What is Integrated Weed Management
Increased world population will demand more food production, which can only be achieved by increasing crop yields and applying a sustainable approach, i.e. more production with rational use of available resources, which also implies responsible use of land and water and enhanced food diversity. Efforts are needed to reduce crop losses due to pests through the implementation of Integrated Pest Management (IPM) (resistant crop varieties, rational use of pesticides, biocontrol and better cultural practices) without harmful side-effects.
Among the pests, weeds are considered an important biotic constraint to food production. Their competition with crops reduces agricultural output (quantity and quality), and increases external costs by spreading them across farm boundaries. It is also a major constraint to increased farmers’ productivity, particularly in developing countries. The labour needed for hand- weeding one hectare of land is not within reach of the small farmer’s family.
An integrated weed management approach to land management combines the use of complementary weed control methods such as grazing, herbicide application, land fallowing, and biological control. The resulting combinations provide the best possible solutions to weed problems for land managers. By studying the impact of each of the above methods individually as well as in combination, sustainable management systems can be devised to suit different regions and catchment areas (from http://www.csiro.au/org/IWM.html).
Herbicides applied at the wrong time of the year can be ineffective. This wastes both time and money for the farmer. Mature plants have already produced large amounts of seeds which simply add to the soil seed bank reserves (CSIRO).
The strategic fallowing of land may provide windows of opportunity for perennial pastures to establish. This helps them out-compete weed species. A common characteristic of weedy species is that they are generally first level colonisers. That is, when other vegetation is removed by overgrazing, clearing or ploughing and the ground is left bare, weeds establish quickly and reproduce effectively to maintain that niche. Trials where land is fenced off show that some weed species have difficulty competing with other vegetation (CSIRO).
Biological control agents are generally most effective when established in gullies and rocky knolls of hills. These areas are usually inaccessible and too costly to spray so they can provide a safe haven for agents to retreat to from pastures when there is too much disturbance, such as grazing (CSIRO).
Herbicide tolerant crops
The discovery of herbicide-resistant weeds in the early 1970s triggered an interest in mimicking this unintentional development for use in crop breeding. The concomitant progress in molecular genetics made it possible to incorporate resistance genes from unrelated organisms into an otherwise susceptible crop. In other words, we were now able to adapt the biology of the crop to the chemistry of an herbicide, whereas we previously had to adapt chemistry to biology. It must, however, be noted that herbicide-resistant crops (HRCs) were first produced by methods of traditional breeding, whereas the major current HRCs have been produced by genetic engineering, the technology which has unintentionally placed these crops in a fierce debate between those in favour, and those against, the introduction and commercial use of genetically modified (GM) crops.
Ecological importance of IWM
In most ecosystems, herbicides have become one of the most important components in weed control. There are two reasons to explain the increased use of herbicides, the first being the widespread adoption of high-yielding varieties which created economic incentives for farmers to reduce weed infestation; and the second is the availability of cheap herbicides, indicating that the cost of weed control by herbicides in wet-seeded rice is less than one-fifth of the cost of a single hand-weeding in Illoilo, Philippines (Moody, 1991). Similar situations exist in West Java, Indonesia and the Mekong Delta, Vietnam (Pandy and Pingali, 1996).
Because of the availability of cheap herbicides, it is expected that herbicide usage will continue to increase, both in developed countries, and even in developing countries, where herbicides are currently used sparingly and farm wages are relatively low. However, this does not indicate a lack of importance for hand-weeding. Manual weeding is still the dominant weed control method in many parts of Asia, since management options for weed control are limited under diverse agro-ecological conditions (Kim, 2000).
All the recommended herbicides commonly used are known to be very safe not only for humans and cattle, but also for the environment when they are used properly. Sulfonylurea type herbicides have been intensively used in far-east Asia since 1990 because of their high efficacy against a broad spectrum of paddy weeds even at extremely low doses. However, intensive and repeated application of this type of herbicide has resulted in several negative effects, as follows:
- Evolving resistant weeds (Valverde et al. 2000)
- Residual effects on the following crops
- Disappearance of some susceptible weeds such as Brasenia schreberi and Sagittaria aginashi, which affects weed biodiversity (Itoh, 2000).
All these factors may well provide sufficient reason to attract public concern and anxiety regarding the negative effects of herbicides that might originate from intensive herbicide application in the environment. In this regard, an alternative to such a heavy dependence on herbicide is needed. Such an alternative might be found in the use of integrated weed management, which can reduce herbicide use in different cropping systems.
During recent years, organic agriculture has hugely increased in many regions of the world, and this trend has not yet ceased. The total area of organically grown crops is estimated to be around over 8 million ha (reference) (Organic agriculture should be seen as a process depending highly on naturally-occurring biological processes. Therefore, the objective is to stimulate these processes to obtain maximum suppression of pest problems. Obviously, the use of chemical pesticides is contrary to the concept and the practice of organic agriculture, and human intervention in the process should therefore be carried out only very carefully.
Weed management in organic agriculture is not an easy task, particularly in areas where labour for hand-weeding is short or not affordable. However, the principle should be the same as in any conventional cropping system, i.e. weed competition needs to be prevented in order to obtain maximum crop yields. This necessarily implies weeding with non-chemical materials but, this has to be carried out exactly at the right time to eliminate weeds during the so-called ‘critical period of weed competition. Organic systems also require the use of preventive methods before growing the crop and to establish a reasonable crop rotation. Stale seed bed preparation in order to kill the weeds mechanically or manually is a very good option to delay the start of weed competition. The use of cover crops and green manure, in addition to increasing soil fertility, may help to control some weed species. The most common methods used to prevent weed competition in organically grown crops are high seeding rates, narrow seed spacing/cross seeding, and companion cropping with small-seeded legumes. Rotational sequences for crops grown in tropical and sub-tropical areas should be developed to guarantee vigorous growth of the crops as well as fewer pest problems, including weeds.
Weed control in Conservation Agriculture
The so-called Conservation Agriculture (CA) is gaining a lot of recognition among farmers all over the world. Grain crops as wheat, barley, maize, rice and soybean are grown in some parts of the world under this system, which consists of the judicious use of crop rotation with minimum or zero tillage, including the use of green manures and cover crops. This approach is beneficial to effectively protect and increase soil fertility. It is wrong to identify conservation agriculture as the practice of zero or minimum tillage. In fact these procedures are part of the system, but if they are implemented in areas of monocropping then this cannot be considered conservation agriculture. The use of herbicide-resistant crops (HRCs) combined with the application of broad-spectrum herbicides make the process of conservation agriculture easier, but it also runs the risk of bringing about new weed problems, either by a shift in the weed populations or the presence of species able to evolve resistance to the herbicides in use. Again it is not always necessary to use HRCs and herbicides, particularly when the system has been practised for ten years or more.
Although this development is real, several weeds still remain which seriously affect the production of a number of crops such as rice, with its incidence of red/weedy rice, and Echinochloa spp., parasitic Orobanche weeds found in sunflower, faba beans and solanaceous crops; parasitic Striga spp. in cereals in Africa south of the Sahara, and Imperata cylindrica in many areas in Africa.
The switch to zero tillage or direct seeding practices concerns many producers when it comes to controlling weeds. The loss of tillage as a method of weed control means that producers must adjust crop rotations, herbicide use, and other cultural practices to compensate. Perennial weeds may become a serious problem to overcome, and there is a need to implement additional cultural methods, such as the use of covers. Some annual weeds, such as wild oat and volunteer canola, also grow well under zero-till conditions (see section on Conservation Agriculture).
Weeds exist in many different forms and with different life spans; there are annual, biennial, and perennial weeds. Weeds are not always bad, and in low density will result in no, or small yield losses. Heavy manifestations of weeds, or establishment of perennial weeds, can result in large yield losses or can even take land out of production until the weeds are controlled. Weeds can also reduce yield quality and can be toxic when ingested by animals or humans. Weeds can also cause environmental damage and loss of agricultural biodiversity, by competing for inputs. Integrated Weed Management aims at:
- Preventing weeds from spreading by:
- Cleaning farm machinery and vehicles before transporting, to avoid risk of spreading weeds;
- Cleaning the hair and feet of animals before moving to new areas;
- Controlling the weeds in feed and bedding grounds;
- Using only well stored and rotted manure (4-5 months), possibly improve decomposition;
- Making sure that soil disturbances are immediately reseeded
- When possible, practicing weed control on all aspects of the farm, including irrigation canals, drainage ditches, fence lines, stockyards, and farm roads.
- Improving knowledge of the identification and effects of different types of weeds.
- Monitoring and map the spread of weed populations and the resulting damage
- Making control decisions based on full knowledge of potential damage, cost of control methods, and the environmental impact of the control strategy.
- Using combinations of (preferably biological) weed control strategies to reduce the weed populations, these can include: winter cover crops, mulching, crop rotations, natural competition (e.g. ryegrass), livestock grazing, proper seedbed preparation, selecting locally adapted varieties, proper fertilizer application, stimulating bio-control by insects, mowing, hand weeding, and try to avoid, when possible herbicide application, tillage, and burning
- Evaluate and monitor the effectiveness and (environmental) effects of control strategies.