| Mechanical grain harvesting of maize(Zea mays L.)is the inevitable direction of the development of harvesting technology,and it is also the key to the realization of full mechanization.At present,the general situation is the lack of both maize varieties that are suitable for mechanical harvesting and dedicated harvesting machinery.Therefore,the outstanding problems of mechanical harvesting of maize are the high grain broken rate resulted from high grain moisture content,and the high yield loss rate due to severe plant lodging,which are key factors restricting the development of mechanical grain harvesting technology.In this study,the field experiment and the research on cultivation techniques of mechanical harvested maize were carried out during 2015-2019 in the Honghe Hani and Yi Autonomous Prefecture of Yunnan Province,which belongs to the concentrated production area of summer maize in the multi-cropping systems of the Yunnan-Guizhou Plateau.From the perspectives of stalk-ear-grain mechanical strength,physiological characteristics and component structure of grain,the harvesting mechanism of mechanical harvested maize was explored.In addition,the influence of reduced-nitrogen and dense-planting cultivation techniques on maize yield and key characteristics of mechanical harvesting was analyzed,and the response mechanism of mechanical harvested maize yield to different straw returning modes was discussed.The aim of this study is to provide a theoretical basis and technical approach for the evaluation criteria and cultivation system of maize varieties suitable for mechanical harvesting.The main findings are as follows:(1)In this study,static crushing strength of grain(crushing resistance),and connection strength between the grain and cob(threshing performance)showed a direct causal relationship with the grain broken rate,and both these factors affected the broken rate.The relationship between the grain crushing strength(x1)and broken rate(y)could be described by the following binary regression relationship:(R2=0.8827,P<0.001),and the regression formula was y=0.0021x12-1.5443x1+293.78.The connection strength between the grain and cob(x2),and broken rate(y)could be depicted by a power regression relationship(R2=0.7656,P<0.001)and the regression formula was y=0.007x24.12.The relationship between cob crushing and broken rate was lower than the aforementioned(R2=0.1644,0.01<P<0.05).In addition to the association between plant lodging and grain loss rate,the stripped rotor of harvester header was easy to cause grain loss before ear threshing.Further,the loss rate was greater for grains that were easy to thresh,as a whole the grain impurity rate significantly declined as the water content of the non grain organs reduced.(2)The results indicated that the stem density,moisture content,bending strength and rind puncture strength were responsive to stalk-rot resistance.However,these indices were in agreement with stem resistance only after the physiological maturity of maize.In particular,the stem rind puncture strength exhibited a closer correlation with the stem resistance in the late stage of physiological maturity.It was also demonstrated that stem density directly affected bending strength and rind puncture strength,whereas indirectly affected the elastic modulus.The elastic modulus includes both load and displacement;therefore,it can prompt the reliability of stem strength evaluation,and is the best description of stalk toughness.The attenuation amplitude of the elastic modulus is always the largest whether it is caused by the aggravation of the disease,the growth stage delay,or the internode position rise,which significantly improves the sensitivity of stalk strength evaluation.Our results showed that the elastic modulus was correspondent with stem resistance from the silking stage of maize,which not only effectively improved the accuracy of the stalk strength evaluation,but also was extremely beneficial to improve the efficiency of maize variety selection.(3)The coefficient of determination of grain crushing strength to grain broken rate(R2=0.805,n=105,P<0.001)is greater than that of grain moisture content(R2=0.724,n=105,P<0.001).Grain starch content(x)is the material basis of grain crushing strength(y),and they exhibit a very significant exponential regression relationship(y=6.8612e0.0064x,r=0.9554,P<0.001).The activity of sucrose synthase is more closely related to the starch accumulation rate compared with that of starch branching enzyme,indicating that sucrose synthase may be the key factor to initiate starch accumulation.Although the grain protein accumulation rate is not significantly correlated with the glutamine synthetase activity(P>0.05),the grain protein content is significantly correlated with the grain crushing strength(P<0.01).Moreover,the binding protein in starch granules may be closely related to their formation,structure,and properties,significantly altering the texture and properties of endosperm.(4)Grain cracking pressure is closely associated with starch accumulation.The crystal structure and physical properties of endosperm starch play a decisive role in the formation of grain mechanical strength.The crystalline type of starch changes significantly as the grain filling progresses.Compared with the crystalline region of X-ray diffraction spectrum,the amorphous region occupies a larger proportion of the starch crystal structure,ranging from 83.8%to 87.0%.The area of the amorphous region,the total area of diffraction spectrum,the relative crystallinity,and the starch particle size directly contribute 0.826**to the grain cracking pressure,while the area of crystalline region and the intensity of diffraction peak indirectly affect the grain cracking pressure through other factors(P>0.05).Grain cracking pressure increases with the increase of starch particle size,and they show a very significant exponential regression relationship(r=0.9357,P<0.001).Furthermore,the contribution of starch particle size to the grain cracking pressure is significantly greater than that of starch relative crystallinity.During grain filling,the shape of starch granules gradually changes from a small spherical type to a large polygonal type,which may create structural conditions for the improvement of the mechanical strength of grains.(5)The results revealed that nitrogen fertilizer management pattern had no significant effect on the yield,but the nitrogen partial productivity was 33.7%higher in nitrogen fertilizer reduction plus deep application in holes between plants and rows(RDNP)than in conventional nitrogen fertilization(CNP).The RDNP significantly reduced the incidence and severity of maize stalk rot by 4.8 percentage points and 26.8%,respectively,but had little effect on the incidence of ear and kernel rots compared to CNP.The grain yields of densification I(D2,67 500 plants per hectare)and densification Ⅱ(D3,82 500 plants per hectare)were greater than that of commonly used rare-planting(D1,52 500 plants per hectare),but the sustaining yield increase rate of D3 was significantly lower than that of D2.The incidence and severity of stalk rot and ear and kernel rot were relatively high in D3 than in D2 and D1,while there was no significant difference between D2 and D1.The combination of RDNP and D2 was able to increase the maize yield,improve nitrogen partial productivity,and control the occurrence of maize stalk rot and ear and kernel rot effectively,so as to reduce the risk of lodging and grain quality safety.(6)As the increase in rounds of residues application,compared with straw deep burial with plowing(SDBP)and straw shallow burial with rotary tillage(SSBRT),straw mulching with minimum tillage(SMMT)continuously increased the soil moisture content in the 0-30 cm soil layer at the early stage of maize growth,increased the soil alkaline-hydrolyzed nitrogen content in the 0-20 cm and 40-60 cm layer and reduced the soil compaction under 40 cm layer,which were more conducive to the root system growth.Maize yield of the SMMT increased by 5.4%compared with that of the previous season,while the yield of SDBP and SSBRT decreased by 16.7%and 12.7%respectively,compared with those of the previous season.The data show that starting from 30 days after silking,maize kernels from the SMMT group had significantly less water content than those from the SDBP and SSBRT groups.This difference became larger as maize matured.At harvest,the SMMT group grain water content was less than SDBP and SSBRT by 6.0%and 10.9%,respectively(calculated with the averages between data from 2017 and 2018.Same below).Additionally,grain mechanical strength in the SMMT group was significantly higher than that in the SDBP(by 14.1%)and SSBRT(by 12.3%)groups;the SMMT group also had significantly lower grain-cob connection strength(13.3%less than SDBP,and 26.6%less than the SSBRT group).Moreover,SMMT can also increase the dehydration rate of corn grains at a later stage,leading to the harvested grains to be of low moisture content,high hardness,and easy to thresh,which create conditions for reducing grain breakage rate.In conclusion,it is recommended to employ SMMT method during crop mechanical harvesting,which is of great significance to reduce the impact of machinery on soil compaction,improve soil fertilizer supply capacity,realize grain mechanical harvest and steadily increase maize yield according to the red soil region of rain fed and dry farming in Yunnan Guizhou Plateau of Southwest China. |
PDF Full Text Request