WEEDS are harmful plants within a crop because they compete for nutrients, water, space, light and significantly reduce crop yield. Weeds also harbour pests and diseases, interfere with harvest operations, and increase the cost of cleaning and drying of the produce. Among 300,000 species of plants on earth about 250 are harmful and are known as weeds.
For economical and profitable crop production, it is necessary to manage weeds. For that purpose herbicides are used to kill or inhibit weeds. The first widely used herbicide was . ..
For economical and profitable crop production, it is necessary to manage weeds. For that purpose herbicides are used to kill or inhibit weeds. The first widely used herbicide was 2,4-dichlorophenoxyacetic acid (2,4-D), developed by a British team during the World War II.
Development of herbicides was welcomed by farmers as a means to control weed and ensure a better crop. Herbicides though help increase yield and bring consistency to crop production, its non-judicious use results in environmental degradation and health problems. Toxic residues of herbicides from plants or soil enter in the food chain and may prove hazardous even to the future generations. These toxic herbicide residues in environment and food chain have resulted in dangerous diseases not only in human beings but also animals. Exposure to herbicides has been strongly associated with neurological, reproductive, immunological defects, hyperactivity, pulmonary fibrosis, kidney damage, leukaemia and damage to blood/brain barrier in children, and birth defects in newborns due to maternal exposure.
It has also been found that in some cases herbicide can cause some weed species to dominate fields because it develops resistance against herbicides. In addition, some herbicides destroy weeds that are harmless to crops, resulting in a potential decrease in biodiversity on farms. Environmental, health and resistance development issues, therefore, stress the need for non-chemical or organic weed control techniques.
Organic weed management is a system involving an entirely different approach for managing a farm. A farmer who manages weeds organically must be familiar with the type of weeds and their growth pattern to determine which methods are needed to control it. Organic weed control can have different approaches and can only be successful when several approaches are integrated into the weed management programme.
The first step in organic weed control is to reduce weed pressure by managing its seed bank. To achieve this objective, thoroughly compost animal manure must be used to kill weed seed and by checking its mixing with crop seeds. Cultivation is critical to weed control on organic farms, and a variety of tools are needed according to the type of weed, crop and soil situation. Over the season, different tools are needed as the crops and/or weeds grow. Blind cultivation controls very small weeds. As crops grow, soil can be thrown into the row to bury in-row weeds using rolling cultivators.
Cover crops suppress weeds by competing for light, soil moisture and nutrients, and by allelopathy, in which plants or their residues release substances that inhibit germination or growth of other plants. Cowpea, pea, oats, clovers, mash bean and mung bean are some crops that can be used successfully as cover crops. Cover crop residues on the soil surface suppress weeds by shading and cooling the soil. Crop rotation is the practice of growing a series of dissimilar types of crops in the same space in sequential seasons to avoid the build up of pathogens and pests that often occur when one species is continuously cropped. The success of rotation systems for weed suppression appears to be based on the use of crop sequences that create varying patterns of resource competition, allelopathic interference, soil disturbance, and mechanical damage to provide an unstable and frequently inhospitable environment that prevents the proliferation of weed species. Diverse crop rotations are essential to build a healthy, sustainable organic system and break weed cycles. In general, it is best to alternate legumes with grasses, spring-planted crops with fall-planted crops, row crops with close-planted crops. Mulching or covering soil surface can prevent weed seed germination by blocking light transmission. Allelopathic chemicals in the mulch also can physically suppressing seedling emergence. There are many forms of mulches available.
The concept of biological weed control is based on the premise that certain biotic factors differentially influence the distribution, abundance, and competitive abilities of different plant species. Biological weed control is, therefore, an approach using living organisms to control or reduce the population of a selected, undesirable, weed species, leaving the crop unharmed. Biological control of weeds by using plant pathogens is accepted as a practical, safe, environmentally beneficial weed management method applicable to agro ecosystems.
Why are some weeds successful?
Weeds become successful because of their characteristics that give them the ability to:Set seed before the crop ripens.
Produce large quantities of seed (e.g., Cyperus difformis can set 100,000 seeds per plant).
Seeds survive in the soil.
Reproduce vegetatively, which aids their dispersal and makes them difficult to control.
Mimic the crop (e.g., red rice ? which cannot be distinguished from the crop in early stages but which sets seed and then shatters before the crop is harvested).
Grow vigorously which allows them to out-compete the crop.
Integrated weed management:
Integrated weed management makes use of a combination of different agronomic practices to manage weeds, so that reliance on any one weed control technique is reduced. Reducing the reliance on one or two specific weed control techniques means that those techniques or tools will be effective for future use. The object of integrated weed management is to maintain weed densities at manageable levels while preventing shifts in its populations to more difficult-to-control ones. Losses caused by weeds will be minimized without reducing farm income.
Controlling with one or two techniques gives the weeds a chance to adapt to those practices. For example, the use of herbicides with the same mode of action, year after year, has resulted in weeds that are resistant to those herbicides.
Integrated management uses a variety of techniques to keep weeds ?off balance?. Weeds are less able to adapt to a constantly changing system that uses many different control practices, unlike a programme that relies on one or two control tools. Integrated weed management practices in rice include:
Thorough land preparation can significantly decrease the incidence of weeds in rice by destroying all weeds and weed seeds to provide weed-free conditions at the time of planting, and providing a good environment for rapid growth of rice seedlings. Water management: Many weeds cannot germinate or grow in flooded soils, making water management an extremely effective tool for controlling, particularly grasses and sedges. When the transplanted seedlings have established themselves (approximately one week after transplanting), completely flood the plot to a depth of 3?-4? to inhibit weed growth. As the rice grows, gradually increase the depth to 6?. The soil must be submerged completely and uninterruptedly if flooding is to be effective.
Hand weeding is time consuming and tedious. When weeds are large enough to be gripped, they are pulled out of the soil and discarded. Smaller weeds can be hand-pulled. Early hand weeding is better, since any delay will enable the weeds to absorb nutrients. A common fallacy is that small weeds do not affect the rice but they certainly do, as a simple weeding demonstration will show.
Hand hoeing is used as a method of weed control, particularly where line-planting is practiced. Hand hoeing is faster than hand weeding and works well against creeping perennials.
Since every crop has its own characteristic weeds, continued cultivation of the same crop in one plot allows these weeds to build up. Rotation of rice with kharif crops may result in reduced infestations of water tolerant weeds in the subsequent crops.
Crop rotation with allelopathic crops and rice cultivars:
Some crops such as sorghum, pearl millet and maize may drastically suppress the weed population and reduce its biomass. Pearl millet may exhibit residual weed suppression in the following crop. The inclusion of these fodder crops before the rice crop in a rice-wheat rotation may provide satisfactory weed control and can minimize the use of herbicides. It is obviously necessary to evaluate whether these crops can be grown successfully.
The importance of herbicide use is closely related to the cost and availability of labour. Herbicides are one of the first labour saving technologies to be adopted as labour costs rise. As a consequence, the use of herbicides varies considerably between countries. Herbicides replace hand weeding and enable direct seeding rather than transplanting which is less labour demanding. Direct seeding is linked to the use of herbicides, as without their use the weeds grow so rapidly in the stages before the fields can be flooded, that manual means of control are often not feasible.
Herbicides are also used in the transplanted systems. The costs involved with herbicide use are likely to remain a major constraint to their widespread adoption. Herbicides may be classified as non-selective or selective, and pre-and-post emergence. Most herbicides used in rice production are selective, controlling some or most weeds, while having a limited effect on the crop. Selectivity is not necessarily dependent upon the compounds, but also on the rates, timing and methods of application, and hence it is important to follow the manufacturer?s recommendations.
Non-selective herbicides such as glyphosate are sometimes used before establishing rice, on weed infestations such as wild rice which are difficult to control with selective herbicides.
Pre-emergence herbicides are applied to the soil and control weeds before they emerge, while post-emergence are applied to weeds after they emerge. Among the amide group are the herbicides butachlor, pretilachlor and propanil. Butachlor can be applied either as pre-emergence or early post emergence to give control over a wide range of annual grasses and some broadleaf weeds.