By Amar Matloob and Dr. Abdul Khaliq
FOOD SECURITY in Pakistan is threatened by ever increasing population, declining availability of fresh water, sky rocketing prices of agricultural inputs and uncertainty of climatic optima. Rice is a major food cash crop in the country and is traditionally cultivated by raising nursery and then transplanting it in standing water. This type of rice culture helps in weed control especially. However, this method requires huge inputs like irrigation water and extensive labor. Transplanted rice is gigantic user of water. Current production system consumes about 2500-3500L of fresh water to produce 1 kg of paddy. One can undoubtedly conclude that it is not the paddy but our fresh water that is being exported. In addition, the standing water in rice fields is a major source of methane emission and posing a threat to the environment. Increasing water crisis is forcing farmers and researchers to find out ways to decrease water use in rice production and have higher yields by enhancing its use efficiency. This situation has led to look for new ways of cultivating rice that require less water than transplanted rice. Direct seeding offers a promising solution for this by saving water and labor. Direct seeding of rice has potential for attaining high water productivity and eliminating the edaphic conflict in rice-wheat cropping system of Punjab. Fairly good yields can be obtained if crop is properly managed.
Sustainability of dry direct seeded rice is however adversely affected by weeds. Weeds are serious biological constraint to rice production and greatest competition due to them occurs in dry direct seeded method of rice stand establishment. It is estimated that direct seeded rice yield is reduced by 60 per cent and even 100 per cent due to huge weed infestation. Carpet weed (Itsit) and Jungle rice (Swanki) being the most competitive species though the rest of weed flora comprising of grasses like goose grass, barnyard grass, crow foot grass and sedges like purple nutsedge, flat sedge and umbrella sedge can also be harmful. The proper time and method of weed control are also a complex phenomenon in direct seeded rice crop as the weeds and rice emerge simultaneously. A direct seeded crop generally lacks a head start over weeds that a 25-30 day old transplanted seedling may have over weeds.
The lack of suitable genotype, proper sowing equipment, technical knowhow, increased incidence of paddy blast and unavailability of a selective post emergence herbicide for direct seeded rice are the limiting factors in its large scale adoption. Moreover, basmati (fine) varieties are seldom being used for direct seeding across the globe. Most of the rice growing and consuming nations have coarse rice varieties which perform better in direct seeding culture. The problem of sacrificing our fine rice to coarse rice to adopt direct seeding is too bitter to swallow as fine rice is the main export commodity and fetch a major share of foreign earnings in global market.
The herbicide-based weed management is becoming the most popular method of weed control in dry seeded rice. However, while herbicide application certainly controls several weeds, it does not eliminate others, thereby provoking a weed shift of tolerant species. In some areas it is believed that herbicide use will solve all weed problems. Experience shows, however, that although herbicide use alleviates the problem of labour for weeding, incorrect use of herbicides may bring about other environmental problems. The advent of herbicide-resistant species is an increasingly worrying problem for farmers, extension workers and policy-makers in many rice-growing countries.
The only way to avoid these problems is the implementation of improved weed control within the context of integrated weed management (IWM), with particular emphasis on the weed eco-biology of the prevailing species. This is an important prerequisite for achieving the expected growth and yield in direct seeded rice and obtaining the necessary reduction in weed stand, including weed seed bank. The ultimate objective of IWM is to maintain weed densities at manageable levels while preventing shifts in its populations to more difficult-to-control ones. Reducing the reliance on one or two specific weed control techniques means that those techniques or tools will be effective for future use IWM seeks to combine two or more control actions which will interact to provide better control than any one of the actions might provide. However, even if multiple control actions do not interact, their additive effects can mean the difference between success and failure.
Season, magnitude, weed types and timing of their association with the rice, fertilization practices, competitive ability of the rice cultivar, weed and rice density dynamics, relative growth stages of rice and weeds, moisture and nutrient availability and cultural practices are few aspects related to weed-crop competition in dry seeded rice. An integrated weed management approach involving cultural practices, crop rotations, stale seed bed practices, selection of suitable competitive cultivars, weed smothering with green manure crops and the use of herbicide mixtures is essential in responding to changes in weed community structure.
Direct seeding can be broadly categorized into dry direct seeding, wet seeding and water seeding. Considering the acute water shortage that the country is facing, dry direct seeding of rice seems most striking. It involves drilling of dry or primed seed in 22.5 cm apart rows or broadcasting the same just like an ordinary wheat crop. Land should be prepared with a principal aim to achieve a weed free seed bed of fine tilth. Land preparation through tillage operations such as plowing, disking, harrowing, and land leveling contributes to cryptic weed seed and seedling mortality at depths within the soil profile and influence the dormancy status of buried weed seed populations through exposure to altered temperature, gaseous and water regimes. Land should be disturbed a minimum as it brings more and more number of weed seeds to the surface thus breaking their intimate dormancy and providing initial flush of weeds that can be rarely tolerated by the crop.
As sustainability of direct seeded rice is greatly determined by the presence or absence of weeds thus weed control should be started as early as possible. Weed management for dry seeded paddy crop should start from the land preparation and stale seed bed or suicidal germination should be employed. Stale seedbeds also known as false seeding technique refers to cultural method of weed control. After seed bed preparation, the land is left unsown to allow weed seeds to germinate and emerge above the soil surface. The rice then can be sown after removal by either mechanical (harrows) or chemical (non-selective herbicides).
Seed priming techniques can provide good stand establishment and early seeding vigour. This can prove as key factor in imparting competitiveness against weeds as crop can quickly conceal its susceptible period which in turn sets the foundation of good yield. Priming can be done with water or osmolytes like calcium chloride and potassium chloride.
The next crucial factor is the optimum field capacity at the time of sowing. Lack of moisture at this stage can be fatal. Sowing seed at appropriate moisture level at optimum rate and depth should be the ultimate goal. A seed rate of 50 kg ha-1 and depth of 3-4 cm is fairly adequate. Lower seed rate would yield less number of productive tillers while increasing seed rate would lead to intra-specific competition for resources and mutual shading resulting in unhealthy stand. Sowing of crops in narrow rows (15 cm apart) could also reduce the space available for weeds to thrive.
Fertilization should be carried out in such a way that end beneficiaries are crop plants but not the weeds which utilize and accumulate nutrients in greater quantities than crop. Nitrogen should be applied in two or three split doses as crops initial requirement is low and most of the applied pool goes to build weed biomass. Side or band placement beside or below the seed rate has been found better than broadcasting. Beside the use of macronutrients, micronutrients like zinc and boron are important as they help to increase metabolic efficiency of crop plants and result in better seed set and quality of the produce.
Under field conditions, dry seeded paddy yield are directly proportional to the duration of weed free period. A weed competition up to 20 days after sowing could drastically reduce the number of panicle bearing tillers. In this context, pre-plant incorporation or pre-emergence application of pendimethalin could serve the purpose. This later on must be followed by a selective post emergence herbicide to tackle weeds that will emerge later on with subsequent irrigations. The combination of pre and post emergence herbicide is efficient in controlling weeds than the sole application of either single of them.
Hand hoeing/pulling twice, 25 and 45 days after sowing, however can benefit crop in two ways, i.e., by effectively controlling weeds and eliminating the chance of crop injury/stress resulting due to herbicide. However, its large scale adoption is difficult to achieve due to increasing cost and scarcity of labour. Mechanical weed control with tractor drawn implement can also be done provided crop is drilled with a seed drill permitting the use of hoeing or inter-culturing equipment.
Biological weed control with tadpole larvae and shrimps have been practiced in transplanted rice but can not be adopted for dry seeded rice due to differential water regimes at the time of sowing. Any other biological weapon comprising of pathogen or predator is currently lacking. Moreover this process is slow, risky and fragile as selectivity is rarely possible under field conditions.
Certain crops when included in rotation have the potential to reduce weed count in the next coming crop. Dry seeded rice crop must be grown after wheat, sunflower, brasssica spp. and sorghum to avail the Allelopathic benefits of these crops. Research has shown that there is wide variation in allelopathy among rice cultivars themselves. Allelopathic rice can suppress both monocot and dicot weed species. The use of allelopathic rice varieties having weed suppressing potential and competitive ability against weeds is also crucial.
Studies have examined the competitive ability of rice cultivars with the weeds. The results indicate that it is not necessary to develop high erect cultivars (normally susceptible to lodging) in order to achieve a high level of competitiveness; modern high-yielding, semi-dwarf cultivars are also able to compete with weeds efficiently.
Rice may be successfully intercropped with legumes like cow pea, sunn hemp, sesbania and soybean. Experiments have shown that rice intercropped with any of the above mentioned crops yielded as much as rice treated with herbicide. Intercropping smothered weed stands comprised of worst rice weeds.
Weed management in dry seeded rice will only be improved if farmers take into consideration the ecology of major weeds and their interaction with rice. Reduction of inputs for weed control will only be possible if there is a sustainable reduction of weed seed bank. There are several new developments in weed control which require implementation as part of an integrated weed management approach. Policy-makers need to pay attention to the problems posed by weeds in dry seeded rice as an important issue affecting rice productivity. Support to weed research programmes and farmers training in improved weed management in dry seeded rice is required to make this technique feasible.
The writers are associated with Weed Science and Allelopathy Lab, Department of Agronomy, University of Agriculture, Faisalabad. They can be contacted at agronomistryk@gmail.com