WEEDS reduces the yield and quality of a crop. It also raises the cost of production. In case of paddy, the yield drops by 15-20 per cent and sometimes, even 50 per cent.
A crop badly infested with weeds, normally fails in totality. Every year there is huge loss of paddy crop while the quality of produce from weed infested fields is also inferior. These also obstruct harvesting.
Weed-crop competition: Weeds compete with desirable plants. Competition denotes a relationship between the same or different species which leads to the flourishing of one at the expense of the other or at the expense of both.
While weed competition with rice does not normally lead to death of either species, it almost results in decreased yields. Weeds are known but the extent of problem they pose is not recognized. Farmers acknowledge the problem in their fields but the high labour cost of hand weeding discourages adequate response.
The competition between weeds and rice depends on the following influences:
i. Relative growth stages of rice and weed.
ii. Nature of stand establishment (transplanting versus direct seeding).
iii. Density of planting.
iv. Rice variety.
v. Moisture and nutrient availability.
In irrigated systems, rice seedlings are transplanted into puddled soil. This gives rice a substantial head-start on weeds and initially, competition is minimal. Competition increases as growth progresses, particularly in stands of direct-seeded rice, since weeds germinate at the same time and compete for light and nutrients with the rice seedlings. Weed competition generally takes three forms:
Competition for light: Weeds that are shorter than rice crop throughout growth period compete a little or not at all with rice for light. However, weeds that are taller can reduce the light available to rice by as much as 50 per cent. Since sunlight provides the main source of energy utilized by plants for manufacturing food, shading by tall weeds can significantly stunt growth and reduce yields.
Competition for water: Where water is plentiful, competition between rice and weeds is minimal but during shortage the situation is quite different. If weeds consume significant portion of water then tillering, flowering, and grain filling are delayed or impeded.
Competition for nutrients: Weeds have a large nutrient requirement. They are big feeders and can, if left uncontrolled, absorb more of the soil nutrients than the crop. Fertility increases, though fertilization is generally accompanied by increase in weeds, which can result in larger yield reductions.
Reduction of grain quality: Weed seeds in grain reduce the price. Weed seeds in grain can also cause uneven moisture in the grain causing loses in quality due to the formation of molds and/or due to cracking losses during milling.
In addition to competing with rice for sunlight, water, and nutrients, weeds pose another problem. Many weed species act as alternate hosts for insect pests and disease-causing organisms, and their presence among the crop or along bunds and peripheries can increase losses due to insect or disease attack.
Why are some weeds successful?
Weeds become successful because of their characteristics that give them the ability to:
i. Set seed before the crop ripens.
ii. Produce large quantities of seed (e.g., Cyperus difformis can set 100,000 seeds per plant).
iii. Seeds survive in the soil.
iv. Reproduce vegetatively, which aids their dispersal and makes them difficult to control.
v. 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).
vi. 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:
Land preparation: 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: 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: 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.
Crop rotation: 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.
Herbicides: 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.
By Dr M. Farooq, Dr Shahzad Basra & Basharat Ali Saleem