Chemical Control of Weeds and its Implications

Chemical Control of Weeds and its Implications

A weed is an undesirable plant that grows on cultivated ground and which provokes crop yield loss. Since ancient times, farmers have applied different control methods to eradicate them. However, they are still present in every agro-ecosystems. [1] In the past mechanic control was the only practice known until the use of the first chemical methods. [2] The beginning of chemical techniques was based on the use of inorganic salts such as sodium chloride. The first herbicide was an organic substance called DNOC (4,6-dinitro-o-cresol), which was discovered during the 1930s in France. [3] Since then, more than 200 active principles have been developed and approved. [4] With this progress, labor hand requirements for weed control have been reduced. [5] Nowadays, herbicides dominate the pest market and represent more than 40% of total world pesticide amounts used. [6]

Herbicide use is associated with reductions in crop yield loss due to weed interference and an increase of 30% in total production, reflecting an economic benefit to the whole society. [7] However, extensive herbicide use has been related to several problems. Among them the following topics could be highlighted:

  • Environmental behaviour of herbicides: the life of a herbicide does not end in the weed, as much as 55% of an applied herbicide is lost in other environmental sinks (i.e. soil, atmosphere and bodies of water) and it can provoke toxicological effects on wildlife.
  • Human risk: the capacity of a herbicide to cause adverse effects in humans depends on its active ingredient and its formulation. Although the manipulation of pesticides requires strict safety norms, the exposure of a person to pesticides is possible due to truck and aircraft spills during their application. [8]
  • Impact on biodiversity: Herbicides used to control target weeds also affect other plant and algal species that support ecological functions in agro-ecosystems. [9] These species provide shelter and food for many types of beneficial insects (i.e. pest controllers), birds and wild animals. Thus, loss of biodiversity alters trophic chains, genetic pools and ecological services of agro-ecosystems.
  • Herbicide-resistant weeds: the wide and continual use of an herbicide leads to the selection of herbicide-resistant weeds. [10] These plants have specific mechanisms that allow them to reduce toxic effects by limiting herbicide absorption or transport into the plant, degrading the active principle, compartmentalizing into vacuoles or changing the target enzyme of the herbicide. [11] Currently, the cases of herbicide-resistant weeds have been increasing and the success of chemical control is at risk.

These impacts on the agro-ecosystems can be minimized when herbicide applications are combined with other control techniques such as cultural, mechanical and biological methods in programs of integrated weed management.

Controlling Plant Height without Chemicals

The height of greenhouse plants can be controlled by a number of non-chemical cultural methods. Interest in these techniques has grown because of the tighter controls placed on the use of agricultural chemicals and the public’s negative perception of chemicals in general. The Worker Protection Standards recently developed by the USEPA control the use of plant growth regulators (PGRs) and limits have been set on how soon workers may reenter greenhouse areas treated with PGRs. Reentry intervals (REIs) for PGRs range from 12 to 48 hours. What follows is an outline of other methods for controlling plant growth which may be effective alone or in combination with low levels of PGRs.

Scheduling and Cultivars

Plants started too soon often need to be «held back» by PGRs. Some growers start plants early to spread out transplanting to match the availability of labor and space or cutting production from stock plants. The quality of the earliest plants may suffer from efforts made to hold them back and/or the plants may be past maturity at marketing time. Buying-in cuttings or plugs rather than trying to grow-your-own may help keep crop production on the proper schedule.

Cultivar selection is also important. Some bedding plants are available in tall, medium, and short types which look very similar except for height. Unless customers are fussy about height, using smaller cultivars is an easy way to «control» height.

A review of the basic cultural recommendations and schedules for a crop may be an useful and «eye-opening» exercise. For example, the PGR requirements for most of today’s poinsettias are rather minimal compared to the Heggs assuming the /proper crop timing, light, and temperature requirements are met. But when the cuttings are potted and pinched early or if the plants are crowded the number of PGR applications generally increases.

Light Intensity

One of the eaiest ways to reduce height and the need for PGR treatment is to maximize the amount of light plants receive to reduce «stretch.» This means adequate spacing, clean glass, and fresh plastic covering. For some plants supplemental HID lighting may be feasible. One of the most common reasons for frequent use of PGRs is that the grower is not taking full advantage of all the available natural light.

DIF Temperature Control By now most growers have heard of the DIF technique of temperature control developed by Dr. Royal Heins and colleagues at Michigan State University. In fact, some growers in our area are using some form of DIF on a regular basis. DIF is defined as the difference between day temperature (DT) and night temperature (NT). Stem elongation is promoted by warmer days than nights (+ DIF) and inhibited by warmer nights than days (- DIF). Plants become taller as DIF becomes more positive and plants become shorter as DIF becomes smaller or more negative.

Significant height control and reduction in PGR use is possible by reducing the difference between DT and NT as much as possible. Another approach to using DIF is the «cool morning pulse.» A cool morning pulse is created by reducing the greenhouse temperature 5-10 F lower than the NT for 2- 3 hours at dawn. This approach reduces plant height as much as a negative DIF and may be the most easy DIF treatment to make.

DIF seems to be effective on most greenhouse plants, but research continues with many different species. Some responsive and non-responsive species are listed in Table 1. DIF, like a PGR, has its greatest effect on height during the period of most rapid stem elongation. DIF does not have to be applied continuously throughout a crop cycle to be effective, but rather only during the period of most active vegetative growth.

Table 1. Response of some plants to DIF.

Large response
Easter lily Dianthus
Chrysanthemum Tomato
Poinsettia Snap bean
Salvia Watermelon
Celosia Sweet corn
Fuchsia Oriental lilies
Impatiens Asiatic lilies
Portulaca Gerbera
Hypoestes Petunia
Snapdragon Geranium
Small or no response
Squash Platycodon
French marigold Tulip
Hyacinth Narcissus

A note of caution: DIF treatments affect the rate of crop development as well as stem elongation. Growers using DIF should determine the effect of their DIF treatment on the average daily temperature. A DIF treatment raising the average daily temperature would speed crop development, while a treatment lowering the average daily temperature would slow crop development.


One of the oldest and most common ways of attempting to prevent stretching is to withhold fertilizer or water. Some growers try to hold back plants using low temperature in combination with nutrient and/or water stress. Low fertility or mild water stress can be successful if carefully controlled. However, there are risks — too much growth inhibition, development of nutrient deficiency symptoms which are unsightly and hard to correct, or damage to the plants from water stress.

The nutrients which have the most effect on the size of greenhouse plants are nitrogen (N) and phosphorus (P). The biggest effect of withholding a water-soluble fertilizer is N deficiency. Unfortunately if N deficiency conditions go on too long the plants will be too small and also very yellowed. A P deficiency is somewhat more difficult to create than a N deficiency. However, if carefully managed a mild to moderate P deficiency will result in a desirable reduction in growth and no foliar symptoms. In fact, a mild P deficiency actually makes many plants appear greener! A well-known fertilizer company promotes the «phosphorus starvation» technique for growth control and markets two water-soluble fertilizers, 20-1- 20 and 20-2-20, for this purpose. Routine use of these fertilizer supposedly results in shorter, stockier plants than fertilizers with higher P analysis (e.g., 15-16-17, 20-10-20). Growers should be warned that low P is said to reduce bract diameter of poinsettias, so low P fertilizers should not be used on this crop.

I conducted research on the potential of nutrition to control growth with support of the Massachusetts Flower Growers Association and the New England Greenhouse Conference. The main impetus for this project is that growers have no recommendations to follow if they try to use low fertility as a growth control method. The objective is to develop methods which reduce plant height while avoiding too much stunting, deficiency symptoms, and undesirable delays in crop development. There are many different practical approaches which could be tried, but I have had a chance to try only a couple.

In one experiment I grew ‘Red Elite’ geranium in a commercial mix amended with 20% superphosphate in a range of 0 to 32 oz./cu. yd.. The fresh weight of the tops at the end of the experiment followed a typical fertilizer rate response pattern (Fig. 1). Plants grown with no superphosphate or 2 and 4 oz./cu. yd. were darker green in color, but only slightly shorter than the higher P treatments. Perhaps more growth control did not occur because there was enough P in the starter charge to carry the plants during the early, active stage of growth.

Figure 1. Growth of geranium is affected by superphosphate level.

In another experiment I grew ‘First Lady’ marigold and withheld fertilizer for 10 days from a different group of plants periodically as the plants grew. The biggest effect on growth measured at the end of the experiment occurred when fertilizer was withheld during periods 10-20, 20-30, or 30- 40 days after transplanting the seedlings (Fig. 2). The effects of no fertilizer during these periods was apparent mainly in leaf size, stem thickness, and branch development rather than height. These two experiments demonstrate that using mild or moderate nutrient deficiency to control growth is not as straightforward as might be expected. I plan more trials beginning this fall.

Future Height Control Techniques

Mechanical Conditioning. It has been known for a long time that mechanical stresses such as repeated brushing, shaking, or bending caused by air movement or contact with animate or inanimate objects can reduce plant growth. Recent research conducted by Dr. Joyce Latimer at the University of Georgia has demonstrated the commercial potential of this technique for controlling the height of vegetable transplants, particularly tomato. This work was stimulated, in part, by the fact that B- Nine is no longer registered for use on edible crops. One system of mechanical conditioning adapted to commercial greenhouses involves drawing a bar across the

Figure 2. Growth of marigold is affected by when fertilizer is withheld for short periods at different times during the crop.

tops of the plants once or twice a day. The bar is set low enough to contact the plants, but not so low that the plants are injured or uprooted. Thirty to 40% reductions in height have been reported with this system. Other systems involve periodic shaking, blowing air treatments, or water sprays. For this to become useful to flower growers research is needed to determine the response of flower crops.

Light filters. Plant physiologists have found that changing the ratio of red to far-red light can influence stem elongation and branching. Red light inhibits stem elongation compared to far-red light which promotes stem elongation. Red light also promotes branching by stimulating lateral bud growth. In nature there are daily and seasonal changes in the red:far-red ratio. Natural light in the middle of the day and in the summer has a higher proportion of red than sunrise and sunset and the winter. The shading effect of plant canopies also changes the ratio increasing the proportion of far-red light. This is an important factor in why plants stretch when they are spaced too closely. Current research is being directed at developing greenhouse coverings which alter the red:far-red balance to control plant height and branching.

Lab 18: Use of Physical Agents to Control of Microorganisms


The next two labs deal with the inhibition, destruction, and removal of microorganisms. Control of microorganisms is essential in order to prevent the transmission of diseases and infection, stop decomposition and spoilage, and prevent unwanted microbial contamination.

Microorganisms are controlled by means of physical agents and chemical agents. Physical agents include such methods of control as high or low temperature, desiccation, osmotic pressure, radiation, and filtration. Control by chemical agents refers to the use of disinfectants, antiseptics, antibiotics, and chemotherapeutic antimicrobial chemicals.

Basic terms used in discussing the control of microorganisms include:

1. Sterilization
Sterilization is the process of destroying all living organisms and viruses. A sterile object is one free of all life forms, including bacterial endospores, as well as viruses.

2. Disinfection
Disinfection is the elimination of microorganisms, but not necessarily endospores, from inanimate objects or surfaces.

3. Decontamination
Decontamination is the treatment of an object or inanimate surface to make it safe to handle.

4 . Disinfectant
A disinfectant is an agents used to disinfect inanimate objects but generally to toxic to use on human tissues.

5 . Antiseptic
An antiseptic is an agent that kills or inhibits growth of microbes but is safe to use on human tissue.

6. Sanitizer
A sanitizer is an agent that reduces, but may not eliminate, microbial numbers to a safe level.

7 . Antibiotic
An antibiotic is a metabolic product produced by one microorganism that inhibits or kills other microorganisms.

8 . Chemotherapeutic antimicrobial chemical
Chemotherapeutic antimicrobial chemicals are synthetic chemicals that can be used therapeutically.

9 . Cidal
An agent that is cidal in action will kill microorganisms and viruses.

10 . Static
An agent that is static in action will inhibit the growth of microorganisms.

These two labs will demonstrate the control of microorganisms with physical agents, disinfectants and antiseptics, and antimicrobial chemotherapeutic agents. Keep in mind that when evaluating or choosing a method of controlling microorganisms, you must consider the following factors which may influence antimicrobial activity:

1. the concentration and kind of a chemical agent used;

2. the intensity and nature of a physical agent used;

3. the length of exposure to the agent;

4. the temperature at which the agent is used;

5. the number of microorganisms present;

6. the organism itself; and

7. the nature of the material bearing the microorganism.


Microorganisms have a minimum, an optimum, and a maximum temperature for growth. Temperatures below the minimum usually have a static action on microorganisms. They inhibit microbial growth by slowing down metabolism but do not necessarily kill the organism. Temperatures above the maximum usually have a cidal action, since they denature microbial enzymes and other proteins. Temperature is a very common and effective way of controlling microorganisms.

1. High Temperature

Vegetative microorganisms can generally be killed at temperatures from 50°C to 70°C with moist heat. Bacterial endospores, however, are very resistant to heat and extended exposure to much higher temperature is necessary for their destruction. High temperature may be applied as either moist heat or dry heat.

a. Moist heat

Moist heat is generally more effective than dry heat for killing microorganisms because of its ability to penetrate microbial cells. Moist heat kills microorganisms by denaturing their proteins (causes proteins and enzymes to lose their three-dimensional functional shape). It also may melt lipids in cytoplasmic membranes.

1. Autoclaving

Autoclaving employs steam under pressure. Water normally boils at 100°C; however, when put under pressure, water boils at a higher temperature. During autoclaving, the materials to be sterilized are placed under 15 pounds per square inch of pressure in a pressure-cooker type of apparatus. When placed under 15 pounds of pressure, the boiling point of water is raised to 121°C, a temperature sufficient to kill bacterial endospores.

The time the material is left in the autoclave varies with the nature and amount of material being sterilized. Given sufficient time (generally 15-45 minutes), autoclaving is cidal for both vegetative organisms and endospores, and is the most common method of sterilization for materials not damaged by heat.

2. Boiling water

Boiling water (100°C) will generally kill vegetative cells after about 10 minutes of exposure. However, certain viruses, such as the hepatitis viruses, may survive exposure to boiling water for up to 30 minutes, and endospores of certain Clostridium and Bacillus species may survive even hours of boiling.

b. Dry heat

Dry heat kills microorganisms through a process of protein oxidation rather than protein coagulation. Examples of dry heat include:

1. Hot air sterilization

Microbiological ovens employ very high dry temperatures: 171°C for 1 hour; 160°C for 2 hours or longer; or 121°C for 16 hours or longer depending on the volume. They are generally used only for sterilizing glassware, metal instruments, and other inert materials like oils and powders that are not damaged by excessive temperature.

2. Incineration

Incinerators are used to destroy disposable or expendable materials by burning. We also sterilize our inoculating loops by incineration.

c. Pasteurization

Pasteurization is the mild heating of milk and other materials to kill particular spoilage organisms or pathogens. It does not, however, kill all organisms. Milk is usually pasteurized by heating to 71°C for at least 15 seconds in the flash method or 63-66°C for 30 minutes in the holding method.

2. Low Temperature

Low temperature inhibits microbial growth by slowing down microbial metabolism. Examples include refrigeration and freezing. Refrigeration at 5°C slows the growth of microorganisms and keeps food fresh for a few days. Freezing at -10°C stops microbial growth, but generally does not kill microorganisms, and keeps food fresh for several months.


Desiccation, or drying, generally has a static effect on microorganisms. Lack of water inhibits the action of microbial enzymes. Dehydrated and freeze-dried foods, for example, do not require refrigeration because the absence of water inhibits microbial growth.


Microorganisms, in their natural environments, are constantly faced with alterations in osmotic pressure. Water tends to flow through semipermeable membranes, such as the cytoplasmic membrane of microorganisms, towards the side with a higher concentration of dissolved materials (solute). In other words, water moves from greater water (lower solute) concentration to lesser water (greater solute) concentration.

When the concentration of dissolved materials or solute is higher inside the cell than it is outside, the cell is said to be in a hypotonic environment and water will flow into the cell (Fig. 1). The rigid cell walls of bacteria and fungi, however, prevent bursting or plasmoptysis. If the concentration of solute is the same both inside and outside the cell, the cell is said to be in an isotonic environment (Fig. 2). Water flows equally in and out of the cell. Hypotonic and isotonic environments are not usually harmful to microorganisms. However, if the concentration of dissolved materials or solute is higher outside of the cell than inside, then the cell is in a hypertonic environment (Fig. 3). Under this condition, water flows out of the cell, resulting in shrinkage of the cytoplasmic membrane or plasmolysis. Under such conditions, the cell becomes dehydrated and its growth is inhibited.

The canning of jams or preserves with a high sugar concentration inhibits bacterial growth through hypertonicity. The same effect is obtained by salt-curing meats or placing foods in a salt brine. This static action of osmotic pressure thus prevents bacterial decomposition of the food. Molds, on the other hand, are more tolerant of hypertonicity. Foods, such as those mentioned above, tend to become overgrown with molds unless they are first sealed to exclude oxygen. (Molds are aerobic.)

For more information on antigens, antibodies, and antibody production, see the following Learning Objects in your Lecture Guide :


1. Ultraviolet Radiation

The ultraviolet portion of the light spectrum includes all radiations with wavelengths from 100 nm to 400 nm. It has low wave-length and low energy. The microbicidal activity of ultraviolet (UV) light depends on the length of exposure: the longer the exposure the greater the cidal activity. It also depends on the wavelength of UV used. The most cidal wavelengths of UV light lie in the 260 nm — 270 nm range where it is absorbed by nucleic acid.

In terms of its mode of action, UV light is absorbed by microbial DNA and causes adjacent thymine bases on the same DNA strand to covalently bond together, forming what are called thymine-thymine dimers (see Fig. 4). As the DNA replicates, nucleotides do not complementary base pair with the thymine dimers and this terminates the replication of that DNA strand. However, most of the damage from UV radiation actually comes from the cell trying to repair the damage to the DNA by a process called SOS repair. In very heavily damaged DNA containing large numbers of thymine dimers, a process called SOS repair is activated as kind of a last ditch effort to repair the DNA. In this process, a gene product of the SOS system binds to DNA polymerase allowing it to synthesize new DNA across the damaged DNA. However, this altered DNA polymerase loses its proofreading ability resulting in the synthesis of DNA that itself now contains many misincorporated bases. In other words, UV radiation causes mutation and can lead to faulty protein synthesis. With sufficient mutation, bacterial metabolism is blocked and the organism dies. Agents such as UV radiation that cause high rates of mutation are called mutagens.

The effect of this inproper base pairing may be reversed to some extent by exposing the bacteria to strong visible light immediately after exposure to the UV light. The visible light activates an enzyme that breaks the bond that joins the thymine bases, thus enabling correct complementary base pairing to again take place. This process is called photoreactivation.

UV lights are frequently used to reduce the microbial populations in hospital operating rooms and sinks, aseptic filling rooms of pharmaceutical companies, in microbiological hoods, and in the processing equipment used by the food and dairy industries.

An important consideration when using UV light is that it has very poor penetrating power. Only microorganisms on the surface of a material that are exposed directly to the radiation are susceptible to destruction. UV light can also damage the eyes, cause burns, and cause mutation in cells of the skin.

2. Ionizing Radiation

Ionizing radiation, such as X-rays and gamma rays, has much more energy and penetrating power than ultraviolet radiation. It ionizes water and other molecules to form radicals (molecular fragments with unpaired electrons) that can disrupt DNA molecules and proteins. It is often used to sterilize pharmaceuticals and disposable medical supplies such as syringes, surgical gloves, catheters, sutures, and petri plates. It can also be used to retard spoilage in seafoods, meats, poultry, and fruits.

For more information on antigens, antibodies, and antibody production, see the following Learning Objects in your Lecture Guide:


Microbiological membrane filters provide a useful way of sterilizing materials such as vaccines, antibiotic solutions, animal sera, enzyme solutions, vitamin solutions, and other solutions that may be damaged or denatured by high temperatures or chemical agents. The filters contain pores small enough to prevent the passage of microbes but large enough to allow the organism-free fluid to pass through. The liquid is then collected in a sterile flask (Fig. 5). Filters with a pore diameter from 25 nm to 0.45 µm are usually used in this procedure. Filters can also be used to remove microorganisms from water and air for microbiological testing (see Appendix E).




2 plates of Trypticase Soy agar, 2 plates of 5% glucose agar, 2 plates of 10% glucose agar, 2 plates of 25% glucose agar, 2 plates of 5% NaCl agar, 2 plates of 10% NaCl agar, and 2 plates of 15% NaCl agar.


Trypticase Soy broth cultures of Escherichia coli and Staphylococcus aureus; a spore suspension of the mold Aspergillus niger.

A. OSMOTIC PRESSURE PROCEDURE (to be done by tables)

1. Divide one plate of each of the following media in half. Using your inoculating loop, streak one half of each plate with E. coli and the other half with S. aureus (see Fig. 6). Incubate upside down and stacked in the petri plate holder on the shelf of the 37°C incubator corresponding to your lab section until the next lab period.

a. Trypticase Soy agar (control)
b. Trypticase Soy agar with 5% glucose
c. Trypticase Soy agar with 10% glucose
d. Trypticase Soy agar with 25% glucose
e. Trypticase Soy agar with 5% NaCl
f. Trypticase Soy agar with 10% NaCl
g. Trypticase Soy agar with 15% NaCl

2. Using a sterile swab, streak one plate of each of the following media with a spore suspension of the mold A. niger (see Fig. 7). Incubate the plates upside down at room temperature for 1 week.

a. Trypticase Soy agar (control)
b. Trypticase Soy agar with 5% glucose
c. Trypticase Soy agar with 10% glucose
d. Trypticase Soy agar with 25% glucose
e. Trypticase Soy agar with 5% NaCl
f. Trypticase Soy agar with 10% NaCl
g. Trypticase Soy agar with 15% NaCl



5 plates of Trypticase Soy agar


Trypticase Soy broth culture of Serratia marcescens


1. Using sterile swabs, streak all 5 Trypticase Soy agar plates with S. marcescens as follows:

a. Dip the swab into the culture.

b. Remove all of the excess liquid by pressing the swab against the side of the tube.

c. Streak the plate so as to cover the entire agar surface with organisms.

2. Expose 3 of the plates to UV light as follows:

a. Remove the lid of each plate and place a piece of cardboard with the letter «V» cut out of it over the top of the agar.

b. Expose the first plate to UV light for 1 second, the second plate for 3 seconds, and the third plate for 5 seconds.

c. Replace the lids and incubate the plates upside down at room temperature until the next lab period.

3. Leaving the lid on, lay the cardboard with the letter «V» cut out over the fourth plate and expose to UV light for 30 seconds. Incubate the plates upside down at room temperature with the other plates.

4. Use the fifth plate as a non-irradiated control and incubate the plates upside down at room temperature with the other plates.

NOTE: Do not look directly at the UV light as it may harm the eyes.



2 plates of Trypticase Soy agar


Trypticase Soy broth cultures of Micrococcus luteus


1. Using alcohol-flamed forceps, aseptically place a sterile membrane filter into a sterile filtration device.

2. Pour the culture of M. luteus into the top of the filter set-up.

3. Vacuum until all the liquid passes through the filter into the sterile flask.

4. With alcohol-flamed forceps, remove the filter and place it organism-side-up on the surface of a Trypticase Soy agar plate.

5. Using a sterile swab, streak the surface of another Trypticase Soy agar plate with the filtrate from the flask.

6. Incubate the plates at 37°C until the next lab period.


A. Osmotic Pressure

Observe the 2 sets of plates from the osmotic pressure experiment and record the results below.

Plate Escherichia coli Staphylococcus aureus Aspergillus
5% NaCl
10% NaCl
15% NaCl
5% glucose
10% glucose
25% glucose

+ = Scant growth
++ = Moderate growth
+++ = Abundant growth
— = No growth

No comments

Добавить комментарий

Your e-mail will not be published. All fields are required.