Weed Management

Weed Description

There are approximately 200 plant species that make up the world’s most important crop weeds.

These species are spread over approximately 12 plant families. These 200-odd species have some common characteristics, which make them important weed species. These characteristics include high seed viability, rapid seed emergence, rapid initial plant growth, strong competitors and the production of large numbers of seed.

Weeds do not fall outside the classification of plants. The major groupings important in weed management, are broad-leaved weeds and grass species. Approximately two thirds of the important weeds are broad-leaved weeds, the rest are grasses, sedges or ferns.

Weeds can be further classified according to the time it takes for the plant to complete its life cycle. There are three groups of weeds classified according to the time for completion of the lifecycle as Annual weeds, Biennial weeds and Perennial weeds. Approximately two-thirds of this important weed species are annuals, with the rest predominantly perennials with only a small proportion being biennial plants. The lifecycle of a plant is defined as the time from seed emerging to the time of seed production i.e. from seed to seed.

Annual Weeds

Annual weeds complete their life cycle in one growing season, thus in less than one year. Due to their short lifecycle annuals produce large numbers of seed and grow very rapidly. These plants are sensitive to weed control practices.

Annual weeds can further be divided into two groups. Summer annuals, which germinate in the spring or early summer, grow in the summer, mature and form seed in the autumn. Summer Annuals complete the cycle seed to seed across two calendar years but in one growing season. Winter annuals germinate in autumn or early winter and mature and form seed in spring or early summer. So, winter annuals complete the cycle in one calendar year and in one growing season. Annuals do not live for time periods exceeding 12 months and reproduce primarily through seed.

Biennial Weeds

Biennial weeds live longer than one year but not longer than two years. Biennial weeds usually have a fleshy tap root system for the storage of food sources. Biennials are not abundant in South Africa.

Perennial Weeds

Perennial weeds reproduce every year through vegetative organs such as rhizomes and stolons, bulbs and tubers, root and stem cuttings and can generally also form seed. Perennials can be sub-divided into two groups; namely simple perennials and creeping perennials. Simple perennials reproduce only by means of seed. Should the shoot of a simple perennial be damaged, new buds can form. Creeping perennials reproduce through seed and vegetative parts. Vegetative reproduction usually occurs through creeping surface roots or stolons, underground stems or rhizomes, tubers and bulbs.

Weeds are generally known by common names that may be unique to regions of the country. There are approximately 5 species of weeds that are all known as the “Khakie-bos”. Within the scientific community this problem of confusing species is overcome by specific scientific, Latin names. It is useful to learn the Latin names of weeds rather than common name, to avoid the potential confusion of species.

Weed Management

The influence of weeds on a crop and natural vegetation and the rate and extent of weed growth is more predictable than that of crop pest. Weeds can therefore be better managed than the other pests and diseases. Weed control includes prevention, control and eradication.

Prevention

Prevention are measures taken to prevent a new weed from entering an area. Preventive measures must also be taken to prevent weed seed from forming. Prevention includes avoiding:

• Crop seed contaminated with weed seed
• Spreading plant parts with implements
• Using contaminated manure
• Using contaminated irrigation water.
• The development of seed on the weed plant
• Spreading of vegetative reproductive plant parts

Control

Weed control is the general objective in weed management. Controlling weed with chemicals or mechanically, is the most predominant weed control techniques used. Weed control is performed to reduce the numbers of weeds to such levels that they do not adversely affect the yield of a crop or interfere with the harvesting process. A balance must be created between weed control costs and economic advantages in the form of larger profits.

Eradication

Is the elimination of weed plants and their reproductive capacity from an area. The only way in which eradication can be performed in soil is by means fumigation. The application of fumigants is expensive and therefore only profitable when used in small areas of high-production soils or systems with the production of high value horticultural and specialised vegetable crops.

Weed management can be done by applying various methods:

Preventative Weed Management: Most weeds produce large amounts of seeds per growing season that build up as a ‘seed bank’ in the soil. The seed of many weed species also have complicated dormancy behaviour with the result that all the seed do not germinate when conditions are favourable. Weed seed produced in the same season can therefore germinate over an extended period of several years. It is therefore important to prevent weeds from producing seed. Suitable cultivation techniques should be applied to ensure weeds are controlled early, before they become competitive. The use of mulch is effective in preventing seed germination and weed growth. Pruned leaves and branches, straw, bark mulch, or black plastic will control weed growth considerably. However, you should ensure that especially pruned leaves and branches are well composted before applying as mulch, as these could carry diseases that could infect the crop.

Weed control by way of cultivation practices is only relevant in the agriculture sector. It must be kept in mind that management practices of natural fields and pastures could also aid in managing encroachment and invader species. Weed control through cultivation practices starts with the crop selection. The choice of a crop determines the weed control programme to be used. Crop choice may also guide the programme developer as to the weed spectrum he could expect to encounter. Crops that germinate rapidly and produce leaf covering will develop faster than the competing weeds, get a head start and keep on suppressing it. Once a crop is selected, the time of planting must be considered as some crops, adapted to lower temperatures, may get a head start on some weeds that are not adapted to it. Early planting may favour crops, but a weed, adapted to lower temperatures, may need chemical control measures for extended periods which may lead to increased input costs. Plant spacing is also of importance as dense crop stands in narrow rows will naturally suppress weed growth by depriving them of light. The growth of the crop can be manipulated through fertilisation and irrigation. The addition of extra fertilisation is not always effective in overcoming the weed competition since the weed may also react positively to fertilisation.

Biological Control: Some insects and animals can be used in certain crops to keep weeds under control.

Mechanical Weed Control: In fruit orchards weeds between rows are slashed by means of hand- lashers or machine lashers pulled by tractors. In agronomic crops weeds between rows can be removed using specialised machinery. Weed slashing involves walking along the rows of the crop, slashing the weeds with a hoe, leaving the weeds on the ground as mulch. Mechanical weed control measures include:

Chemical Control; A very effective way of controlling weeds is by using chemicals known as herbicides. An herbicide is a chemical specifically designed to control killing plants. Herbicides are developed with specific modes of action. In broad terms, there are two main groups of weeds, the broad-leaved weeds and grasses. An herbicide like ‘Roundup’ will kill both narrow as well as broad leaf weeds, including annual crops.

An herbicide is a mixture of chemicals prepared to suppress plant growth or kill plants. The wide use of herbicides in crop fields and on range-land is based primarily on herbicide selectivity which can be used to control weeds without causing damage to the crop plants. Herbicides are made up of chemical compounds known as active ingredients (a.i.) which affects the life processes within weed plants in a way that it die off. The herbicides are sold as herbicide formulations, which are a combination of active ingredient and other chemicals. A formulation can be in a liquid form, as powders or granules. These formulations also contain solvents to disperse the active ingredient; stabilisers to prevent the active ingredient from reacting with other chemicals and to prolong its shelf-life; emulsifiers to enable the active ingredient to mix with water or oils; adjuvants to improve its entry into the plant; dyes, an inert substance that show where the herbicide has been applied. The addition of dyes to a formulation is especially handy where invader species are controlled. The herbicide is sold as a formulated product or product. The formulated product may require dilution before it can be applied. The dilution of the product is mixed with a suitable carrier to ensure an even distribution of the active ingredient over the target plants. Most active ingredients are formulated so that they mix with water, the most common carriers. Diesel is also used in specific situations, such as the application of Garlon® in cut stump treatments.

When using herbicides to kill weeds, care must be taken not to accidentally use a broad leaf herbicide on a broad leaf crop or vice versa. Herbicides can be highly poisonous and must be used very carefully, and only with competent handlers and equipment.

It is important to know the desired end result of an application before the final decision is made on the type of herbicide that will be used. Herbicides can be classified in a number of ways, but the most useful classifications are those that aid in the selection process; (a) their ability to be transported or translocated in a plant, (b) the compound’s ability to selectively kill weed species without damaging desired plants, (c) their persistence in the soil environment and the mobility of a herbicide in the soil, (d) the time of application, relative to the growth stage of the crop.

Classification According to Crop Growth Stage

It is important to remember that when a weed control programme is based on the use of herbicides, the crop’s growth stage will be the main decision-making feature. When one considers herbicide application schedules in agriculture they include: preplant, pre-emergence and post-emergence application.

Pre-plant application take place before the crop is planted and the herbicide can be incorporated into the soil by implements.

Pre-emergence application take place after of the crop is planted but before the crop emergences. The herbicide must be washed into the soil by rain or irrigation.

Post-emergence applications occur directly to the plant foliage. This aids in the sense that the weed problem is already visible and soil factors do not influence activity. When alien invaders and industrial weeds are controlled chemically, the terms Pre-and post-emergence have another meaning as it refers to the development stage of the weed as no crop plants are present. Thus, when weed is controlled by using a Post emergence application, it is done directly onto the plant. Direct application onto vegetation can be done by using different methods:

Foliar application:  Foliar application occurs when the herbicide mixture is applied to the foliage (leaves and stems) of the plant. The volume of herbicide mixture required depends on the density and height of the plants. In this type of application, leaves are sprayed to the point of run-off.

Stem and stump applications: Alien invaders, mainly perennial plants may produce regrowth after being injured or cut down, it is therefore important that all dormant buds near the soil surface and the roots are killed. In achieving this, the herbicide mixture is applied to the stems and stumps to ensure efficient uptake of the herbicide.

Stem applications: Stem applications are divided into three main groups. These are basal stem-, partial frill- and stem injection: -

Basal stem application is suited for plants with a thin bark which in general do not exceed 20 cm diameter. The herbicide mixture is applied to the bark to a height of approximately 30 cm from the ground. The application can be made by using a paintbrush or sprayer. The herbicides are absorbed through the bark of the plant from where it is translocated to its site of action.

Partial frill application requires a ring of evenly spaced cuts to be made through the bark of the plant. The cuts can be made with an axe. Ideally the cuts should be made low on the stems. A suitable herbicide mixture is then administered into each cut by using a syringe or hand-held sprayer. The herbicide will eventually be taken up in the plant. Take care as not to spill the solution as it may affect other plants. This method is time consuming and tedious.

Stem injection is used only in the control of the Cactus family because of the growth habit of these plants. The presence of thorns and thick cuticles layers handicap other application techniques. A hole is made into the lower part of the stem by means of a thrusting spike or drill. The herbicide mixture is then sprayed into the hole.

Stump application the plants are cut down as close to ground level as possible by using a chain saw or brush cutter, leaving a clean cut. The stump should be cut to provide a short stump with a level surface and with the bark still intact. The herbicide must be applied within 2 hours. The herbicide mixture can be applied either to the outer ring of the cut surface (close to the bark) or to the cut surface and sides of the stump, as well as any exposed roots parts.

Soil application the herbicide is applied to the soil around the base of the target plants. Herbicides designed for this type of application are available either as a granule that is applied as it is, or as a formulation mixed in a carrier. This type of application is more suited for industrial weed control where total weed control is aimed at. It can however also be applied in the invader control and encroachment control sectors, but only under certain conditions. The compounds used in encroachment control are selective compounds, which normally require only a single application. These compounds generally have a prolonged soil activity and thus with residual activity. It is important when soil application is considered, that the compound will suit for the area where it is to be applied. This type of application should not be done close to crop fields or when desirable plants are in close proximity. The compounds should not be applied more than once a year to the same area. Remember; always follow the instructions on the label to the letter!

Classification According to Herbicide Translocation in Plants

Herbicides are classified according to the ability of being translocated within a plant:

Contact herbicides cause injury, limited to a small area, to the leaves where they come in contact with the plant. They cannot be translocated within the plant and will not reach the root system of the weed. These compounds will kill annual plants but only cause defoliation in perennial plants. Contact herbicides are applied to the plant as a whole.

Systemic herbicides can be translocated or “transported” in the plant to the sites where active growth occurs. These compounds can be applied either as foliar sprays or soil applications. Systemic herbicides are ideally suited for the control of perennial weeds, invader species and encroachment species.

Classification According to Herbicide Selectivity (ability to select weeds)

Herbicide selectivity is one of the most important concepts in modern weed control. The ability to choose specific weeds makes it possible to control a weed plant within a crop, without damaging the crop plant. Similarly, one is able to control an invasive tree within grassland without any detrimental effects to the grass. A selective herbicide therefore effects weed growth, but does not affect the desired plants. Selective herbicides are however only selective within limits and is affected especially by environmental conditions and the sensitivity of the plant to the compound. It is especially important within a crop situation that the selected care is taken to ensure that an herbicide can indeed be safely applied to the specific crop cultivar. The instructions on the herbicide formulation label must always be strictly adhered to. Always ensure that the herbicide is certified as safe for use on a specific crop cultivar. This can be done by checking the herbicide label and consulting with both the relevant seed and chemical companies. Herbicide selectivity is affected by environmental conditions which cannot be manipulated during crop cultivation. An herbicide can only be effective if it is absorbed by the target plant. Once absorbed, it is translocated to the metabolic sites of action. Varying environmental conditions (temperature and water availability) affects the extent and rate of absorption of herbicides by the plant. Soil type and soil structure also affects the selectivity of herbicides because the extent of adsorption of the herbicide to soil will determine the amount available for root uptake. Certain plant characteristics such as; plant age, plant growth rate, plant morphology, plant physiology, plant biophysical processes, biochemical processes and plant genetics can also play a role in the uptake and translocation of herbicides. In general, younger plants are more sensitive to herbicides than older plants as the uptake of herbicides are more efficient and the growth rates of younger tissues are higher. Young plants growing vigorously will have a larger up-take and transporting ability, thus be the more susceptible to herbicides. The most important factor of a plant, its genetic make-up, must always be taken into account as plants may be naturally resistant to certain herbicides. This natural resistance may stem from specific degradation pathways which the plant may posses or a mechanism which could prevent herbicide activity. Resistant weed biotypes will only develop if the herbicide is present in the environment of the weed, enabling the genetic make-up to build up a resistance to the already existing compound. The repeated exposure to this herbicide then eventually leads to the killing of all the susceptible individual plants and leaving only the resistant plants to propagate. When a weed management programme is developed the potential development of resistant weeds must thus be considered. Rotation of the herbicides with different modes of action on fields could aid in reducing the risk of resistant weed populations to develop. Regarding their classification, herbicides are generally placed in two groups concerning selectivity. These two groups are; the grass herbicides and the broad leaf herbicides. Grass herbicides will kill primarily grass weeds and will not damage broad-leaved weeds / plants. Broad-leaf herbicides will kill primarily broad-leaved weeds and not the grasses. (Remember however, selectivity is not absolute, as explained above.)

Classification According to Herbicide Residual Activity (persistence) in Soil

The duration of herbicides remaining active in soil is determined by the amount applied and the rate of loss. It is therefore important that it stays active long enough to allow effective weed control, but not too long to damage the follow-up crop. The activity of herbicide remainders is thus critical when deciding on which compounds to use. Herbicides have withholding periods for specific crops which are specified on the label. Herbicide residual activity is of major importance in the crop situation. The duration of active herbicide remains in soils will determine the time a farmer has to allow to pass before a new crop is planted. This time is determined by soil type as well as the follow-up crop to be planted. In the case of certain crops the herbicide label specifies the time period, which has to laps before a follow-up crop can be planted, as the withholding period. The withholding period is normally specified in days and it refers to the period of time that has to laps, before a follow-up crop can be planted. These specifications must be followed at all times, or damage to the follow-up crop may occur. Some herbicides remain active in soil for some time (weeks to months) and are referred to as residual herbicides. This herbicide group is usually used for industrial weed control purposes. They allow a single application once or twice per year whilst keeping an area weed free. Residual herbicides are also ideally suited for the control of encroachment species as long as the compound is selective towards the tree species, i.e. the grasses are not damaged due to the application. Residual herbicides are usually systemic compounds. Non-residual herbicides are those that do not last for long periods in soil. These compounds could have either a systemic or contact activity. Non-residual herbicides are normally applied to the target plants by means of foliar application.

Classification According to Herbicide Behaviour in Soil

When herbicides are applied to soil the compounds become absorbed to soil particles or soil colloids. Colloids are microscopic particles in soil which carries an electrical charge. Nutrients and herbicides are also electrically charged and can thus bind to soil particles. Mobile herbicides though, will not become absorbed with the colloids and will thus leach away through the soil profile. Immobile herbicides on the other hand will bind more easily to soil particles and limited leaching will thus be expected. Absorbed molecules are temporarily unavailable to plants in both cases. In general, the following applies:

• Soils with high organic matter require higher herbicide doses
• Soils with high clay content require higher herbicide doses
• Herbicides will remain for a longer period of time in soils with high clay content and high organic matter than in sandy soils.