Weed Management

Growing of green manuring, puddling and efficient water management reduce the occurrence of weeds under transplanted condition. When FYM or other organic sources are applied, 2 hand weddings (one at 20-25 days and another at 40-45 days after transplanting) or two mechanical weeding with conoweeder will result in efficient management of weeds. In direct seeded crop 2-3 hand weedings up to the age of 45 days will result in satisfactory control of weeds.

The important weed species, which act as constraints in production potential of rice are given below:

SI. No.	Scientific name	English name	Local name
1.	Echinochloa colonum	Wild rice	chotta sawank
2.	Echinochloa crusgalli	Banyard grass	sawank
3.	Cyperus rotundus	Purple nut sedge	motha
4.	Cyperus irri	Yellow sedge	chhatriwala dila
5.	Cyperus difformis	Common sedge	motha
6.	Eclipta alba	false daisy	jal bhangra
7.	Celosia argentia	Cock's comb	chilmil, sarai
8.	Dactylotenium aegypticum	Crowfoot grass	makra
9.	Setaria glauca	Cattail millet	laptana, banra
10.	Scripus spp.	Club rushes	-
11.	Panicum spp.	-	-
12.	Paspalum spp.	-	-

Fig: Echinochloa colona	Fig: Paspalum distichum
Fig: Echinochloa glabrescens	Fig: Echnochloa crus-galli (L.)
Fig: Ischaemum rugosum	Fig: Leersia hexandra
Fig: Cyperus difformis L	Fig: Leptochloa chinensis (L.)
Fig: Cyperus iria L.	Fig: Fimbristylis miliacea (L.),
Fig: Ageratum conyzoides (L.)	Fig: Sphenoclea zeylanica
Fig:Trianthema portulacastrum	Fig: Eclipta alba L.
Fig: Eclipta prostrata (L.)	Fig: Amaranthus spinosus L
Fig: Eclipta erecta L.	Fig:Marsilea minuta L.
Fig:Ludwigia octovalvis	Fig: Monochoria vaginalis (Burm. f.)

Critical period of crop-weed competition

The critical period also differs with the cultivars used. For examples, 20 days of weed free growth appears best in short-statured variety of rice. For an intermediate-statured variety, the weed free period should be extended to the first 30 days after planting of rice. Generally weed competition is less under flooded condition where rice is transplanted as compared to upland situation.

Preventive measures

• Always use well rotten and decomposed organic manure.
• Avoid feeding of grain screenings or hay containing weed seeds without destroying their viability by grinding or cooking, otherwise weed will spread from dung or manure.
• Clean all the implements & machinery properly after their use in infested areas & before using in clean areas.
• Keep irrigation and drainage channels free from weeds.
• Watch seedlings in nursery carefully so that they do not get mixed with weed seedlings and get carried to the fields.
• Use of weed free seeds/seedlings for sowing/transplanting from a reliable source. It is a preventive method against introduction of weeds.

Summer ploughing

            Ploughing in summer exposes the underground parts like rhizomes and tubers of perennial and obnoxious weeds to scorching summer sun and kills them. It also helps in improving the soil physical conditions. Conventional tillage that includes 2-3 ploughings followed by harrowing decreases the weed problem in upland rice.

Field preparation

            Better land preparation (2 ploughing at 15 DBS and 2 at sowing), timely sowing (last week of June), optimum management of nutrients and an additional hand weeding markedly decreased the infestation of all categories of weeds in rainfed upland rice. In lowland rice, puddling operation incorporates all the weeds in the soil, which would decompose in course of time & that is the reason for reduced weed infestation in transplanted rice as compared to direct seeded rice.

Planting method

            Sowing should be done 2-3 days after irrigation in upland rice. Weeds already present in soil start germinating with 2-3 days. Sowing operation with seed drill removes some of germinating weeds as blade harrow is run to cover the seeds. During this process, all the surface soil to a depth of 2-3 cm is disturbed, thereby uprooting the germinating seeds.

Blind tillage/Stale seedbed

            This method is extensively used to combat weeds in rice in early stages before the sowing or just after sowing but before the crop emergence. In this method, weeds are allowed to germinate and then their above ground parts are destroyed by using various types of blade harrows as soon as the weed appears.

Hand weeding

             Two aspects are important in hand weeding: the no. of hand weeding to be done & interval between two hands weeding. The number of hand weeding depends on crop growth, weed growth & critical period of crop-weed competition, 2-3 manual weeding at 25 and 45 DAS in direct seeded rice and at 15 and 30 DAT in transplanted rice is the best and most effective way of weed management.

Post plant tillage

            Rice rotary weeder is a specific tool for weeding in rice fields. Single or double row weeders are also available like conoweeder. They are worked manually with back and forth movement in between rice rows in wetlands. This rotary weeder is less strenuous than hand weeding. At the time of weeding, it is essential to retain 1.5-2.0 cm depth of water. The 1^st weeding should be done at 10-12 DAT and subsequent weeding can be done once every 10 days. 

Solarization

It involves covering the soil surface with polythene sheets to increase the soil temperature, which would be lethal to weed seeds. A mulching period of 2-6 weeks with clear polythene sheets has been reported to give effective control of many annual weeds. Irrigation prior to solarization has a complementary effect, as moisture-imbibed seeds are more sensitive to heat than dry seeds. The limitation however is that normally weed seeds up to about 5 cm depth are only affected implying that deep prepatory cultivation would nullify the effect. Although this technique is limited by the cost of treatment, it may be made use of in controlling weeds in nursery areas. Mulching by polythene sheets for 32 days decreased emergence of Dactylotenium aegypticum and Cyperus rotundus by more than 90% and the main solarization is restricted to the 0-5 cm layer of soil.

Mulching & mowing

Different types of mulch like straw, hay, saw dust, paper, plastic film etc. when applied to soil surface, do not allow weeds to germinate or to grow as light does not reach to the soil. In bed-planting method of rice seeding, the furrow is covered with straw mulch gave sufficient control of weeds. Spreading of polythene sheets for 32 days in furrow checked the emergence of Trianthema, Cyperus & Dactylotenium spp under direct seeded upland rice.

Water management

Depending upon the method of irrigation, weed infestation may be increased or decreased. Frequent irrigation or rain during initial stages of crop growth reduces several flushes of weeds. In lowland rice, where standing water is present most of the time, germination of weeds is less, which is mainly due to puddling & impounding of water.

 Biological controls

It involves deliberate introduction & establishment of natural enemies in areas where they did not previously occur. Good control of a variety of aquatic weeds by Chinese grass carp (Ctenopharyangdon idella) has received worldwide attention. In Japan, there is limited use of tadpole shrimp (Triopus spp) to control weeds in flooded rice fields. At the IRRI, the cover effect of Azolla spp is being evaluated as a means of weed control in rice. Several species of herbivorous fish feed on aquatic weeds. The most important fish are Tilapia melanopleura, T. zilli, T. nilotica, and Puniux gonianatus. In Japan, farmers also used some species of ducks for weed control in natural rice farming.

Synthetic derivatives in naturally occurring compounds as herbicides

• Methoxyphenone: is the herbicide developed as a synthetic analog of the microbial toxin, anisomycin by employing bio-rational chemical synthesis of an herbicide based on a natural chemistry. In Japan, this herbicide is marketed as a selective herbicide for the control of banyard grass (Echinochloa crusgalli) in rice is easily degraded in soil.
• Natural herbicides: Solutions containing rice hull extract inhibits the germination of barnyard grass (Echinochloa crusgalli ). Japanese  farmers uses rice bran (200 g/m²) for weed control.

Many crop residues including straw from wheat, barley, rye and sorghum contain allelopathic compounds. Volatilization, leaching, root exudation, and decomposition of plant residues release these compounds.

Corn gluten meal (CG) is a by-product of corn wet milling process and has been patented as a natural pre-emergence herbicide.

Crop rotation

Crop rotation helps to interrupt the life cycle of weeds and thereby can eliminate or at least reduce difficult weed problems. Weeds like Cyperus rotundus (motha) can be controlled effectively by including lowland rice in crop rotation in place of upland rice. Add the smoothering crops in rotation like fodder grasses and majority of pulses controls the weeds very effectively. In continuously transplanted rice fields under irrigated conditions, Scripus maritimus persists. When upland crops are rotated with rice, the population of this weed has been reduced drastically even without adoption of weed control measures.