| Soil seedbank is the main source of weed infestation.This study sampled the representative direct-seeding rice(DSR)fields in the middle and lower reaches of the Yangtze River,compared species composition and vertical distribution of weed seedbanks under different rice planting systems including dry direct-seeding rice(DDSR),water direct-seeding rice(WDSR)and machine transplanting rice(MTR);determined characteristics of weed seedbanks of DSR fields.Leptochloa chinensis is a serious weed species in DSR fields.This study surveyed occurring patterns of Leptochloa chinensis,and explored integrated management strategies of this weed,referring to convenient and simple agronomic methods,non-chemical methods and chemical herbicides.Main results are:In 12 sites of rice fields with DDSR system sampled,seeds of 37 rice’s companion weed species referring to 16 families were observed,and the average weed seed density was 1.413×105 seeds/m2;among these 37 weed species,Cyperus difformis held the largest weed seedbank,followed by Ludwigia prostrate,Lindernia procumbens,Leptochloa chinensis,Fimbristylis dichotoma,Monochoria vaginalis,Ammannia baccifera,Digitaria sanguinalis,and so on.In eight sites of rice fields with WDSR system sampled,seeds of 30 rice’s companion weed species referring to 13 families were observed;and the average weed seed density was 1.182×105 seeds/m2;among these 30 weed species,Mazus japonicas held the largest weed seedbank,followed by Cyperus diformis,Leptochloa chinensis,Lindernia procumbens,Monochoria vaginalis,Ammannia baccifera,Cyperus iria,and so on.In 12 sites of rice fields with MTR system sampled,seeds of 33 rice’s companion weed species referring to 11 families were observed,and the average weed seed density was 1.063×105 seeds/m2;among these 33 weed species,Leptochloa chinensis held the largest weed seedbank,followed by Cyperus difformis,Mazus japonicas,Lindernia procumbens,Monochoria vaginalis,Rotala indica,Ammannia baccifera,Echinochloa crusgalli,and so on.Moreover,weed seeds in rice fields mainly distributed in the soil within a depth of 0-10 cm,and seed abundance decreased significantly with.the increase of soil depth.In DDSR,81.77%seeds distributed in 0-10 cm soil(out of 0-20 cm soil);in WDSR and MTR,the percentage was 74.56%and 81.48%,respectively.The rank of Leptochloa chinensis occurrence in rice fields with different planting system was:DDSR>WDSR>MTR.The rank of Leptochloa chinensis occurrence in rice fields with different tillage system was:non-tillage>shallow tillage>deep tillage.Leptochloa chinensis occurrence in rice fields with straw incorporation was significantly lower than those without straw incorporation.The rank of Leptochloa chinensis occurrence in rice fields with different rotation system was:oilseed rape-rice rotation>wheat-rice rotation>rice(one growing season per year).In DDSR fields,ca.5 d after sowing was the critical period for Leptochloa chinensis management with pre-emergence herbicides and 15-20 d after sowing was the critical period for its management with post-emergence herbicide.In WDSR fields,<5 d after sowing was the critical period for Leptochloa chinensis management with pre-emergence herbicides and ca.20 d after sowing was the critical period for its management with post-emergence herbicide.The period from June to July was critical period for Leptochloa chinensis management in DSR fields,and Leptochloa chinensis seedlings emerged in August and later failed to set seed.Among the 12 rice varieties tested,Yangjingnuol,Wuyunjing 27 and Yangdao 6 showed high ability to inhibit the germination and seedling growth of Leptochloa chinensis.Water irrigation with a depth not submerging young leave of rice 6 d after sowing showed high efficacy of Leptochloa chinensis control.Fertilization right after rice sowing showed low efficacy of Leptochloa chinensis control,while applying tillering fertilizer 30 d after sowing may contribute to its control for improving the competitiveness of rice.Allelopathic effects of straws from 5 oilseed rape varieties and 5 wheat varieties against Leptochloa chinensis were investigated,respectively.By coverring straws of oilseed rage or wheat simulating on-field environment,we found that oilseed rape variety”Shilifeng" and wheat variety "Zhengmai 9023”had higher inhibitory effect on the germination and growth of the Leptochloa chinensis.Hence,in WDSR fields,we can use the previous crop straws coverage to control Leptochloa chinensis.When the amount of residual straw of "Zhengmai 9023" and "Shi Lifeng" reached 3.9 t/ha.,the germination and seedling growth of Leptochloa chinensis were strong inhibited.Hence in DDSR fields,wheat and oilseed rape residue could be covered on soil for Leptochloa chinensis control.Whole-plant bioassay of 15 herbicides on the efficacy of Leptochloa chinensis suggest that pre-emergence herbicides including triafamone,pretilachlor,oxaziclomefone,pendimethalin and oxadiazon,and post-emergence herbicides including metamifop,cyhalofop butyl,bispyribac-sodium,fenoxaprop-p-ethyl,fluazifop-p-butyl and molinate hold high efficacy on this weed species.To broaden spectrum,improve efficacy and reduce costs of chemical control of weeds in DSR fields,we tested the combination effect of 3 herbicides(cyhalofop,bispyribac and pyrazosulfuron).We found that the optimum formulation of these 3 herbicides was:cyhalofop:bispyribac:pyrazosulfuron(10:4:1).Applying the above herbicide mixture when grasses on 3-5-leaf stage,the safety index of this herbicide agent on japonica rice was 4.83,and the safety index of indica rice was 10.82. |
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