian toxicity.

Front 43

What are the symptoms of plants affected by the Group 8s (thiocarbamates)?

Back 43

Affected grassy weeds frequently fail to emerge. Cell elongation in the shoots slows dramatically. Shoots, if they emerge, often appear swollen and bright green. Shoots often emerge from the sides of the coleoptile and can form a loop.

Thiocarbamates affect shoots more than roots and may induce abnormal emergence of the first leaf from the coleoptile which itself is inhibited.

Front 44

What are the implications of Group 8s (thiocarbamates)? (there are 4)

Back 44

Thiocarbamates must be incorporated to position the herbicide in the layer of soil containing weed seeds and to reduce losses due to volatilization;

Cereal crops must be planted below the layer containing herbicide;

Persistence in soil is relatively short due to losses by volatilization and microbial degradation; and

Mammalian oral toxicity is low.