Non-chemical weed management in organic rice

Field experiment was conducted at Tamil Nadu Agricultural University, Coimbatore during Rabi 20122013 (October-February) to identify the ideal weed management technique in organic rice production. The experiment was laid out in randomized block design with three replications. The treatments comprised of fourteen different weed management practices. The results revealed that application of rice bran at 2 t ha on 3 DAT followed by a hand weeding on 35 DAT kept the total weed density (15.00 No.m) and weed dry weight (3.38 g m) below the economic threshold level and also increased the yield (4816 kg ha) of organic rice production.


INTRODUCTION
Organic weed control encourages weed suppression rather than elimination.This is done by promoting soil health through a combination of crop rotation, cover crops, biologically based bio-fertilizers, compost and mulch.Proper management through organic methods offer varied benefits over chemical herbicides, including increased biodiversity, improved soil nutrition and structure, and protection of ground and surface water.Surender-Rao and Sitaramayya (1997) reported that growing of Azolla as a dual crop with rice resulted in significantly more tillers, longer panicles and more spikelets.Solaiappan and Veerabadran (1997) have also reported that the green manure intercrops had no effect on the yield of associate kharif rice but the subsequent rabi rice yield got increased.Use of rice bran, which contributes nutrient after decomposition and suppresses weeds in rice fields (Yan et al., 2007).The higher mineral nitrogen concentration in top soil at early growth stage of rice, which was induced by rice bran applied at 7 DAT for weed suppression, brought about higher grain yield of rice as reported by Yan et al. (2007).The present study aimed to confirm this possibility in weed control and evaluate the influence on yield performance in organic rice farming system employing non-chemical weed management practices.

MATERIALS AND METHODS
Field experiment was conducted at Tamil Nadu Agricultural University, Coimbatore during Rabi 2012-13 (October-February) to identify the ideal weed management technique in organic rice production.The experiment was laid out in randomized block design with three replications ).The rice variety Co (R) 48 was used as test cultivar sown on 5 th October 2012 at 30 × 10 cm.Weed observation like weed flora, weed density and dry weight taken at 20, 30 and 50 DAT.The number of total tillers, productive tillers, number of panicles per hill, panicle length and panicle weight were counted from the five tagged plants and mean value was calculated.Grain from each net plot was cleaned, sun dried and weighed at 14% moisture content and the grain yield was expressed in kg ha -1 .The straw obtained from each net plot area was sun dried and weighed.The straw yield was expressed in kg ha -1 .The data were statistically analyzed by the analysis of variance method as suggested by Gomez and Gomez (2010).Wherever the treatment differences were found significant, critical differences were worked out at 5% probability level and the values are furnished.

Weed occurrence
In the experimental field, all the three groups of weeds consisted of two grasses, two sedges and three broad leaved weeds were present.).Sedge weed density (Figure 2) was distinctly lower (6.00 m -2 ) in hand weeding twice (T 13 ) at 20 DAT.Azolla as dual crop with rice and cono weeder incorporation twice (T 4 ) and Azolla as dual crop with rice and rotary weeder incorporation twice (T 3 ) recorded significantly lower (6.33 and 4.00 m -2 ) density of sedges.In respect of the sedge weed density, the result was significant at all the stages of observation.The distribution of sedges, C. difformis and C. iria, was very high in the total weed density in all the three stages (20, 30 and 50 DAT).BLW density was distinctly lower of 3.00 and 5.00 No.m -2 (Figure 3) in application of rice bran at 2 t ha -1 fb hand weeding dual crop with rice and cono weeder incorporation twice (T 4 ).Among different weed management practices the total weed density was lower in application of rice bran at 2 t ha - 1 on 3 DAT followed by hand weeding on 35 DAT methods of weed control significantly (Table 2) reduced the weed density over unweeded control.Rice byproducts meal was also found to be quite suppressive to annual weed growth (Kuk et al., 2001).Grasses, sedges and BLW density was distinctly lower (4.67,4.00 and 5.00 m -2 ) in application of rice bran 2 t ha -1 fb one hand weeding at 50 DAT (T 12 ).Distinct reduction of total weed density by application of rice bran at 2 t ha -1 3 DAT + HW on 35 DAT might be due to the control of weeds at the germination phase and significant reduction at later stages as late germinating weeds were controlled by one hand weeding at 35 DAT.The suppressive effect of rice bran application to soil surface after transplanting on paddy weed was considered to be associated with a decline in redox potential and dissolved oxygen concentration was reported by Muroi et al. (2005) and Nakayama et al. (2002).Similar observation was reported by Kim et al. (2001) who found that rice bran application had little suppression effect on E. crus-galli, but suppressed M. vaginalis, Scirupus juncoides, and Cyperus spp to a substantial degree.Total weed density was distinctly higher (57.33, 82.00 and 102.33 No. m -2 ) in unweeded control (T 14 ) at 20. 30 and 50 DAT.Obviously unweeded control (T 14 ) resulted in higher grasses, sedges and broad-leaved weeds density due to unchecked and increased weed growth at all the growth stages of the crop.

Total weed dry weight
Data pertaining to the application of rice bran at 2 t ha -1 fb hand weeding (T 12 ) recorded distinctly lower grasses, sedges and BLW dry weight of 1.13, 1.12 and 1.13 g m -2 at 50 DAT respectively.As shown in Table 3. Considerable reduction in total weed dry weight was recorded lower in application of rice bran at 2 t ha -1 on 3 DAT + HW on 35 DAT (3.92, 5.34 and 3.38 g m -2 ) at 20, 30 and 50 DAT.The application of rice bran significantly decreased the total weed density and weed dry weight as compared with weedy control was reported by Khan et al. (2007).This is next to rice bran application Azolla as dual crop with rice and cono weeder incorporation twice 20 and 40 DAT at 30 DAT might be attributed to the minimum number of total weeds with lesser biomass in the cropping period.Weed dry weight was reduced due to the efficient weed control and lesser weed population as compared to other treatments.

Weed control efficiency
Weed control efficiency (WCE) indicates the magnitude of effective reduction of weed dry weight by weed control treatments over weedy check.This was highly influenced by different weed control treatments.Application of rice bran at 2 t ha -1 3 DAT + HW on 35 DAT recorded higher weed control efficiency of 83.0, 91.4 and 96.3%, respectively at 20, 30 and 50 DAT and it was on par with Azolla as dual crop with rice and cono weeder incorporation twice on 20 and 40 DAT More reduction of weed dry weight by reducing the weed density in these treatments resulted in higher Weed Control Efficiency.

Yield parameters
Application of rice bran at 2 t ha -1 3 DAT + HW on 35 DAT recorded significantly high values of yield components viz., number of productive tillers (192.2 m -2 ), panicle length (20.3 cm), panicle weight (3.10 g) and number of filled grains (110.8) which was in conformity with Kato et al. (2010).
The possible reason might be that the weed free situation at early stage favoured the vigorous growth of seedlings without crop weed competition due to prolonged control of weeds.The competition free environment has increased the capacity of source (LAI) and sink and in turn the length of panicle, panicle weight and number of filled grain panicle -1 of rice (Table 4).

Yield
Significantly higher grain yield of rice (4816 kg ha -1 ) was recorded with the application of rice bran at 2 t ha -1 on 3 DAT and hand weeding on 35 DAT which was in conformity with the findings of Yan et al. (2007) and Kato et al. (2010).The favourable conditions created through the efficient weed control resulted in lesser weed competition between crop and weeds.This favoured the crop to produce more leaf area and plant dry matter production.The increase in number of productive tillers, panicle length and number of filled grains panicle -1 resulted in the higher grain yield in application of rice bran at 2 t ha -1 on 3 DAT and hand weeding on 35 DAT (Table 5).Rice bran application under deep flooding significantly increased both spikelet number per panicle and panicle number, leading to substantial increase in total spikelet number per unit area and grain yield as compared to deep flooding with no rice bran reported by Yan et al. (2007).The straw yield was recorded 7108 kg ha -1 with the application of rice bran at 2 t ha -1 on 3 DAT + HW on 35 DAT might be due to efficient weed control, high nutrient uptake by the crop and increased dry matter production.Distinctly lower grain and straw yields were recorded in unweeded control.This was due to severe competition between crop and weed for different resources viz., light, moisture, space and nutrients (Table 1).

Economics
The mean data on cost of cultivation (Rs.ha -1 ), gross return (Rs.ha -1 ), net return (Rs.ha -1 ) and benefit-cost (B:C) ratio is presented in Table 6.Lower cost of cultivation was recorded in cono weeder four times (Rs.31,825 ha -1 ), rotary weeder four times (Rs.31,825 ha -1 ).The moderate cost of cultivation was recorded in application of rice bran 2 t ha -1 fb hand weeding and higher cost of cultivation was registered with Azolla as dual crop with rice and manual incorporation twice (Rs.37,980 ha -1 ) and was followed by Azolla as dual crop with rice and rotary weeder incorporation twice (Rs.36,165 ha -1 ), Azolla as dual crop with rice and cono weeder incorporation twice,(Rs.36,165 ha -1 ) and hand weeding twice (Rs.35,455 ha -1 ).Azolla as dual crop with rice and manual incorporation on 20 and 40 DAT recorded higher cost of cultivation which might be due to application of Azolla and high labour requirement for manual incorporation.The net return was markedly higher in application of rice bran at 2t ha -1 fb hand weeding (Rs.49,862 ha -1 ).Application of rice bran at 2 t ha -1 on 3 DAT and hand weeding on 35 DAT increased the grain and straw yield and consequently enhanced the net income.The result is in line with the findings of Yan et al. (2007) and was lower net return was recorded in unweeded control (Rs.15, 191 ha -1 ).The B:C ratio was higher in application of rice bran at 2 t ha -1 fb hand weeding (2.45) and lower B:C ratio was registered under unweeded control.

Conclusion
This study was concluded that organic rice responded favourably to non-chemical weed management practices.Application of rice bran at 2 t ha -1 on 3 DAT followed by hand weeding on 35 DAT positively influenced the growth and yield parameters along with nutrient uptake.     .
Rice bran application at 3 DAT for weed suppression significantly increased mineral nitrogen concentration in the top soil during tillering stage, providing much more available nitrogen for rice growth.It also increased the both spikelet number per panicle and panicle number, leading to substantial increase in total spikelet number per unit area and grain yield.Hence, the non-chemical weed management practice of application of rice bran at 2 t ha -1 on 3 DAT followed by hand weeding on 35 DAT in rice has been found to be an ideal option to control weeds and also improve yield of rice besides being economically competitive and productive under organically grown situations.

Figure 1 .
Fig 1.Effect of non-chemical weed management practices on grasses weed denisty

Figure 2 .Figure 3 .
Figure 2. Effect of non-chemical weed management practices on sedges weed density.

Table 1 .
Properties and composition of grain.

Table 2 .
Weed flora of the experimental field.

Table 3 .
Effect of non-chemical weed management practices on total weed density (No.m -2 ) at 20, 30 and 50 DAT in organic rice production.

Table 4 .
Effect of non-chemical weed management practices on total weed dry weight (g m -2 ) at 20, 30 and 50 DAT in organic rice production.
Figures in parenthesis are original value, HW: Hand weeding.

Table 5 .
Effect of non-chemical weed management practices on yield parameters of organic rice production.

Table 6 .
Effect of weed management practices on economics of organic rice production.