Nitrogen and herbicides interactions in direct-seeded rice- Punjab, India

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Effect of nitrogen rates and weed control methods on weeds abundanceand yield of direct-seeded riceGulshan Mahajana; Jagadish Timsinab

a Department of Plant Breeding and Genetics, Punjab Agricultural University, Ludhiana, India b

International Rice Research Institute, Los Baños, The Philippines

Online publication date: 16 May 2011

To cite this Article Mahajan, Gulshan and Timsina, Jagadish(2011) 'Effect of nitrogen rates and weed control methods onweeds abundance and yield of direct-seeded rice', Archives of Agronomy and Soil Science, 57: 3, 239 — 250To link to this Article: DOI: 10.1080/03650340903369384URL: http://dx.doi.org/10.1080/03650340903369384

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Effect of nitrogen rates and weed control methods on weeds abundance

and yield of direct-seeded rice

Gulshan Mahajana* and Jagadish Timsinab

aDepartment of Plant Breeding and Genetics, Punjab Agricultural University, Ludhiana,India; bInternational Rice Research Institute, Los Banos, The Philippines

(Received 1 April 2009; final version received 11 September 2009)

Field experiments were conducted at the Punjab Agricultural University,Ludhiana, India, during 2007–2008 to examine if nitrogen (N) rates and weedmanagement practices using herbicides in combination with hand-weeding (HW)can improve crop competitiveness against weeds, and increase the yield, waterproductivity and profitability of direct-seeded rice (DSR). Increasing Napplication rate up to 150 kg ha71 caused significant improvement in grainyield when the weeds were well controlled either by Pendimethalin þ BispyribacNa or by Pendimethalin þ Bispyribac Na þ 1 HW, respectively; however, underpoor weed control condition (Pendimethalin þ 1 HW), it resulted in a drasticreduction in yield. The highest net returns ($1083) and water productivity(0.60 kg m73) were observed for N application of 150 kg ha71 and weeds werecontrolled with Pendimethain þ Bispyribac Na þ 1 HW. We suggest thatPendimethalin þ Bispyribac Na þ 1 HW is the best integrated weed manage-ment strategy to control weeds, and to increase yield, water productivity, andprofitability. We propose that the simulation modelling tool be utilized in futurefor evaluating options for improving sustainable N and weed managementpolicies especially when crop, weeds, soil, weather and management data areavailable for calibration and validation of appropriate and existing simulationmodel of rice.

Keywords: Bispyribac Na; hand-weeding; integrated weed management; Nmanagement; Oryza sativa; Pendimethalin

Introduction

The shortage and rising cost of labour and high volume of water use for puddling intransplanted rice (TPR) are the main reasons to seek alternatives to growing floodedTPR. In the Indian Punjab, rice production is increasingly constrained by labour andwater crisis (Mahajan et al. 2009a). The groundwater resources in Punjab have beendeclining at a very fast rate (0.1 m to 1.0 m year71) because in that state irrigatedrice cropping has been practised in alluvial coarse-textured soils using largelygroundwater resources since the early 1970s (Hira et al. 2004). Further labourshortage during rice transplanting time is prompting a search for a productionsystem that uses less labour and water to produce rice. Dry direct-seeded rice (DSR)is such an alternative, which has replaced some areas under TPR in Southeast Asia,

*Corresponding author. Email: [email protected]

Archives of Agronomy and Soil Science

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particularly in The Philippines, Malaysia and Thailand (Pandey and Velasco 2002).There has long been interest in growing DSR in Asian rice production system, butDSR is beset with weed problems (Johnson and Mortimer 2005; Singh et al. 2006).DSR warrants the frequent use of herbicides for weed control, but if the sameherbicide is used for a long time, weed species resistant to that herbicide may buildup and eventually the control measure will fail. Thus, effective weed control oftenrequires a combination of cultural, mechanical and chemical control measures, suchas an integrated weed management approach to delay herbicide resistance andreduce the herbicide load on the agro-ecosystem (Rao et al. 2007).

Crop fertilization is one important component of integrated weed managementand it is observed that nitrogen (N) fertilizer plays an important role in thecompetitive balance between weeds and rice (Raun and Johnson 1999; Camara et al.2003). Although crop competitiveness may improve with improved nutrient status,some weeds are more effective in utilizing the available excess nutrients than thecrops (Raun and Johnson 1999). Other authors also reported that weeds are capableof absorbing nutrients faster and in relatively bigger amounts than crop plants(Iqwal and Wright 1997; Ali et al. 2003; Blackshaw et al. 2005), and that in thepresence of a high weed population density fertilizer application may stimulate weedgrowth so greatly that the crop growth will be suppressed. Therefore, manipulationof crop fertilization is a promising agronomic practice in reducing weed interferencein crops (Cathcart et al. 2004; Blackshaw et al. 2005).

Since the concept of aerobic rice (dry DSR) in the north-western Indo-GangeticPlain (IGP) is new, relatively few insights exist into N use and crop-weed dynamics.Furthermore, there are no fertilizer recommendations made for DSR. In dry DSR,the dominant form of N is nitrate and the alternate moist and dry soil conditionsprevalent in this system can stimulate nitrification and denitrification processesresulting in a loss of N through N2 and N2O. Moreover, the relatively low uptake ofN can cause low fertilizer-N recovery under aerobic conditions versus floodedconditions. Beldar et al. (2005) reported that out of 150 kg N ha71 applied to theDSR crop, only 22% was taken by the crop while 31% was left in the soil and rootsafter harvest. They reported that intensive weed control measures adopted during theexperiment reduced the growth and N uptake by the weeds; both might have causedreduction in ANR (Apparent nitrogen recovery) of aerobic rice. These all suggestthat dry DSR crops may require more fertilizer rates than the flooded TPR; hence,higher N rates (120–180 kg ha71) were selected in this study.

Zoschke and Quadranti (2002) noted that the effect of fertilization and of N inparticular, on weed interference with crop is not fully understood. We hypothesizethat sustainable weed management in DSR can be accomplished by appropriateN management and herbicide practices. For example, Pendimethalin (Pendi) at1.0 kg ha71 as pre-emergence has been quite effective and economical for DSR(Jayadeva and Bhairappanavar 2002; Singh et al. 2006). Likewise, Bispyribac Na(Bis) at 30 g ha71 as post emergence has been found to be very effective against mostof the weeds in DSR (Rao et al. 2007; Mahajan et al. 2009b).

Herbicides move to the agro-ecosystem to low species diversity and impose highselection pressure. This necessitates integrated weed management approaches inDSR rather than relying totally on chemical control methods (Moody 1992; Raoet al. 2007). Furthermore, altered soil fertility and herbicide use can markedly affectcrop-weed competition (Cathcart et al. 2004; Abouziena et al. 2007). Limitedinformation is available on the interaction of N rates and weed management

240 G. Mahajan and J. Timsina


practices on weed crop dynamics, which may alter grain yield, profitability and waterproductivity in DSR. It is thus imperative to develop best management options forDSR in response to N and herbicide application so that grain yield, waterproductivity and profitability can be increased directly by N supply and indirectly byresulting in decreased crop-weed competition. Hence, the objective of this study wasto examine if N rates and weed management practices using herbicides incombination with one hand-weeding (HW) can improve crop competitivenessagainst weeds, and increase the yield, water productivity and profitability of DSR.

Materials and methods

Experimental site

Field experiments were conducted at Rice Section, Punjab Agricultural University,Ludhiana (308560 N, 758520 E and 247 m a.s.l.), India, during 2007 and 2008 toevaluate the effect of N fertilizer and weed control methods on productivity of riceand growth of associated weeds. The field has a history of rice-wheat rotation for thepast 20 years, with a hard pan below 25 cm in the soil profile. The climate of the areais semi-arid with an average annual rainfall of 400–700 mm (75–80% of which isreceived during July to September), minimum temperature of 0–48C in January, andmaximum temperature of 41–458C in June. The soil is loamy sand (Entisol, TypicUstipssamint, Fatehpur Series) with pH 7.3, total nitrogen 0.32 g kg71, organicmatter 2.8 g kg71, and Olsen P 8 mg kg71.

Experimental treatments and observations

The experiment was established with a split-plot design having three replicates andthe crop was sown in the second week of June every year. The main plots includedthree N fertilizer rates (120, 150, and 180 kg ha71) and subplots three weedcontrol methods. The three weed control methods were: (1) Pendimethalin 1 kgha71 þ Bispyribac Na 30 g ha71; (2) Pendimethalin 1 kg ha71 þ Bispyribac Na30 g ha71 þ 1 HW (45 DAS); and (3) Pendimethalin 1 kg ha71 þ 1 HW (30 DAS).Pendimethalin was applied as pre-emergence at three days after sowing (DAS) andBispyribac Na was applied as post-emergence at 18 DAS. Both the herbicideswere applied using a knapsack sprayer with a flat fan nozzle and water as a carrier at375 l ha71. The land was prepared with two ploughings with a disc harrow and oneplanking and seed at 50 kg ha71was direct-seeded in rows 25 cm apart from row torow. An indica pureline cultivar PR-115 (120–125 days growth duration) of rice(Oryza sativa L.) was sown. Nitrogen was applied through urea in four equal splits(basal, 21 DAS, 42 DAS, 63 DAS). Flash irrigation was applied just after sowing;thereafter, it was applied at 5 DAS, 14 DAS, and then followed by intermittentirrigation at three-day intervals up to 14 days before harvest.

Chlorpyriphos (2.5 l ha71) and Tilt 25 EC (250 ml ha71) were used to controlinsect pests and diseases. Weed abundance (biomass and density) was recorded withthe help of a quadrant (0.5 6 0.5 m) placed randomly at two spots in each plot 15days before maturity (just after last irrigation) of the crop. During last irrigation (15days before maturity) weeds were cut at ground level, washed with tap water, andthen separated and counted species wise. Thereafter those weeds were first sun-driedand then oven-dried at 708C for 48 h, and then weighed for weed dry matter. Atharvesting, grain yield was recorded at 14% moisture from each plot with net

Archives of Agronomy and Soil Science 241


sampling area of 6 m2 (4 6 1.5 m) and then expressed in t ha71. At the same time,five hills were selected randomly from each plot for measuring agronomic parametersincluding number of productive tillers m72, panicle weight, grains/panicle andspikelet sterility (%).

Water productivity

Depth of irrigation water (50 mm) was computed by monitoring water level beforeand after irrigation using a fixed scale installed in each plot having gross area of10.08 m2 (4.8 6 2.1 m). Water (irrigation plus rain) productivity was calculated as:WPIþR [kg grain ha71 mm71]: grain yield/(irrigation þ rainfall) and then convertedto kg m73: WP [kg m73] ¼ 0.1 6 WP [kg ha71 mm71]. The mean rainfall receivedduring the crop season was 405 mm and mean water used was 1105 mm includingrainfall.

Economic analysis

Human labour uses for tillage, seeding, irrigation, fertilizer and pesticide application,weeding and harvesting were recorded separately for each treatment. Time (h)required to complete operations for each treatment was expressed as person-day/ha(8 h equal 1 person-day). Similarly, time required by tractor to complete tillage(h ha71) and time and diesel required for each irrigation (h ha71 and l h71,respectively) were also noted. The cost of cultivation was calculated based on thecost of seed, fertilizer, pesticides, and human labour ($2.7 d71) and tractor ($6 h71).Gross income was determined on the basis of minimum support price for rice offeredby the Government of India. Finally, net income was determined as the differencebetween gross income and total cost.

Statistical analysis

Analysis of variance was performed to determine the effects of N and weed controlmethods and their interactions on weeds and grain yield of DSR using CROPSTATversion 7 (IRRI, Manila, Philippines). Means were compared using least significantdifferences (LSD) at 5% probability level. The results for 2007 and 2008 were notsignificantly different and hence were pooled for analysis.

Results and discussion

Effect of N rates on weeds abundance and grain yield

Weed flora was dominated by Echinochloa colona, E. crussgali, Digitaria sanguinalis,Cyperus iria, Eleusine indica, and Eclipta alba, etc. The weed density of grasses andsedges increased with increase in N fertilizer rate from 120 kg ha71, whereas thedensity of broad leaf weeds was not influenced significantly with increasing N ratefrom 120 kg ha71 (Table 1). These results are in conformity with the reports ofZoschke and Quadranti (2002), Kim et al. (2006) and Andreasen et al. (2006) whoreported that the influence of N rate differed among weed species. In our study, itwas observed that with application of 150 kg N ha71, total weed density increasedby 85% compared to that at 120 kg N ha71. Further, increase in N rate from 150 kgha71 failed to influence the total weed density significantly (Table 1).



The mean dry weight of weeds found to be increased significantly with increasingN rate from 120–180 kg ha71. Therefore, a strong linear relationship was found withincreasing level of N and weed dry matter (Figure 1). According to Ampong-Nyarkoand De Datta (1993) increase in fertilizer dose may benefit weed growth to a greaterextent than crop growth. Likewise, some authors (Jornsgard et al. 1996; Shipley andKeddy 1988; Harbur and Owen 2004) reported that weeds absorbed the nutrientsfaster and in larger amounts than crop plants, and as a result, flourished well andtherefore registered higher dry matter with higher amount of N application.Averaged over weed control treatments, grain yield increased significantly withincrease in N application rate from 120 to 150 kg ha71 (Table 2). A further increasein N rate above 150 kg ha71 did not increase the grain yield significantly. Aquadratic relationship was found between N rate and grain yield (Figure 1).

Table 1. Effect of varying N level and weed control measures on weed abundance.

Treatments

Grassweed

density*(No. m72)

Sedgesweed

density**(No. m72)

Broad leaveweed

density***(No. m72)

Totalweeddensity

(No. m72)

Totalweed drymatter(g m72)

N levels120 kg ha71 8 7 4 20 31.4150 kg ha71 21 9 6 37 39.8180 kg ha71 29 13 5 46 47.6LSD (0.05) 4.5 3.0 NS 9.8 10.0P 4 F p 5 0.0001 p ¼ 0.0007 p ¼ 0.26 p 5 0.0001 p ¼ 0.019

Weed control measuresPendi þ Bis 8 3 3 19 17.4Pendi þ Bis þ 1 HW 7 2 2 11 8.1Pendi þ 1 HW 43 10 10 73 93.2LSD (0.05) 4.7 5.4 3.7 10.0 10.4P 4 F p 5 0.0001 p 5 0.0001 p ¼ 0.001 p 5 0.0001 p 5 0.0001

Pendi, Pendimethalin; Bis, Bispyribac Na; HW, hand-weeding; P ¼ probability. *Main grassy weeds:Echinochloa colona, E. crussgali, Digitaria sanguinalis, Eleusine indica; **Main sedges: Cyperus iria;***Main broad leaf weed: Eclipta alba.

Figure 1. Showing relationship of grain yield and weed dry matter with increasing N rate.1 q ¼ 100 kg.



It was found that weed dry matter remained statistically the same with Napplication of 120 and 150 kg ha71, as a result crop N-use efficiency increased withN application of 150 kg ha71 and ultimately resulted in more yield. But furtherincrease in N level from 150 kg ha71 resulted in significant increase in weed biomassas a result grain yield did not increase further. Weeds have combined competitivecharacteristics such as high growth rate, high N-use efficiency and luxuryconsumption with high photosynthetic rate (Ampong-Nyarko and De Datta1993). This enabled the weeds to remain more competitive than rice at higher Nrate (180 kg ha71), so no further response to N was observed with increasing N ratefrom 150–180 kg ha71. It is quite possible that at higher N rate, N uptake by thecrop may also be reduced due to more competition offered by weeds which caused anincrease in spikelet sterility at higher N doses. It was observed that spikelet sterilityincreased by 22% (Table 2) at higher N level (180 kg ha71) as compared to lowerlevel of 120 kg N ha71.

Effect of weed control method on weeds abundance and grain yield

Weed control treatments influenced the weed density and weed biomass sig-nificantly (Table 1). The treatments Pendimethalin þ Bispyribac Na andPendimethalin þ Bispyribac Na þ 1 HW were found to be more effective thanPendimethalin þ 1 HW in reducing the total weed density and weed dry matter(Table 1). This happened due to the emergence of many weeds after the hand-weedingin the latter, but in the plots where the Bispyribac Na was applied, no such incidencewas observed. Bispyribac Na is a selective herbicide effective for the control ofgrasses, sedges, and broad leaf weeds in rice, and is also effective as a soil or foliartreatment (Schmidt et al 1999). It is a member of the pyrimidinyl oxybenzoic chemicalfamily and inhibits the enzyme acetohydroxy acid synthase, also known asacetolactate synthase (ALS), in susceptible plants. Pendimethalin þ BispyribacNa þ 1 HW recorded lowest weed biomass among all the weed control treatments

Table 2. Effect of varying N level and weed control measures on paddy yield (t/ha) and itscomponents.

Treatments

Paddyyield

(t ha71)Panicles

(No. m72)

Grains/panicle(No.)

Spikeletsterility(%)

Panicleweight(g)

N levels120 kg ha71 4.52 397 99 19.1 2.4150 kg ha71 5.39 413 105 21.5 2.5180 kg ha71 5.34 378 105 23.5 2.5LSD (0.05) 0.5 NS NS 0.8 0.9P 4 F p ¼ 0.0005 p ¼ 0.15 p ¼ 0.26 p 5 0.0001 P ¼ 0.007

Weed control measuresPendi þ Bis 5.32 417 104 19.7 2.6Pendi þ Bis þ 1 HW 6.11 452 112 19.4 2.6Pendi þ 1 HW 3.82 312 92 25.1 2.1LSD (0.05) 0.3 31 4 1.3 0.08P 4 F p 5 0.0001 p 5 0.0001 p 5 0.0001 p 5 0.0001 P 5 0.0001

Pendi, Pendimethalin; Bis, Bispyribac Na; HW, hand-weeding; P ¼ probability.



resulting in 81.3% and 91.3% reduction in dry matter accumulation by weedscompared to Pendimethalin þ Bispyribac Na and Pendimethalin þ 1 HWtreatments, respectively (Table 1). However, weed dry matter in Pendimethalin þBispyribac Na and Pendimethalin þ Bispyribac Na þ 1 HW treatments werefound to be statistically the same. The superiority of Pendimethalin þ BispyribacNa þ 1 HW over Pendimethalin þ Bispyribac Na was due to poor control ofDigitaria sanguinalis by Bispyribac Na in the Pendimethalin þ Bispyribac Natreatment; in the Pendimethalin þ Bispyribac Na þ 1 HW treatment, D. samgunalisnot controlled by Bispyribac were removed by hand-weeding at later stages. Theseresults are corroborating with the study of McDonald and Dernoeden (2006) whoreported that Bispyribac Na can selectively control annual grasses. Grain yield wasfound highest with Pendimethalin þ Bispyribac Na þ 1 HW (6.1 t ha71) and itincreased to the tune of 14.8% and 60% as compared to Pendimethalin þ BispyribacNa and Pendimethalin þ 1 HW, respectively (Table 2). This increase in yield resultedfrom effective control of weeds with Pendimethalin þ Bispyribac Na þ 1 HW, whichproduced more panicles/sqm and grains/panicle, and ultimately resulted inmore yield. It was noticed that with the application of Pendimethalin þ BispyribacNa þ 1 HW panicles/m2 and grains/panicle increased to the tune of 44.8% and21.7%, respectively, as compared to Pendimethalin þ 1 HW (Table 2). Earlier Risiet al. (2004) and Mahajan et al. (2009b) found more grain yield in rice due to bettercontrol of weeds with Bispyribac Na.

Interactive effect of weed control methods and N fertilizer rate on weeds abundance andgrain yield

The data on interactive effect of weed control method and N fertilizer rate onweed density in Figure 2 revealed that with increasing N supply from 120–180 kgha71, the weed density remained statistically same in Pendimethalin þ BispyribacNa þ 1 HW and Pendimethalin þ Bispyribac Na. However, with Pendimethalin þ1 HW, weed density increased with increasing N supply from 120 kg ha71 and itincreased to the extent of 110.2 and 151.3% at 150 and 180 kg N ha71, respectively.

Figure 2. Total weeds density (No. m72) in response to varying levels of N and weed controlmeasures.



The same trend was noticed with weed above ground dry matter (Figure 3). InPendimethalin þ 1 HW when the crop was supplemented with 150 kg N ha71, thedry matter production by weeds increased to the extent of 44.7% as compared to Nsupply of 120 kg ha71. With N application of 180 kg ha71, however, it increased by69.6% as compared to N supply of 120 kg ha71. Increasing N supply from 120–180 kg ha71in the Pendimethalin þ 1 HW treatment, however, did not cause anysignificant increase in yield, while in the Pendimethalin þ Bispyribac Na andPendimethalin þ Bispyribac Na þ 1 HW treatments, the responses were observedup to N application of 150 kg ha71 (Figure 4). This was due to poor weed control inPendimethalin þ 1 HW as a result increasing N supply favoured the crop weedcompetition towards weeds and ultimately no N response was observed athigher N doses. On the other hand, in Pendimethalin þ Bispyribac Na andPendimethalin þ Bispyribac Na þ 1 HW, due to good weed control higher N dosefavoured crop weed competition towards crop so registered higher yield with Nsupply of 150 kg ha71. The results reveal that in the presence of weeds, increased N

Figure 3. Total weeds dry matter (g m72) in response to varying levels of N and weed controlmeasures.

Figure 4. Grain yield (t ha71) in response to varying levels of N and weed control measures.



rate had a positive effect on weeds rather than on the crop. Our study supportsprevious findings indicating that many weeds are equally or more responsive thancrops to higher soil N levels (Lintell-Smith et al. 1992; Supasilapa et al. 1992;Blackshaw et al. 2003). Thus, indiscriminate N additions have the potential to benefitweeds at the expense of crop, if not controlled properly.

Water productivity

The data on water productivity presented in Figure 5 shows that in Pendimethalinþ 1HW, water productivity did not improve with increasing N supply from 120 kgha71. However, in Pendimethalin þ Bispyribac Na and Pendimethalin þ Bispyr-ibac Na þ 1 HW it increased with increasing N supply from 120–150 kg ha71.Water productivity increased from 0.46–0.6 kg m73 with increase in N supplyfrom 120–150 kg ha71 in Pendimethain þ Bispyribac Na þ 1 HW, while inPendimethalin þ Bispyribac Na it increased from 0.42–0.51 kg m73 as the Nincreased from 120–150 kg ha71, respectively. The highest water productivity wasfound in Pendimethalin þ Bispyribac Na þ 1 HW and the crop was supplementedwith 150 kg N ha71. This was due to better weed control and N-use efficiency bythe crop which caused more yield and ultimately resulted in higher waterproductivity. Zimdahl (1999) reported that competition for water is oftenconsidered to be the most important source of weed-crop competition and underintensive crop-weed competition water uptake by the weeds is more as compared tocrop. Likewise, in our study due to poor weed control in the Pendimethalin þ1 HW treatment; increasing the N rate did not cause any improvement in waterproductivity because water competition was favoured towards weeds as comparedto crop due to the more competitive nature of weeds.

Economic analysis

In all the weed control methods, the return was highest when the crop wassupplemented with 150 kg ha71 of N (Figure 6). Return was highest ($1083.4) whenthe crop was supplemented with 150 kg ha71 of N and the weeds were controlled

Figure 5. Irrigation water productivity (kg m73) in response to varying levels of N and weedcontrol measures (Vertical bars represent standard error).



with Pendimethalin þ Bispyribac Na þ 1 HW. In the Pendimethalin þ BispyribacNa treatment increasing N supply from 120–150 kg ha71 caused increase in netreturns from $691.4 to $919.4. Net return with Pendimethalin þ 1 HW was lower atall the levels of N as compared to other weed control methods. Higher grain yield inthe Pendimethalin þ Bispyribac Na þ 1 HW treatment due to effective control ofweeds resulted in more net returns.

Conclusion

Our study confirmed that grain yield, water productivity and net returns were higherwhen the crop was fertilized with 150 kg ha71 of N and weeds were controlled byPendimethalin þ Bispyribac Na þ 1 HW. We conclude that in north western IGP,and particularly in the Indian Punjab where the soil are coarse textured, use of higheramount of N fertilizer up to 150 kg ha71 has the potential to increase the yield of dryDSR. Higher N would also increase the degree of competition against weeds, if notcontrolled properly as was observed in the Pendimethalin þ 1 HW treatment. Morestudies will be required comparing varying rates of N to come up with robustfertilizer recommendations for dry DSR. The study also revealed that some weedspecies would be highly responsive to higher N rate, and this needs to be deepened.Greater understanding is required on the ecology of weeds in response to varyinglevels of N and weed control techniques so that reliance on herbicides could bereduced by following integrated weed management approaches. Therefore, adjust-ment of N fertilization together with identification of suitable herbicide moleculesand their doses and timing of applications should be considered an importantcomponent of long-term weed management and sustainable crop productionsystems. Information gained in the current study will be used to develop moreintegrated programs for weed management in dry DSR and to increase its yield,water productivity, and profitability.

A significant step toward improved weed management system for DSR followedby wheat (i.e. rice-wheat system; Timsina and Connor 2001) will be the synthesis of

Figure 6. Net returns (US$) in response to varying levels of N and weed control measures(Vertical bars represent standard error). 1 US$ ¼ Rs. 45 (Value adjusted to nearest US$),hand-weeding 2.7 US$ day71, Pendimethalin: 15.0 US$ ha71, Bispyribac Na: 25 US$ ha71,human labour charges (US$ 2.7/day) and tractor charges (US$ 6/h).



such findings and extrapolating the results with similar climate and soil environ-ments in Punjab and the IGP of South Asia. Simulation modelling could be a usefultool for evaluating options for improving sustainable N and weed managementpolicies and for extrapolation of field results, especially when crop, weeds, soil,weather and management data are available for calibration and validation ofexisting and appropriate simulation models of rice. Punjab Agricultural University(PAU) has skilled modellers to refine and use the existing models of rice. We arecurrently evaluating the CERES-Rice and ORYZA2000 models (Bouman et al.2001; Timsina and Humphreys 2006) by seeking assistance from other modellers ofPAU that will help us to extrapolate our findings to other similar environments ofinterest.

Acknowledgements

The authors are highly thankful to Dr D.E. Johnson, Sr. Scientist (Weed Science), IRRI, ThePhilippines, and Dr Pritpal Singh, Asst Statistician, PAU, for the useful comments for theimprovements of this article.

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Nitrogen and herbicides interactions in direct-seeded rice- Punjab, India

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