Germination ecology and management of chinese sprangletop [Leptochloa chinensis (L.) Nees.] in wet seeded rice

Abstract

An investigation entitled ‘Germination ecology and management of Chinese sprangletop [Leptochloa chinensis (L.) Nees.] in wet seeded rice’ was undertaken at the College of Agriculture, Vellayani during 2017-2020. The objectives were to study the habitat, germination ecology and distribution of Leptochloa chinensis in major rice growing tracts of Kerala; to test the efficacy of tank mix combinations of herbicides for the management of the weed; to test the sensitivity of the weed to herbicide combinations and to assess the mode of action of the herbicide combinations. The phytosociological survey was conducted to document the habitat, composition and distribution of L. chinensis in different rice tracts of Kerala viz., Palakkad, Kole and Kuttanad after selecting three severely infested padasekharams in each tract during 2018 and 2019. L. chinensis was found to occur in all the major rice growing tracts of Kerala and registered summed dominance ratio of 13.05, 12.40 and 17.49, respectively in Kuttanad, Kole and Palakkad (Kharif). Appraisal of weed vegetation analysis indices displayed the highest weed species richness (17) and Simpson’s diversity index in Kole and the lowest Shannon Wiener diversity index (2.09) in Palakkad. L. chinensis was the dominant weed in all these tracts with an abundance of 17.33, 17.0 and 16.36, respectively and the weed inhabited both upland and lowland situations, either in crop lands, field bunds, stream banks or waterways. Profuse growth of the weed was observed along the inner bunds separating individual fields. The weed was a prolific seed producer with seed production potential ranging from 7400-33,941 seeds per plant across the surveyed locations. Germination ecology experiments encompassed studies on weed phenology and germination of Chinese sprangletop. L. chinensis is an erect or creeping, annual or perennial grass that can grow upto a height of 120-150 cm, propagates both by seed and slips with very minute seeds (thousand seed weight of 0.10-0.18 g). The weed was noticed to develop through five phenological stages, viz., emergence, tillering, heading, flowering and maturity with an average duration of 10.6, 41.5, 73.5, 78.5 and 95 days, respectively. Investigations on germination ecology revealed that light was not an absolute requirement for germination of seeds of L. chinensis, but stimulated germination by 23 per cent. When exposed to alternating temperatures in light/dark, seeds germinated at 15°C to 35°C. The highest germination occurred at 25/15°C (87.2%), while at 35/25°C it was only 70.31 per cent. Germination of L. chinensis was significantly influenced by moisture regime; with zero germination under continuous flooding or with thin layer of water (3 cm) and 70 per cent germination on irrigating at alternate days. Seedling emergence was also significantly affected by seed burial depth. Seedling emergence was high (85%) for seeds placed on the soil surface, while no emergence was observed at burial depths of 2 cm or beyond. Slips placed at the surface recorded 100 per cent sprouting and the time taken for 50 per cent emergence increased with increase in burial depth. The seeds germinated upto nine months after harvest with the germination declining over time. The field experiments on management of L. chinensis were conducted during 2018 and 2019 Kharif at Integrated Farming System Research Station, Karamana. The experiment was laid out in randomized block design with 10 treatments and three replications. The treatments included T1: cyhalofop butyl @ 0.08 kg ha-1 , T2: penoxsulam + cyhalofop butyl (6% OD) @ 0.15 kg ha-1 , T3: cyhalofop butyl + carfentrazone ethyl @ 0.08 + 0.02 kg ha-1 , T4: bispyribac sodium @ 0.025 kg ha-1 , T5: bispyribac sodium + cyhalofop butyl @ 0.025 + 0.08 kg ha-1 , T6: bispyribac sodium + fenoxaprop-p-ethyl @ 0.025 + 0.06 kg ha-1 , T7: fenoxaprop-p-ethyl @ 0.06 kg ha-1 , T8: stale seedbed followed by (fb) glyphosate + oxyfluorfen @ 0.8 + 0.15 kg ha-1 at 15-20 days after land preparation fb cyhalofop butyl + carfentrazone ethyl @ 0.08 + 0.02 kg ha-1 , T9: unweeded control and T10: hand weeding twice at 20 and 45 days after sowing (DAS). The weed management practices had significant influence on L. chinensis count, dry matter production, control efficiency and nutrient removal at different stages of observation. Count of L. chinensis was zero in T6, T7, T8 and T10 at 15 days after treatment application (DATA) during both the years. The lowest count and dry matter production and the highest pooled control efficiency of L. chinensis (97.55 and 96.78 per cent, respectively at 30 and 45 DATA) was registered in T7. It was statistically on par with all other treatments except T4 and T9. Bispyribac sodium was not effective in controlling L. chinensis and resulted in lower control efficiency of 52.63, 38.68 and 42.43 per cent, respectively at 15, 30 and 45 DATA. However, its combination with fenoxaprop-p-ethyl (T6) or cyhalofop butyl (T5) resulted in higher control efficiency of L. chinensis at all stages of observation and recorded 100, 96.06 and 95.96 and 97.23, 92.37 and 94.55 per cent, respectively at 15, 30 and 45 DATA. Among the herbicide treatments, T6 registered the least total weed dry matter production at all stages and resulted in the highest pooled weed control efficiency of 98.20 and 89.70 per cent which was on par with T2 (96.61 and 83.86%) and T5 (95.40 and 84.81%) respectively at 30 and 45 DATA. The highest pooled grain yield (5.03 t ha-1 ) also was registered in T6 which was on par with T2 and T5 with 4.79 and 4.76 t ha-1 . Pooled data revealed that season long weed competition in wet seeded rice (WSR) with L. chinensis as a major weed caused a yield reduction of 59.95 per cent. Compared to the unweeded control, herbicidal treatments enhanced grain yield by 23.89-58.84 per cent, whereas herbicide combinations increased grain yield by 56-59 per cent in WSR. Pooled mean of the economics of cultivation registered maximum net returns per hectare (₹ 63,657 ha-1 ) and B:C ratio (1.81) in T6 followed by T2 (₹ 56,995 ha-1 and 1.73) and T5 (₹ 56,044 ha-1 and 1.72). The sensitivity of L. chinensis to herbicide combinations was tested at the field recommended dose and its lower doses using whole plant bioassay technique after identifying best combinations viz., bispyribac sodium + fenoxaprop-p-ethyl @ 0.025 + 0.06 kg ha-1 , bispyribac sodium + cyhalofop butyl @ 0.025 + 0.08 kg ha-1 , penoxsulam + cyhalofop butyl @ 0.15 kg ha-1 and cyhalofop butyl + carfentrazone ethyl @ 0.08 + 0.02 kg ha-1 . The experiment was laid out in completely randomized design (CRD) with eight treatments and three replications. Lower dose of bispyribac sodium + fenoxaprop-p-ethyl @ 0.020 + 0.04 kg ha-1 recorded the least survival (0%) of L. chinensis whereas, bispyribac sodium + cyhalofop butyl @ 0.020 + 0.06 kg ha-1 , cyhalofop butyl + carfentrazone ethyl @ 0.06 + 0.01 kg ha-1 and penoxsulam + cyhalofop butyl @ 0.10 kg ha-1 registered 26.66, 30.0 and 86.66 per cent survival, respectively. The experiment on mode of action of tank mix herbicide combination was laid out in CRD with four treatments and five replications. The treatments included T1: ALS inhibitor alone (bispyribac sodium @ 0.025 kg ha-1 ), T2: ACCase inhibitor alone (fenoxaprop-p-ethyl @ 0.06 kg ha-1 ), T3: ALS + ACCase inhibitor (bispyribac sodium @ 0.025 kg ha-1 + fenoxaprop-p-ethyl @ 0.06 kg ha-1 ) and T4: Control. Treatment T3 recorded lower amino acid content (0.1775 mg mL-1 ) than T1 (0.2904 mg mL-1 ) confirming that the combined application of an ALS + ACCase inhibitor could provide better control of L. chinensis compared to sole application of ALS inhibitor. The experiment on assessing the differential response of L. chinensis and Echinochloa colona to the broad-spectrum herbicide bispyribac sodium, revealed that amino acid content of L. chinensis was not influenced by the increasing concentration of bispyribac sodium. High content of amino acid was registered in L. chinensis (0.2904 and 0.3234 mg mL-1 ) compared to E. colona irrespective of the concentration of bispyribac sodium. However, in E. colona, the amino acid content (0.1520 and 0.0627 mg mL-1 ) was found to decrease with increasing concentration of bispyribac sodium. The present study identified L. chinensis as a major weed in all the major rice growing tracts of Kerala indicating its invasive potential under diverse environmental conditions owing to its prolific seed production, sprouting from weed slips on soil surface, extended period of seed viability and different mode of propagation. The results revealed that early and continuous flooding, deep tillage for burial of seeds and slips into the soil beyond 5 cm could suppress its emergence. The study identified fenoxaprop-p-ethyl @ 0.06 kg ha-1 as the most effective herbicide against L. chinensis to be sprayed at 15-18 DAS in WSR. In areas where L. chinensis is a dominant weed in the WSR, tank mix application of bispyribac sodium @ 0.025 kg ha-1 + fenoxapropp-ethyl @ 0.06 kg ha-1 at 15-18 DAS could be recommended for broad spectrum weed management. Whole plant bioassay of the above combination proved effective at its still lower dose and the differential response of L. chinensis to bispyribac sodium indicated herbicide combinations for managing the complex spectrum of weeds in wet seeded rice.