Weed Management Practices in Transplanted Winter Rice

Introduction

It has become a major challenge to meet food demand for the burgeoning population now than ever before. Agriculture is at the forefront of national and international agenda to assume food security and sound management of natural resources. Rice (Oryza sativa L.) is the second most important staple food crop, just after wheat, for more than half of the world’s population, including regions of high population density and rapid growth. Globally, India ranks first in rice in terms of area and second in terms of production next to China. India occupies 21% of global rice production from 28% of rice area. Among all the states, West Bengal is the leading state contributing 13.8% of India’s total rice production, which occupies the first position in terms of production and second in terms of area.

There are several factors for its low productivity and out of that, losses due to weeds are the most critical one. In recent years, rice production has increased with the introduction of high-yielding varieties, but their maximum yield potential has not been fully realized owing to improper weed management. Weeds are the most severe and widespread biological constraints that compete with rice plants for natural resources viz., light, nutrients, moisture, and space and thus reducing the yield significantly. Mukherjee and Singh (2005) opined that uncontrolled weeds compete with the rice and cause grain yield losses up to 76% under transplanted conditions. Thus, effective weed management at the  initial stages [0-40 days after transplanting (DAT)] is an important key factor for obtaining higher crop productivity.

Weed flora in rice field

The weed flora in lowland rice is very much diverse and dynamic over times and places. Depending on the types of rice culture and season, rice field is colonized by terrestrial, semi aquatic or aquatic weeds. Weeds belonging to various species of grasses, sedges and broad-leaved weeds are found to be associated with rice culture. The dominant grassy weeds are Echinochloa sp. and Cynodon dactylon; sedges are Cyperus sp. and Fimbristylis miliacea and broad-leaved weed species are Ammania baccifera, Marsilia quadrifolia, Monochoria vaginalis and Ipomea aquatic etc.  under puddle condition of sandy clay loam soil during kharif season. According to Singh et al. (2007), the broad leaved constituted 34.1 %, grasses 42.2 % and sedges 23.6 % of the total weed population under weedy conditions. The wet seeded rice infested with composite weed flora comprising of 51.5 % grasses, 30.9 % sedges and 17.5 % broad leaved weeds (Ravisankar et al., 2008). The detail of predominant weeds has been depicted in the Table1.

Table 1. Weed flora prevalent in the field of transplanted winter rice

Crop - weed competition

Weeds possess antagonistic effect on rice crop for several growth factors such as space, light, water and nutrients. Competition begins when crop and weeds grow in close proximity to each other and supply of a single necessary factor falls below the demand as both the crops as well as weeds have the same requirement for their growth and development. Once this occurs, the factors necessary for plant growth cannot be used effectively even though they may be present in abundance and the overall effect would be a reduction in the biomass and reproductive potential of the competitors. Sharma et al. (2003) found that the outcome of the competition would depend on the type of competing species, their population, duration and the level of fertility. The competitive advantage of weeds over rice is attributed to some being C₄ plants and more aggressive unlike rice which is a C₃ plant. Weeds have high photosynthetic rates and correspondingly high growth rates, high potential to acclimatize to changing environment and more efficient in seed production.

Critical period of rice-weed competition

The critical period for crop weed competition indicates the critical crop growth stage or the periods during which the field must be kept weed free and weeding results in highest economic return. Critical period of competition between rice and weeds is when the rice is in the vegetative phase and the yield components of rice are being differentiated. According to Thapa and Jha, (2002), critical period for crop weed competition in rice is up to 40 days after transplanting. In rain fed lowland rice, 30-60 days after sowing period is considered as critical period for crop weed competition to avoid grain yield losses (Moorthy and Saha, 2005). The yield obtained by weeding at this duration is almost similar to that obtained by full season weed free condition.

Effect of weed competition on rice growth

In a particular environmental condition, a unit area of land can produce a certain amount of total vegetative dry matter. In order to maximize the crop yield, all of this growth should be in the form of crop. The competition between rice and weeds is severe during early growth of rice when yield components like tillers, panicles, grains etc. are being formed. Limited crop growth at this stages ultimately possess negative effect on yield of rice. Crop yield loss and population of weeds is linearly correlated, however, above certain population limits, yield reduction becomes nearly constant due to self-competition among weed plants. Crop yield losses due to weeds are mainly influenced by their intensity as well as type of weed flora present in the field. Any weed growing in association with the crop will reduce the vegetative potential of the crop and ultimately result in loss of yield. Grassy weeds are the heavy competitors of rice crop which is followed by sedges and broad leaved weeds.

Methods of weed control

A sound weed management system involves utilization of all feasible methods of prevention and control in a harmonious combination including the maximization of mortality factors to keep weed population below threshold level and at the same time keep the cost and harmful effects at minimum. Weeds must be kept under check with the use of any of the weed control measures in order to achieve higher benefits from applied inputs. Methods of controlling weeds vary from situation to situation and season to season. These methods are grouped into cultural, manual, mechanical, chemical, and biological. Each of them has their own advantage and disadvantage, any single method is rarely found adequate and effective in controlling weeds. The integrated weed management is very effective and low cost it includes use of weed control methods in combination with each other to check weed population.

1.Cultural method:

Transplanting, the oldest and most widely used method of growing rice, is effective in lowering weed density. Under transplanting, puddling and submerged conditions inhibit weed germination. If flooding happens soon after crop establishment, a better weed control could be achieved. It was also noted that if weed infestation is severe during the early stages of crop growth and is not managed, later submergence will not be successful in weed management. Under transplanted rice, continuous submergence of the crop successfully reduced weed population and weed seed germination. As both the weeds and rice usually emerge simultaneously and farmers are not generally able to use standing water to suppress weeds at the early growth phases of rice, the risks of crop yield loss due to competition from weeds in direct-seeded rice were higher than in transplanted rice. Reducing the row-to-row and plant-to-plant spacing, weeds can also be suppressed. The closely spaced crop do not provide enough space for weed growth and prevents sunlight from entering downward thus effectively buried the weeds growing beneath its canopy.

2.Manual method:

Weed management through manual methods i.e. hand weeding, is very effective. It was found that the maximum grain yield and weed control efficiency were achieved when two manual or hand weeding were given in rice field (Tripathi et al., 2000). According to Pal et al. (2009), the highest grain yield of 5.08 t ha^-1 in Gangetic alluvial soil was obtained by hand weeding at 20 and 40 DAT, since it left limited space for weeds to grow and compete with the crop, preferably at the crucial stage of crop weed competition. But in large-scale cultivation, it is tedious, expensive, and time-consuming. Also in rainy season, Continuous rains and unavailability of man power make manual weeding difficult.

3. Mechanical method:

Interculture with different implements or machines viz., khurpi; kassola/ kodal; wheel hoe, rotary weeder; cono weeder; Japanese paddy weeder; rotating hoe etc., can be used for controlling weeds from different crops. This method can be adopted by unskilled labour and are invariably economical, non-polluting, without residue problems, and relatively safe for operator. The incorporation of weeds in situ through mechanical weeding may aid in the efficient recycling of nutrients that have been depleted. These implements helped to save labour, time and reduce man days required for weeding from 30 to 10 as they become more experienced in handling the weeding implements. In comparison to using a conventional weeding approach, using a cono weeder increased grain yield by 10% during the wet season but only by 3% during the dry (Thiyagarajan et al., 2002). The weeds in the inter-row space were successfully controlled by the rotary weeder, but those in the intra-row space and those near the crop were not. The mechanical weeding using rotating hoe having small toothed wheels, increase the soil pores thus roots and microbes gain access to oxygen more easily and also significantly increase the tiller production. There are some problems encountered with mechanical weeding that incorporation of some weeds like Cynodon sp. and sedges with underground stolon and rhizomes which result in faster regeneration. Beside this, driving of heavy machinery on field leads to soil compaction which is not desirable for plant growth.

4. Chemical method

In general, cultural and mechanical methods of weed control are time-consuming, cumbersome, and laborious apart from being less effective because of a chance of escape and or regeneration of weeds from roots or rhizomes that are left behind. Hand weeding is difficult due to the crop's morphological resemblance to some grassy weeds. Thus, it looks like the only other option is to apply herbicides. Chemical weed control under puddled planted rice culture is an effective way to get rid of grasses, sedges, and broad-leaved weeds by reducing labour costs and increasing grain output. Higher weed control efficiency and crop yield depend on judicious herbicide selection, appropriate timing, dosage and application technique. Herbicide application techniques such spraying, sand mixing and urea mixing need to be made more efficient. Method of sowing, water management, fertilizer management etc. are greatly influence the time of application of herbicides. In transplanted rice, pre-emergence herbicides should be applied at 2-3 DAT whereas in case of direct seeded rice it is 1 DAT under sufficient moisture condition.

Delay in the application of herbicide beyond the date of sowing decreases the phytotoxicity to the crop but increases resistance of germinating weeds. So the complexity of our rice farming system, soil and environmental conditions, as also the farmers’ ignorance of the herbicide technology' necessitates development of relative safe, economic and easily acceptable herbicides. The co-ordinated research effort of several years helped to identify pretilachlor, pyrazosulfuron ethyl, butachlor, thiobencarb, anilofos, pendimethalin, oxadiazon, 2, 4-DEE, bispyribac sodium, oxyflurofen and sulfonylurea etc. herbicides are of promising herbicides for rice.

Herbicide mixtures:

Herbicide residue or herbicide resistance to weeds may result from the repeated application of the same herbicide or chemical belonging to the same family. In contrast to using a single herbicide, the usage of herbicide mixtures (both ready and tank) enabled broad spectrum weed control with a minimal dose. Some herbicide combinations used in transplanted rice are Arozine (Anilofos 24% + 2.4 –DEE 32EC); londax power (Bensulfuron methyl 0.06 % + Pretilachlor 0.60 %); Almix (Metsulfuron methyl 10% and Chlorimuron ethyl 10%); Swachh (Pretilachlor 6% + pyrazosulfuron ethyl 0.15%) etc.

Herbicides that are most commonly used in transplanted rice field are given presented in Table 2.

Table 2. Details of some herbicides

5. Biological method

It includes the deliberate use of living organisms to suppress the population of the targeted weeds and has produced numerous remarkable achievements against weeds. Biotic agents such as insects, mites, nematodes, and plant infections have been utilized to suppress weeds. As far as rice weed concerns, biological weed control has not yet received much attention. To effectively utilise it and assist the farmer community, extensive study and innovative technology are needed in this field. The concept of "bio-herbicides," which contain microorganisms as their active ingredient, has gained momentum. In this approach, only minute fungal spores have been employed thus the resulting compounds are frequently referred to as mycoherbicides. Currently, scientists are using Collego, Devine, Biomal, Dr. Bio Sedge etc. commercially available bio-herbicides, to manage weeds in rice fields, but there has been no success documented to yet. Geese and ducks are used in the case of deep-water rice cultivation to pick weeds and insects. In China, grass carp, a herbivore fish, has attracted a lot of attention, particularly for its ability to eliminate submerged weeds like duckweed, Amaranthus viridis, Corchorus acutangulus, and filamentous algae.

Integrated weed management

An integrated weed management system is the coordinated control of the weed population using reliable, effective, and practical management strategies that are both economically sound and environmentally safe in order to increase and maintain the yield of the rice crop. In rice and rice-rice cropping systems, integrated weed management encompasses using well-accepted high-yielding crop cultivars that resist weed competition, pre-sowing seedbed tillage for effective seed bed preparation, seeding techniques that enhance crop growth and minimise weed growth, optimal plant population, including closer spacing in and between rows, use of cultivars that form a good canopy structure over weeds, precision placement/timing of fertiliser application, judicious use of irrigation water, sound crop rotation use of effective herbicides. Thus, integrated weed management combines all appropriate management strategies with the environment's own natural controlling and limiting factors.

Conclusion

Rice has a dynamic weed flora that changes from place to place and condition to condition which competes with the crop and lowers grain output. In places where labour is expensive and limited, chemical weed management is becoming more important. In order to manage weeds in rice fields, some herbicides, either individually or in combination, have been shown to be an economically feasible alternative to hand weeding. However, an innovative idea called integrated weed management is now emerging. In this case, all efficient management techniques are used in a way that is compatible with maintaining environmental quality while lowering weed populations below economic threshold levels.

References

Moorthy, B. T. S. and Saha, S. (2005). Studies on crop weed competition in rainfed direct seeded lowland rice (Oryza sativa L.). Indian Journal of Weed Science, 7(3-4): 116-118.

Mukherjee, D. and Singh, R. P. (2005). Effect of micro-herbicide on weed dynamics, yield and economics of transplanted rice (Oryza sativa). Indian Journal of Agronomy, 50(4): 292-295.

Sharma, S. D., Singh, S., Singh, D., Narwal, S., Malik, R. K. and Punia, S. S., (2003). Evaluation of ethoxysulfuron and its mixtures against weeds in transplanted rice. Indian Journal of Weed Science, 35(3&4): 201-204.

Pal, D., Ghosh, R. K., Sounda, G., Dolai, A. K, Pramanick, M. and Mallick, S. (2009). Bio efficacy of some promising herbicides in transplanted kharif rice and their influence on soil microflora. Indian Agriculture, 53(3&4): 193-198.

Ravisankar, N., Chandrasekaran, B., Raja, Bin, M. and Chaudhuri, S. G. (2008). Influence of integrated weed management practices on productivity and profitability of we seeded rice (Oryza sativa). Indian Journal of Agronomy, 53(1): 57-61.

Singh, P., Singh, P. and Singh, S. S. (2007). Response of aromatic rice (Pusa Basmati 1) to establishment methods, fertility levels and weed management practices. Indian Journal of Weed Science, 39(1&2): 32-35

Thapa, C. B. and Jha, P. K. (2002). Paddy crop weed competition in Pokhara, Nepal. Geobios, 29(1): 51-54.

Thiyagarajan, T. M., Senthil Kumar, K., Bindraban, P. S., Hengsdijk, S., Ramasamy, V., Velu, D., Durgadevi, K., Govindarajan, R., Priyadarshini, R. Sudhalakshmi, C, Nisha, P. T. and Gayathry, G. (2002). Crop management options for increasing water productivity in rice. Journal of Agriculture Research and Management, 1(4): 169-181.

Tripathi, H. P., Jaiswal, L. M. and Verma, D. K. (2000).Chemical weed control in direct seeded rice under puddle conditions. Oryza. 37(2):64-65.

Authors

Debarati Seal¹, Kabita Mishra², Bijan Behari Barick³, Rakesh Jhadi⁴, Bappa Mandal⁵, Susmita Moi⁶ and Bikas Chandra Patra⁷

(1 to 6 Research Scholar and 7 Professor)

Department of Agronomy, Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, Nadia-741252, West Bengal, India