Integrated Weed Management, Advantages and Limitation of Herbicide Usage

Integrated weed management may be defined as the combination of two or more weed-control methods at low input levels to reduce weed competition in a given cropping system below the economical threshold level. It has proved to be a valuable concept in a few cases, though much is still to be done to extend it to the small farmers’ level.

The Integrated Weed Management (IWM) approach aims at minimizing the residue problem in plants, soil, air, and water. An IWM involves the utilization of a combination of mechanical, chemical, and cultural practices of weed management in a planned sequence, so designed as not to affect the ecosystem. The nature and intensity of the species to be controlled, the sequence of crops that are raised in the rotation, the standard of crop husbandry, and the ready and timely availability of any method and the economics of different weed-management techniques are some of the potent considerations that determine the success for the exploitation of the IWM approach.

Why IWM

• One method of weed control may be effective and economical in one situation and it may not be so in another situation.
• No single herbicide is effective in controlling a wide range of weed flora
• Continuous use of the same herbicide creates resistance in escaped weed flora or causes a shift in the flora.
• Continuous use of only one practice may result in some undesirable effects. E.g. Rice, wheat cropping system – Phalaris minor
• Only one method of weed control may lead to an increase in the population of a particular weed.
• Indiscriminate herbicide uses and its effects on the environment and human health.
• Uses a variety of technologies in a single weed management to produce optimum crop yield at a minimum cost taking into consideration ecological and socio-economic constraints under a given agro-ecosystem.
• A system in which two or more methods are used to control a weed. These methods may include cultural practices, natural enemies, and selective herbicides.

FAO Definition

It is a method whereby all economically, ecologically, and toxicologically justifiable methods are employed to keep the harmful organisms below the threshold level of economic damage, keeping in the foreground the conscious employment of natural limiting factors.

IWM is the rational use of direct and indirect control methods to provide cost-effective weed control. Such an approach is the most attractive alternative from an agronomic, economic, and ecological point of view.

Among the commonly suggested indirect methods are land preparation, water management, plant spacing, seed rate, cultivar use, and fertilizer application.  Direct methods include manual, cultural, mechanical, and chemical methods of weed control.

The essential factor in any IWM programme is the number of indirect and direct methods that can be combined economically in a given situation.  For example, increased frequency of ploughing and harrowing does not eliminate the need for direct weed control.  It is, therefore, more cost-effective to use fewer pre-planting harrowing and combine them with direct weed control methods.

There is experimental evidence that illustrates that better weed control is achieved if different weed control practices are used in combination rather than if they are applied separately.

Good IWM should be;

• flexible enough to incorporate innovations and practical experiences of local farmers.
• developed for the whole farm and not for just one or two fields hence it should be extended to irrigation channels, roadside sides, and other non-crop surroundings on the farm from where most weeds find their way into the crop fields.
• economically viable and practically feasible.

Advantages of IWM

• It shifts the crop-weed competition in favour of crop
• Prevents weed shift towards perennial nature
• Prevents resistance in weeds to herbicides
• No danger of herbicide residue in soil or plant
• No environmental pollution
• Gives a higher net return
• Suitable for high cropping intensity

IWM of Cuscuta in Lucerne

• In fields with a history of Cuscuta (dodder), adopt crop rotations with non-susceptible crops. Grow lucerne only once in three years in such fields.
• Do not move animals and machinery from the dodder-infested fields to the new ones.
• Treat densely infested patches of lucerne with a non-residue herbicide like paraquat.
• Do not feed the Cuscuta-infested crop to the animals.
• Do not collect the lucerne seeds from the crop infested with dodder.

HERBICIDES

In the past 50 years, much has been learned about the use of herbicides and their strengths and weaknesses. After 50 years, it is an appropriate time to re-evaluate their role in agriculture. The use of herbicides is increasing day by day. This is because the other alternative control measures do not provide an effective and economic substitute for herbicides in many situations. The efficacy and safety of herbicides are greatly influenced by soil and climate. These vary greatly between countries as does the legislation controlling their use.

Advantages of herbicides;

On weed control

• They kill unwanted plants.
• They are easy to use
• Herbicides can be used on closely planted crops where other methods cannot be used.
• Most of the time one application of the herbicide is enough whereas other methods have to be continually used.
• They work fast. They can be removed quickly in critical situations.
• Herbicides are relatively cheap, and most of the time cheaper than manual weeding.

On crop growth

• They can destroy plants bearing diseases.
• They help the crops grow by destroying the weed that causes harmful effects which include competition for water, nutrients, and light; interference of weeds with crop growth by the release of toxins; modification of soil and air temperatures and the harbouring of pests.
• They can be safely used as the manual and mechanical removal of weeds can destroy the crop.
• They are relatively safe on lands that may erode.
• Non-selective herbicides can effectively clear fields, where houses and roads can then be built.

Effects of Herbicides on the environment

Herbicides vary greatly in chemical composition and in the degree of threat, they pose to the environment. Many of the herbicides are highly persistent. It is widely recognized that the main reason accounting for residues of certain herbicides like simazine and other triazines in ground and surface water was the widespread use of these herbicides at high doses on hard surfaces.

Soil:

• Some herbicides are non-biodegradable and are harmful for a long period.
• Heavy dose of herbicides affects the microbial population of the soil.
• With herbicides targeting amino acid synthesis in both plants and microbes, there is a possibility that N₂ fixation may be inhibited by the application of certain herbicides.

Water:

• The improper use of pesticides and herbicides may also cause the stormwater infiltration into groundwater.
• When these pesticides and herbicide contaminants dissolve in stormwater they infiltrate the groundwater and then the surface waters, such as ponds, streams, rivers, and lakes.
• These chemicals may also find their way into the soil and deeper groundwater units polluting them.

Living organisms:

• Most herbicides are specifically plant poisons and are not very toxic to animals.
• However, by changing the vegetation of treated sites, herbicide use also changes the habitat of birds, mammals, insects, and other animals through changes like their habitat.
• Herbivores may eat the plants treated with herbicides and then carnivores eat the herbivores.
• The toxic herbicide would be passed up the food chain increasing in concentration each time resulting in cancers and even deaths.

Anxiety about chemical residues in the environment has increased greatly in the last decade. These fears and concerns about possible litigation have led many land managers to reappraise their weed control strategies. Change has also been forced on them by the decrease in the number of approved herbicides as a result of the high cost of registration. In addition, approval has been withdrawn from more toxic and persistent herbicides

Effects of Herbicides on Humans

Among the many effects of pesticides and herbicides, perhaps the most alarming is the danger they pose to human health. People are directly affected by the toxicity of some herbicides, during their occupation (i.e., when spraying pesticides), or indirectly affected when exposed through drift or residues on food, and wildlife.

• Pesticides and herbicides can cause several health problems such as heart congestion, lung and kidney damage, low blood pressure, muscle damage, weight loss, and adrenal gland damage.
• Arbitrary and indiscriminate usage of herbicides and pesticides can result in endometriosis, a common cause of infertility in women.
• Herbicides and pesticides have been suspected by the National Cancer Research Institute as a probable cause of certain cancers (i.e., cancers of the brain, prostate, stomach, and lip, as well as leukaemia, skin melanomas, etc.) especially among farmers.
• The National Academy of Sciences reported some time ago that, infants and children, because of their developing physiology, are susceptible to the negative effects of herbicides and pesticides in comparison to adults.

Effect of herbicides on the crop plant

• An important problem with broadcast applications is that they are non-selective.
• They are toxic to a wide variety of plant species, and not just the weeds.
• If herbicides are not used properly, damage may be caused to crop plants, especially if too large a dose is used, or if spraying occurs during a time when the crop species is sensitive to the herbicide.
• Unintended but economically important damage to crop plants is sometimes a consequence of the inappropriate use of herbicides.

The build-up of resistant biotypes

Apart from their effect on the environment, another major problem with herbicides has been the build-up of herbicide-resistant biotypes where the same herbicide has been used repeatedly for several years. This problem was not foreseen at the start of the herbicide revolution but, since the early 1980s, triazine resistance has developed in most countries where these herbicides have been used. The usefulness of several other herbicides, including paraquat, dichlofopmethyl, and sulfonylurea types has been affected by the development of resistant biotypes.

• Methods of dealing with this problem include prevention of weed seed shedding, crop rotation, herbicide rotation, control of weed escapes, and tillage practices.
• Crop rotation is not relevant in an amenity situation where the ‘crops’ are usually perennial but other control measures may be appropriate in certain situations.
• If weeds are prevented from setting seed, resistant biotypes cannot develop.
• This could be achieved if land managers were made more aware of the threat of resistant biotypes and made greater efforts in intensively managed areas to prevent weeds from shedding seeds by the use of a rotation of herbicides supplemented by physical means such as mulching, hand hoeing, and hand weeding.
• Modern, intensively managed agricultural and forestry systems have an intrinsic reliance on the use of herbicides and other pesticides.
• Unfortunately, the use of herbicides and other pesticides carries risks to humans through exposure to these potentially toxic chemicals, and to ecosystems through direct toxicity caused by non-target species, and through changes in habitat.
• Nevertheless, until newer and more pest-specific solutions to weed-management problems are developed, there will be a continued reliance on herbicides in agriculture, forestry, and for other purposes, such as lawn care.

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