Interaction Effects Of Multiple-species Paddy Field Weeds On Rice Yield And The Eco-economic Thresholds

Rice is one of the main food crops in Shanghai.The high yield of rice is affected by the adverse effects of weeds in rice fields.High frequency of weeding activities and continuous use of highly selective herbicides cannot be done once and for all,but will destroy the soil structure.This leads to changes in the succession of weed community in the field,which in turn leads to a series of problems such as decreased soil fertility,enhanced resistance to weeds and loss of biodiversity.The problems of ecological environment and ecological economic benefits have become more and more concerned with the development of agricultural production,farmland weed control concept has gradually shifted from the traditional weeding standards of“weeds must be thoroughly cleaned in their early growth period”to comprehensive weed management mode of“focus on economic benefits and take ecological benefits into account”.Weed eco-economic threshold and control period scientifically answer the question of under what density and time should weeds be controlled from economic and ecological point of view.While improving weeds control effect and saving farmland management costs,weeding activities based on eco-economic thresholds and control period also provide an opportunity for the maintenance of species diversity and farmland ecosystem stability.Rice field weeds are not only diverse but also have complex and varied community structures.In this study,the mixed weed communities consisting of three dominant weeds in Shanghai direct seeding rice field were used to analyze the effects of compound weeds in paddy field on rice yield under different density and coexistence time,based on the theory and practice.The ecological economic threshold of weeds and the critical period of prevention are explored,and the control indicators that are more in line with the actual occurrence of weeds in rice fields are obtained,which provides a theoretical basis for scientifically improving the weed management system.The main findings of this research are as follows:1)When the direct-seeded rice was interfered by and competed with the mixed weeds,rice yield traits were severely affected by the density of mixed weeds and the coexistence with rice,and the number of effective panicles,the number of grains per panicle and the 1000-grain weight were significantly reduced.The yield loss rate was positively correlated with the density of mixed weeds and coexistence time,and there were significant differences in yield loss among treatments at different weeds density and coexistence time.2)There was a comprehensive effect on the effect of composite weeds density and coexistence time on rice yield.Regardless of weeds density,weeds coexisting with rice within 15 days had no significant effect on rice yield.After 15 days of coexistence,the rice yield reduction trend became more and more obvious.When coexisting for 30 days,rice yield had been significantly different from weed-free treatment;regardless of the coexistence time,the composite weeds had no significant effect on rice yield under low density(≤2 plants/m~2)treatment.After weeds density was higher than 2 plants/m~2,rice yield began to appear significant decrease.3)The effects of Echinochloa crusgali and Cyperus difformis on rice yield were mainly direct effects,and the influence degrees were different.The effect of Echinochloa crusgali on the yield loss rate of rice was even worse,which direct path coefficient was 0.6585,compared with Cyperus difformis increased by 20.2%.According to the results of curve fitting and regression analysis,the logarithmic model y=45.868ln x-39.380 can best fit the relationship between the mixed density of Echinochloa crusgali+Cyperus difformis and yield loss rate,and logarithmic model y=27.522ln x-73.590 can best fit the relationship between the coexistence time of Echinochloa crusgali+Cyperus difformis and yield loss rate.The economic infestation level for manual weeding was 6.60%under actual rice production situation,so the eco-conomic threshold for Echinochloa crusgali+Cyperus difformis was 2.72 plants/m~2,and the control period was 18.4 days.4)The effects of Leptochloa chinensis and Cyperus difformis on rice yield were mainly direct effects,and the influence degrees were different.The effect of Leptochloa chinensis on the yield loss rate of rice was even worse,which direct path coefficient was 0.6872,compared with Cyperus difformis increased by 24.5%.According to the results of curve fitting and regression analysis,the linear model y=4.905x-13.696 can best fit the relationship between the mixed density of Leptochloa chinensis+Cyperus difformis and yield loss rate,and logarithmic model y=13.839ln x-32.394 can best fit the relationship between the coexistence time of Leptochloa chinensis+Cyperus difformis and yield loss rate.The economic infestation level for manual weeding was 6.60%under actual rice production situation,so the eco-conomic threshold for Leptochloa chinensis+Cyperus difformis was 4.14 plants/m~2,and the control period was 16.7 days.In summary,combine the effect of density and coexistence time,mixed weeds in paddy field under low density and short coexistence time have little effect on rice yield,and weeding activities at this time are inefficient.Carrying out weeding activities when the density of mixed weeds is≥2 plants/m~2 and coexisting with rice for 15-30 days,can improve the weeds control effect,save the cost of weeding and avoid the occurrence of subsequent weeds hazard.This also helps to maintain the diversity of weed species in paddy fields and enhance the stability of paddy field ecosystems in response to environmental fluctuations.