Study On Kernel Dehydration And Influencing Factors Of Spring Maize Grain In Cold Climatic Region Of Northeast Of China

Mechanical grain harvest is an important part of modern maize（Zea mays L.）production technology,and the moisture content of grain during harvest has become an important factor affecting the quality of mechanical grain harvesting.There have been many researches on the grain dehydration of maize,but little is known on the dehydration characteristics of maize under cold climatic conditions,which restricts the technology promotion of mechanical grain harvesting in cold climatic region of Northeast China.Experiments focusing on subjects of variety（2015-2017）,density（2017-2018）,sowing date（2017-2018）and film mulching（2018）,were carried out to continuously investigate grains dynamics of moisture content,of which the objectives of clarifying the characteristics of grain dehydration in field,clarifying key factors influencing grain dehydration.And combining with the characteristics of regional heat resources,realizing the resources allocation on maize varieties and regional heat in cold climatic region of Northeast China and providing theoretical basis and technical support for the application and promotion of mechanical grain harvesting technology.The research results mainly included the following four aspects:I.Research on the quality of mechanical grain harvestingQuality investigation of mechanical grain harvesting was conducted in 5 accumulation zones and 9 test sites in cold climatic region of Northeast China.The results showed that grain moisture content during the harvest period consistently sustained higher,ranging from 17.73%to 35.97%with an average of 29.45%.The broken rate of mechanical grain harvest was still higher,varying from 4.22%to 12.86%.Mostly,the broken rate of the maize varieties was over 5%at national standard,with the result that the quality of maize was low.Correlation analysis showed that grain moisture content during the maize harvest period had a significant positive relationship with the broken rate and impurity rate.The fitting equations were y=0.38x-2.5857,R2=0.259**;y=0.0054e^0.1667x,R2=0.181**respectively.The results predicted that the quality of mechanical grain harvesting were affected by both the grain moisture content and the harvest machinery.Compared to the harvest machinery,the impact of the grain moisture content was even greater.II.Analysis on characteristics of grain dehydrationObviously,the process of grain dehydration can be divided into two stages,fast dehydration and slow dehydration.The process of grain dehydration varies with different maize varieties in various maturing periods of different years.In fast dehydration stage,the average dehydration rates were 0.87%·d^-1and0.93%·d^-1 for the early-and middle-late maturing maize varieties,respectively in 2016,these numbers fell to0.61%·d^-1and 0.55%·d^-1 in 2017.In slow dehydration stage,the average dehydration rates were 0.05%·d^-1and 0.11%·d^-1 for the early-and middle-late maturing maize varieties,respectively in 2016,these numbers fell to 0.09%·d^-1and 0.16%·d^-1 in 2017.At the end of the test,the grain moisture content of early maturing varieties was close to the equilibrium moisture content,while the grain moisture content of middle-late maturing varieties was higher than the equilibrium moisture content.The grain moisture content of middle-late maturing varieties were 6.9-21.19%and 3.45-13.39%higher than the equilibrium moisture content respectively in 2016 and 2017.There was a significant difference in the grain dehydration rate between before and after physiological maturity.The grain dehydration rate varied from 0.38 to 1.08%·d^-1 before physiological maturity and from 0.01 to 0.25%·d^-1 after physiological maturity.There was a critical point of grain dehydration at the intersection of two dehydration stages.The grain dehydration rate decreased significantly as the critical point appeared,and the critical point was closely related to the first frost period.The research indicated that the critical point of grain dehydration for middle-late maturing varieties was 2.7days later than the first frost period.On the contrast,the critical point of grain dehydration for early maturing varieties was different,1-7days earlier than the first frost period.However,grain moisture content and temperature varied widely in different years when maize varieties reached the critical point of grain dehydration.To be specific,grain moisture content varied from 16.6 to 41.8%（2016）and from 15.4 to 34.9%（2017）,and the temperature ranged from 6.2 to 14.9℃（2016）and from 4.4 to 8.0℃（2017）.Ⅲ.The influence factors of grain dehydrationThe grain dehydration was influenced by the maize ear traits,cultivation measures and meteorological factors.There was a significant positive correlation between the moisture content of bracts,cobs and stalk and that of grain（r=0.844**,r=0.745**,r=0.815**）.A significant positive correlation was found between the grain moisture content and daily temperature（average,maximum and minimum）,growing degree,precipitation and wind speed（average and maximum）,relative humidity,solar radiation and evapotranspiration.Among them,daily growth degree and average temperature,relative humility and evapotranspiration had a greater influence on the grain dehydration of maize.Notably,the results showed that the determination coefficient of daily growth degree was 0.630.Temperature and relative humidity were two key meteorological indicators which had a great effect on the grain dehydration of maize.The process of maize growth and development was going to happen earlier than would have happened through adopting early sowing,film mulching and other cultivation measures.Meanwhile,reasonable cultivation measures increased the utilization efficiency of light and heat resources in growth period,raised the yield of maize,prolonged the time of grains dehydration,and reduced the grain moisture content at harvest.Results indicated that the grain moisture content of early sowing was 7.37%lower in average than late sowing at harvest.The grain moisture content of film-mulching was 3.09%lower in average than that of bare planting.Ⅳ.The distribution of heat resource and configuration of maize varietyAccording to grain dehydration dynamic,the model MC=90/[1+（T/b）^c]was developed based on the relationship between the thermal time（T）from silking and the grain moisture content（MC）,which could accurately predict the moisture content.Based on the accumulated temperature requirement of the tested varieties,the early-maturing varieties had dropped below the target moisture content of 25%on mechanical grain harvesting before physiological maturity,while the accumulated temperature of the middle-late maturing varieties needed 279.7℃·d to reach the target moisture content of 25%on mechanical grain harvesting.It can be seen that the lack of available accumulated temperature after physiological maturity is the main reason for the high moisture content of grains in the north of Northeast China during harvest period.According to the available accumulated temperature（>0℃）in the growing season,the cold climatic region of Northeast China could be divided into seven accumulative temperate zones（ATZ1-ATZ7）,with the accumulated temperature ranging from 2000℃·d to 3400℃·d.The region could be divided into four zones（ATZ1-ATZ4）,thus optimizing the cultivars’lay-out and maize varieties matching for achieving mechanical grain harvesting on a strategy.Conclusively,the above findings would provide theoretical foundation and technical support for mechanical grain harvesting in cold climatic region of Northeast China.