Weed Control in Direct Seeded Rice Using New Herbicide Combination under Indian Tropical Condition

Direct seeded rice (DSR) is gaining momentum in India due to acute labour shortage during the peak period of transplanting and shortage of water. Weeds are the major biological constrain in DSR and its management contributes a major share in cost of production, as crop and weed emerge simultaneously and exerts competition right from the beginning of the crop. Field experiments were conducted in rabi 2013 and 2014 to study the effect of new herbicide combination, bispyribac sodium + metamifop 14% SE on weed control in DSR and their residual effect on succeeding greengram. Results revealed that the post-emergence (POE) application of herbicide combination, bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter recorded significantly lower total weed density (25.78 and 24.19 plants m -2 respectively, during 2013 and 2014), total weed biomass (24.89 g m -2 and 34.56 g m -2 respectively, during 2013 and 2014) and higher weed control efficiency 1-14, (80.07 and 81.68% respectively, during 2013 and 2014). Application of bispyribac sodium + metamifop 14% SE at 70 g ha -1 along with wetter (100 mL ha -1 ) recorded higher grain yield of 5676 and 6388 kg ha -1 respectively, during both the years. Weedy check recorded the lower grain yield and recorded the higher weed index of 51.83 and 52.85% respectively, during both the years. Succeeding crop of greengram was not affected by the residue of bispyribac sodium + metamifop 14% SE at all the tested doses.


INTRODUCTION
Rice (Oryza sativa L.) is the leading cereal crop of the world and more than half of the human race depend rice for their daily sustenance [1]. Globally, rice yield losses due to pests have been estimated to be 40% of which weeds caused highest yield loss of 32%. The worldwide estimated loss in rice yield from weeds is around 10% [2]. Though India has the largest rice growing area with 44.8 million hectares it stands second after China with respect to production, 106.0 mt in 2013-14. Currently, India produced rice not only to meet the domestic demand but also export to other countries. However, to meet the rapidly increasing population, projected to be 1.6 billion by 2050 calls for stepping up the current production of 106 mt of milled rice to 140 mt [3]. Transplanting is the traditional system of rice cultivation and it is in vogue in many rice growing areas. Such a rice production system requires large amount of water during puddling and transplanting [4]. In order to reduce the use of water, a new technique of crop establishment, direct seeding is now fast replacing traditional transplanting method in areas with good drainage and irrigation facilities [5].
Direct seeded rice cultivation is subjected to greater weed competition than transplanted rice because both weeds and crop seeds emerge at the same time and compete with each other for its existence. Crop competitiveness is the ability of the crop to produce desirable yields in the presence of weeds [6]. In tropics, average rice yield losses from weeds are 35%. Season-long term weed competition in DSR may cause yield reduction up to 80% [7]. Thus, an efficient and timely weed control is crucial for the success of DSR. DSR is successful only when there is good crop establishment as well as adequate weed control methods is available to keep the crop free from weeds [8]. Efficient, cost-effective and timely weed management options remain pivotal to making DSR profitable and commercially acceptable. Such a strategy should help to improve the yield and reduce the production costs as well as minimize the negative effects of weeds on the quality of the produce. Timely and effective weed control has a positive correlation with good crop stand and high grain yield in DSR. Manual weeding though effective have several limitations particularly during peak period which makes it further problematic. In hand weeding, it is difficult to differentiate and remove the grassy weeds especially Echinochloa crusgalli and Echinochloa colonum due to the phenotypical similarities between weeds and rice seedlings in the early stages. Herbicides are considered to be an alternative supplement to hand weeding. The use of herbicides offers selective control of weeds right from the beginning, giving the crop an advantage of good start and competitive superiority over weeds. Hence, chemical weed control in direct seeded rice has gained importance.
In India, the high cost and scarcity of labour have increased the use of herbicides for weed control in almost all crops [9]. In order to control weeds, farmers use both pre and post emergence herbicides [10]. Both pre and post emergence herbicides, if properly used, are quite effective in suppressing weeds in DSR [11]. To the best of our understanding, a very few studies in this line have been conducted in DSR grown in Western Zone of Tamil Nadu, India. Moreover, the rice herbicides presently used in Tamil Nadu are mainly pre-emergence therefore; weeds coming at later stages of crop growth are not controlled as effectively as the weeds at emergence stage. This situation warranted for initiating research efforts to evaluate and identify suitable postemergence herbicides. Continuous use of a single herbicide (pretilachlor) and indiscriminate use of herbicides may lead the buildup of herbicide resistance in weeds. Without any doubt, the development and availability of effective POE herbicides have encouraged farmers to try this new method of crop establishment (DSR) in Tamil Nadu. Currently available rice herbicide have a narrow spectrum of activity and their efficacy is further limited when they are used alone [12]. This rarely provides season long weed control [13]. Control of complex weed flora with a single POE application is really a diffcult task for the DSR farmers [14]. Therefore, the combined application of different herbicides with different mode of action is required for broad spectrum weed control in DSR and for delaying the development of herbicide resistance.
There is a need to focus attention on new herbicide combination to enhance the weed control efficiency, broadening the spectrum of weed control and reduce the cost of cultivation. With changing scenario of weed management, farmers need new herbicides or new herbicide combination with high efficacy, low phytotoxicity, no residual effect and cost effective. Hence the present work is intended to look out the broad spectrum weed control through new POE herbicide combination bispyribac sodium + metamifop 14% SE for weed control in DSR in Tamil Nadu. . The evaporation prevailing during the cropping period ranged from 2.4 to 9.2 mm.

Experimental Site and Initial Soil Characteristics
The soil was clay loam in texture with a pH of 7.4 and an organic matter content of 0.5% with low in available nitrogen (238 kg ha -1 ), medium in available phosphorus (16.8 kg ha -1 ) and high in available potassium (518 kg ha -1 ).

Experimental Design and Treatments
The experiment was conducted in randomized complete block design with 12 treatments and replicated thrice (Table 1). Herbicides included in the study were bispyribac-sodium, metamifop, almix, clincher and a combination of bispyribac sodium and metamifop. These herbicides were applied alone and with wetter as in Table 1.

Experimental Details, Selection of Cultivar and Sowing
In each year, rice (cv. ADT 43, a cultivar with the duration of 120 days) was seeded in the first week of September and harvested in last week of December. Manually operated rice drum seeder developed by Tamil Nadu Agricultural University, Coimbatore was used for sowing the seeds. The seeder has two wheels at both the ends. It drops the seeds at 30 cm apart in continuous row. At a time, eight rows of rice seeds were sown. A seed rate of 40 kg ha -1 was adopted. Before sowing the field was drained to saturated condition, to facilitate easy sowing and uniform establishment of seedlings. A thin film of water was maintained at the time of sowing. For the next 8-15 days, irrigation and drainage of water were alternated to facilitate aeration, adequate moisture for germination of seed and establishments of seedlings. Thereafter, the plots were irrigated to 2 cm depth uniformly in all the treatments after the appearance of hair line cracks, up to panicle initiation stage. After panicle initiation, the plots were irrigated to 5 cm depth on disappearance of ponded water. Irrigation was stopped 15 days prior to harvest.

Treatment Details
All tested herbicides were applied as POE on 10 to 15 DAS. Hand operated knapsack sprayer fitted with a flat fan type nozzle (WFN 40) was used for spraying the herbicides adopting a spray volume of 500 L ha -1 . The herbicides were sprayed by keeping a thin film of water in the field. The field was neither drained nor irrigated for 2 days after application of herbicides. The nontreated control plot was kept undisturbed for the entire cropping period. In the hand treated plot, two hand weedings were given on 25 and 45 DAS. Bispyribac sodium 10% SC 20 200 mL 10-15 DAS T 10 Metamifop 10% SE 50 500 mL 10-15 DAS T 11 Hand weeding twice on 25 and 45 DAS ---------T 12 Non-treated control ---------

Weed flora of the experimental field
To account the general weed flora of the experimental field, species wise observations were carried out at 20 and 40 days after herbicide spray (DAHS).

Weed density
The weed count was recorded species wise using 0.5 m x 0.5 m quadrant from four randomly fixed places in each plot and the weeds falling within the frames of the quadrant were counted and the mean values were expressed in number m -2 . The density of grasses, sedges and broad leaved weeds and also the total weeds were recorded at 20 and 40 days after herbicide application (DAHS) and expressed in number m -2 .

Weed dry weight
The weeds falling within the frames of the quadrant were collected, categorized into grasses, sedges and broadleaved weeds, first shade dried and later dried in hot-air oven at 80°C for 72 hrs. The dry weight of grasses, sedges and broadleaved weeds were recorded separately at 20 and 40 DAHS and expressed in g m -2 .

Weed control efficiency
Weed control efficiency (WCE) was calculated as per the procedure [15]. ) in control plot WD t -weed biomass (g m -2 ) in treated plot

Weed index
Weed index (WI) was calculated as per the method [16].
) from minimum weed competition plot Y = yield (kg ha -1 ) from the treatment plot for which WI is to be worked out.

Grain yield
Grains from each net plot were cleaned, sun dried, weighed and adjusted to 14% moisture content and the grain yield was expressed in kg ha -1 .

Residual Crop Cultivation
To study the residual effect of herbicides, the succeeding crop of greengram (cv. Co 6) was raised without disturbing the layout of the previous experiment. After the harvest of rice crop, greengram was dibbled in rice stubbles. A seed rate of 20 kg ha -1 was adopted for the greengram crop with a spacing of 30 cm x 10 cm.

Statistical Analysis
The data collected was statistically analyzed the following procedure for randomized block design [17]. The data pertaining to weeds were transformed to square root scale of ) 2 (X  and germination percentage was transformed to Arc sine and analysed [18]. Whenever significant difference existed, critical difference was constructed at five per cent probability level.

General Weed Flora of the Experimental Field
A critical analysis of relative proportion of grasses, sedges and broad leaved revealed that during the crop growth period, the population of sedges was higher than that of grasses and broad leaved weeds. Among the grasses, Echinochloa crus-galli (L.) Beauv., Echinochloa colona (L.) Link., Dinebra retroflexa (Vahl.) Panzer. and Panicum repens (L.) were the dominant species and major sedges were Cyperus difformis (L.), Cyperus irria (L.) and Fimbristylis miliacea (L.) Vahl. Among the broad leaved weeds, Marsilea quadrifoliata (Linn.), Ammania baccifera (L.) and Eclipta alba (L.) Hassk. were the dominant species. However, a species-wise result was given for the first five weeds only, as they were the predominant weeds in the experimental trial.

Echinochloa crus-galli
Weedy check plot recorded the higher population of E. crus-galli at 20 and 40 DAHS during both the years (20.36 and 29.45 plants m -1 ). During rabi, 2013 the lower density of E. crus-galli was observed in bispyribac sodium + metamifop treated plot at 70 g ha -1 with wetter (2.30 and 6.54 plants m -2 ) and was statistically similar to bispyribac sodium + metamifop 14 % SE at 70 g ha -1 without wetter (2.86 and 7.86 plants m -2 ) and bispyribac sodium + metamifop 14 % SE at 56 g ha -1 with wetter (3.86 and 7.55 plants m -2 ). POE application of herbicides, almix at 4 g ha -1 (5.63 and 11.19 plants m -2 ) and clincher at 80 g ha -1 (7.21 and 12.77 plants m -2 ) were found to be less effective in controlling E. crus-galli. During rabi 2014, POE application bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter (7.52 and 10.24 plants m -2 ) registered significantly lower density of E. crusgalli which was comparable to bispyribac sodium + metamifop 14% SE at 70 g ha -1 without wetter (7.82 and 13.26 plants m -2 ) and bispyribac sodium + metamifop 14% SE at 56 g ha -1 with wetter (10.76 and 16.78 plants m -2 ). It has also been observed that application of bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter did not allow later flush of E. crus-galli. Thus facilitate the rice crop to attain vigorous growth at the initial stage and in turn provided smothering effect at later stage of the crop. Early POE application of bispyribac sodium 10% SC at 20 g ha -1 was more effective for controlling the grassy weed density at critical stage of crop growth in DSR [19].

Dinebra retroflexa
The non-treated control plot recorded higher density of D. retroflexa at 20 and 40 DAHS (9.56 and 14.23 plants m -2 and 8.24 and 9.45 plants m -2 during 2013 and 2014, respectively). All the tested herbicide treatments reduced the density of D. retroflexa as compared to the non-treated control. During both the years of study, the lower density of D. retroflexa was observed in POE application of bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter and it was statistically similar to bispyribac sodium + metamifop 14% SE at 70 g ha -1 without wetter and bispyribac sodium + metamifop 14% SE at 56 g ha -1 with wetter (Table 2). POE application of bispyribac sodium 10% SC at 20 g ha -1 with wetter was more effective in reducing the density of D. retroflexa as compared to clincher at 80 g ha -1 . It was revealed from the result that all the tested doses of bispyribac sodium + metamifop 14 % SE were more effective in controlling D. retroflexa than the other tested herbicides

Panicum repens
The highest population of D. retroflexa was recorded at 20 and 40 DAHS in the control plot ( with wetter recorded lower density of P. repens and was significantly superior to other herbicidal treatments. The bispyribac sodium + metamifop 14% SE treated plot was almost weed free and did not allow the later flush of weed seedlings to grow due to vigorous growth of the crop. Post emergence application of clincher at 80 g ha -1 recorded lesser density of P. repens as compared to almix at 4 g ha -1 during both the years at both the stages of observation.

Cyperus difformis
C. difformis was one of the dominant sedge present in the experimental field. Different weed control treatments significantly influenced the density of C. difformis at all the stages. During rabi 2013, application of POE herbicide combination bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter recorded significantly lower density of C. difformi at 20 and 40 DAHS (Table 3) ) and metamifop 10% EC at 50 g (18.16 and 24.98 plants m -2 ) were ineffective against sedge weed control compared to herbicide combination. However, the combined application of bispyribac sodium + metamifop 14% SE with wetter at all doses were effectively controlled the sedges present in the experimental plots. The results also indicated the poor control of C. difformis by individual application of almix at 4 g ha Early POE application of bispyribac sodium 10% SC at 40 g ha -1 was more effective against C. rotundus as compared to pretilachlor S at 0.45 ha -1 followed by one hand weeding on 40 days after sowing [19].

Marsilea quadrifoliata
The density of M. quadrifoliata in the non-treated control plot was 17.52 and 32.45 plants m -2 during 2013 and 13.67 and 18.23 in 2014, respectively. All herbicide treatments reduced the density of M. quadrifoliata significantly as compared to the non-treated control ( with wetter registered better control of weeds compared to almix, clincher, bispyribac sodium and metamifop. From the study it was revealed that all the tested doses of bispyribac sodium + metamifop were more effective against grasses and sedges when compared to broad leaved weeds.

Total weed density
Significant variation in total weed density was observed among the herbicidal weed control treatments. During both the years, lesser total weed density was observed with POE application of bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter and bispyribac sodium + metamifop 14% SE at 70 g ha -1 without wetter and it was closely followed by application of bispyribac sodium + metamifop 14% SE at 56 g ha  (Table 3). Bispyribac sodium is pyrimidinyl carboxate group which inhibits the biosynthesis of amino acids. Metamifop is aryloxyphenoxy propionate group which inhibits the activity of acetyl coenzyme-A carboxylase (ACCase) leading to growth retardation of weeds. However, the combined application of both herbicides induces chlorosis selectively in weeds and insufficient chlorophyll production makes it difficult to thrive. The combined application of these herbicides was better than their individual application in reducing the weed density, weed biomass and enhancing the productivity of rice yield. Total weed density was higher in individual application as POE application of clincher at 80 g ha -1 when compared to almix at 4 g ha -1 and it was comparable during both the years of study. POE applications of clincher (alone) effectively control grassy weeds than compared to sedges and broad leaved weeds in the present study. Total weed density in weedy check were 105.20 and 156.13 plants m -2 during 2013; 85.93 and 1132.78 plants m -2 during 2014, respectively at 20 and 40 DAHS. All the herbicide treatments recorded significantly lower total weed density as compared to non-treated control. Sequential applications of pre and post-emergence herbicides provided better weed control than the sole application of pre or post-emergence herbicides in DSR [20].

Total weed biomass
With regard to the total weed biomass, significant variation was observed among the herbicidal weed management practices in DSR. During both the years, lower total weed biomass was observed in POE application of bispyribac sodium + metamifop 14% SE at 70 g ha  (Table 4). Herbicides differed in respect of their efficacy and bispyribac sodium emerged as promising one in averting both density and dry matter accumulation by weeds. The performance of this herbicide could be attributed to reasonable suppression of weeds and selectivity to rice crop as well. It is a member of pyrimidinyloxy benzoic chemical family, inhibits acetolactate synthase enzyme in susceptible plants and thus retarding the synthesis of branch chain amino acids [21].
The effectiveness of bispyribac sodium as a post-emergence herbicide for weed control in DSR was also reported elsewhere [22]. At 20 and 40 DAHS, POE application of bispyribac sodium 10% SC at 20 g ha -1 with wetter recorded lower weed biomass as compared with application of almix at 80 g ha respectively during 2013 and 2014, respectively at 20 and 40 DAHS. All the herbicide treatments recorded lower total weed biomass significantly as compared to the non-treated control.

Weed control efficiency
Adoption of herbicide combination of bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter treatment exhibited lowest weed infestation with higher weed control efficiency than sole herbicide application in the present study. During both the years, it was observed that POE application of herbicide combination bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter resulted the higher weed control efficiency of 87. 43

Response of grain yield
Rice grain yield ranged from 4276 to 5676 kg ha -1 and 4658 to 6388 kg ha -1 , respectively during 2013 and 2014 in herbicide treated plots, while the non-treated control plots recorded the yield of 2734 and 3012 kg ha -1 , respectively during 2013 and 2014 (Table 4). Higher grain yield was recorded in the plots treated with new combination herbicide, bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter (5676 and 6388 kg ha -1 , respectively during 2013 and 2014) and it was statistically comparable with plots treated with the application of bispyribac sodium + metamifop 14% SE at 70 g ha -1 without wetter (5488 and 6232 kg ha -1 , respectively during 2013 and 2014), bispyribac sodium 10% SC at 20 g ha -1 with wetter (5442 and 6076 kg ha -1 , respectively during 2013 and 2014) and hand weeding twice (5256 and 5908 kg ha -1 ,respectively during 2013 and 2014). Higher grain yield in response to efficient weed control are reported elsewhere [23,24]. In both the years, grain yield recorded in the plots treated with almix at 4 g ha -1 (4948 and 5792 kg ha -1 respectively, during 2013 and 2014) and clincher at 80 g ha -1 (4404 and 5248 kg ha -1 respectively, during 2013 and 2014) were statistically similar, but lower than grain yield recorded in the bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter. In direct seeded rice, combined application of bispyribac sodium + metamifop 14% SE with wetter as a postemergence herbicide provide broad spectrum weed [25].

Weed index
The best treatment with the maximum yield was taken as the base to work out the weed index that gives the magnitude of yield reduction due to weed competition in other treatments. New herbicide combination, bispyribac sodium + metamifop 14% SE at 70 g registered maximum grain yield and it was taken as the weed free plot for calculating the weed index. Bispyribac sodium + metamifop 14% SE at 70 g ha -1 without wetter recorded the weed index of 3.31 and 2.44% respectively during 2013 and 2014 ( Table 4). The yield reduction in the treatment bispyribac sodium 10% SC at 20 g ha -1 with wetter and bispyribac sodium 10% SC at 20 g ha -1 were found to be 4. 12  with wetter recorded a weed index of 27.08 during 2014. Non-treated control plots recorded a weed index of 51.83 and 52.85%, respectively during 2013 and 2014. The higher weed index registered in non treated plot might be due to increased weed growth and reduced nutrient availability to the crop. These emphasize the importance of proper weed management for increasing dry matter production of rice with reduced weed indices, thereby increasing the crop growth and grain yield.

Effect on weeds
During both years of study, at 40 days after sowing (DAS), POE application of bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter was found significantly superior in reducing the total weed density in comparison to the other treatments. POE application of bispyribac sodium + metamifop 14% SE at 70 g ha -1 without wetter, bispyribac sodium 10% SC at 20 g ha and metamifop 10% EC at 50 g ha -1 with wetter were found on par with each other (Table  5). Non-treated control registered higher total weed density even in succeeding greengram crop.

Germination
Germination percentage of greengram indicated that there was no significant difference among the treatments (Table 5). It was clear that there was no residual toxicity due to the POE application of herbicide combination bispyribac sodium + metamifop 10% SE at 70, 56 and 42 g ha -1 with wetter during both the years of study.

Number of pods plant -1
Number of pods per plant of greengram showed no significant difference among the weed control treatments. The number of pods per plant in all the treatments was comparable to the observations in that of non-treated control during both the years of study. So, there was no residual toxicity due to new formulation of herbicide combination of POE application of bispyribac sodium 4% SE + metamifop 10% SE at 70, 56 and 42 g a.i. ha -1 + wetter at 100 mL ha -1 on the performance of the succeeding crop (Table 5).

Seed yield of greengram
Yield of greengram raised as succeeding crop showed no distinct variation due to POE application of bispyribac sodium + metamifop 14% SE at 70, 56 and 42 g ha -1 with wetter during both the years (Table 5).   with wetter was found to be safe on the succeeding greengram. This might be due to detoxification of herbicides in soil and the resulting degraded products may not adversely affect the growth and yield of the succeeding crop. The POE application bispyribac sodium + metamifop 14% SE at 70, 56 and 42 g ha -1 with wetter can be safely applied for weed control in DSR without any residual toxicity. However, the impact of continuous application of bispyribac sodium + metamifop 10% SE combination in clay loam soil needs to be investigated. Hence it can be concluded that POE application of bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter can keep the total weed density and weed biomass reasonably at lower level and enhance the productivity of DSR.

CONCLUSIONS
Herbicide combination, bispyribac sodium + metamifop 14% SE at 70 g ha -1 with wetter effectively control of Echinochloa crus-galli (L.) Beauv., Dinebra retroflexa (Vahl.) Panzer and Panicum repens L., among the grasses; Cyperus difformis L. and Cyperus irria L. among the sedges; Marsilea quadrifolia Linn and Ammania baccifera L. among broad leaved weeds with higher weed control efficiency. Hence it can be concluded from the study that POE application of herbicide combination bispyribac sodium + metamifop 14% SE with wetter effectively control all the three major group of weed and maintained a weed free period during the critical stages of crop growth and resulted in higher grain yield in DSR.