Abstract Eight summer maize varieties were evaluated in Funan County， Anhui Province， for yield and grain quality under mechanical grain harvest conditions in 2019 growth season. ZY432， LY35 and JNK728 had higher grain yield potentials than FDC10， LX98， LD575， YD9953 and TT619. Postponed harvest from 28th Sept. to 11th Oct. was positively linked to the grain yield at roughly 1% of daily yield increase. Early harvest gave higher grain water content which was positively related to the percentage of broken grain as y=0.422x-2.984 （R2=0.445）， and to the percentage of foreign substance as y=0.248x-3.245 （R2=0.698）. The relationship between grain water content and the rate of grain loss was negative as y=-0.052x+2.450 （R2=0.089）. There was about 0.89% of grain water content decrease daily between harvests from 28th Sept. to 4th Oct.， and then was a ‘slow-down’ rate of dehydration. Grain weight was increasing for each variety when the harvest was postponed. JNK728 had the highest hundred-grain weight （HGW） and YD9953 had the lowest HGW. To make the mechanical grain harvest of summer maize feasible and acceptable， farmers need to pay attention to the adoption of proper varieties， following the suitable cultivation procedure and the improvement of mechanical facilities and operation.
　　Key words Zea mays L.; Huaihe River Plain; Mechanical grain harvest; Kernel quality; Machine losses
　　 Maize is one of the four most important staple crops in Anhui Province with its annual harvest area over 1 million hectare. Maize is largely produced in the Northern part of the province， which is called the Huaihe River Plain， following a ‘winter wheat-summer maize’ cropping pattern. In the past， maize was produced exclusively by human labors from sowing to harvest. With the development of modern agricultural technology， efficiency in production affiliated by machines was highly promoted. Mechanization is thus the trend of agricultural practices.
　　In China， mechanization has been well developed in wheat and rice production from sowing to harvest. It is less developed in maize production， especially in the direct harvesting of grains by combiners. However， mechanically maize ear harvest has been more widely adopted in many parts of China[1]. Though much advanced over manual harvest， it still falls behind the mechanical grain harvest in efficiencies， as the harvesting cost and time consumption increases about 5.7% and 50%[2]. It is only during the early 21st century that China started its concerns on direct grain harvest of maize. 　　There are many factors affecting the quality of harvested maize grains and the feasibility of mechanical grain harvest， including the suitable varieties， proper farming techniques and the supporting facilities[3-8]. Institute of Crop Science， Chinese Academy of Agricultural Sciences， has made a series of studies on maize mechanical grain harvest during the recent years， from the selection of proper varieties， the illustration of factors affecting grain quality， to the modification of machines[1，6，9-16]. Maize varieties， appealing to such purpose， have been bred by many institutes and officially released since 2017[8，17-19]， and some preliminary cultivation procedures for maize mechanical grain harvest have been suggested accordingly[20-21]， and the modification in machines has also been preliminarily evaluated[22-23].
　　So far， the majority of maize producing farmers in Anhui are not familiar with the direct grain harvest by combiners. The locals mainly harvest the ears by machines and then store the ears for some time. They will later thresh the grains during winter time before marketing. In doing so， they need barn to store the ears and more labor cost. Farmers are now still in short of guidance in mechanical grain harvest. It is thus an important task for scientists to demonstrate them the relevant procedures leading to their witness and willingness of adopting such transformation in maize production. The objectives of the present study， therefore， were to find out the varieties suitable for mechanical harvest， to work out the parameters for grain harvest， and to provide some guidelines in grain harvest in Anhui Province.
　　Materials and Methods
　　Maize hybrid varieties
　　Eight maize hybrids registered for mechanical grain harvest were chosen and seeds were provided by various companies （Table 1）.
　　Experimental design and management
　　Maize was sowed on  June14， 2019， in Funan County， Fuyang， Anhui （32.67° N， 115.67° E）. The previous crop in the experimental field was winter-type wheat. Basal fertilizers were applied at 750 kg/hm2 （N∶P2O5∶K2O=15%∶15%∶15% in formula） before sowing. Each variety was sowed for 6 rows of 100 meters long by a three-row planter.
　　 Field management was following the normal procedure in that region. When maize was mature， a combiner （WorldGroup， mode 4LZ-5.0MAQ， manufactured by Jiangsu Wadder Group Company， modified to harvest maize grains） was adopted to harvest the grains. The three harvest timings were designed on Sept. 28， Oct. 4， and  Oct. 11， respectively. The first harvest was at the time when the majority of maize in that region was under harvesting. Each harvest was taken for 76.32 m2 for every variety. 　　Measurement of parameters
　　After harvest， the fresh weight of grains for each variety was measured directly on spot. Dropped kernels （i.e. grain loss） during harvest were collected randomly， for three times using a circular sampler of 0.107 5 m2. Around 1 kg of the harvested fresh grains was sampled and the percentage of foreign substance （mostly broken cobs） was measured from the samples. The grains were then dehydrated at 60 ℃ in an oven to its stable weight （for over 48 h）. Water content of the harvested grains was calculated as：
　　Water content （%）=（1-GdehydratedGfresh（2））×100（1）
　　Dry weight of harvested grains for each variety was calculated as：
　　Dry grain weight=（Gfresh（1）-Gforeign）×GdehydratedGfresh（2）（2）
　　Wherein Gfresh（1） is the total weight of measured samples including fresh maize grains and fresh foreign substance; Gfresh（2） is the weight of sampled maize grains without foreign substance; Gforeign is the total fresh weight of foreign substance in the harvest; and Gdehydrated is the dry weight of sampled maize grains after dehydration.
　　After dehydration， the broken maize grains were manually picked out and weighted and their percentage out of the whole dehydrated sample was considered as the rate of broken grain. One hundred intact grains were measured， for three times， for 100-grain weight.
　　Statistic analysis
　　Data collected were preliminarily analyzed following PC Excel 2013 program for standard deviation and correlation among some parameters. Results were analyzed by ANOVA， followed by a Duncan multiple-range test using DPS （Data Processing System）. Treatment means were compared by the least significant difference （LSD） at 5% probability （P=0.05）. P value of <0.05 was considered statistically significant.
　　Results and Analysis
　　Yield performance of eight maize varieties
　　There were significant differences in yield among varieties following mechanical grain harvest， both among varieties and among harvesting timings （P<0.05） （Fig. 1a）. The highest yield was achieved by ZY432 at its third harvest （equivalent to 11.17 t/hm2） and the lowest yield was met with TT619 at its first harvest （equivalent to 7.20 t/hm2）. ZY432， LY35 and JNK728 were the tested maize hybrids with better performance in grain yield than others at all three harvest timings.
　　Differences in yield were noticeable among harvest timings， though different varieties responded differently to the harvesting timings. The average yield of hybrids at the first harvest was 62.26 kg per plot， which was increased to 65.84 kg at the second harvest and 70.99 kg at the third harvest （with 5.76% and 7.81% of yield increase， respectively）， corresponding to roughly 1% of daily yield increase from the delayed harvest. 　　Variety LY35 showed only 1.85% of yield increase between the third and the first harvest， whilst for ZY432 there was 36.05% of yield increase and for JNK728 there was 21.92% of yield increase between the third and the first harvest. Consequently， LY35 had relatively broader harvesting period and JNK728 and ZY432 were more suitable for delayed harvest with respect to yield performance following mechanical grain harvest.
　　Changes of maize grain water content at different harvest timings
　　The dehydration process in grains started at different time. At the time of the first harvest， YD9953 and LNK728 had already dropped their grain water content to around 20% （Fig. 1b）. Meanwhile， LD575， TT619 and LY35 showed relatively higher grain water content than 25% at the same time. Water content was significantly different among varieties （P<0.05）.
　　Grain water content all decreased regardless of varieties tested during the process of delayed harvest. At the time of the first harvest， grain water content in eight varieties varied among 19.46-29.42% with its average of 24.08%. As for the second harvest， grain water content dropped synchronously for all varieties to 18.74% on average （4.51% to 6.33% of decrease according to variety）. It was about 0.89% of grain water content decrease daily during the first two harvests. Further dehydration was also noticed from the second to the third harvest， though at a ‘slow-down speed’ （from 18.74% to 17.55% on average）， due to the fact that several humid days occurred during that period.
　　Changes of maize grain weight
　　Grain weight was significantly different among varieties （P<0.05）， but not among harvesting timings for each given variety （P>0.05） （Fig. 1c）. Postponed harvest led to an increased grain weight for all varieties. Measured from the first harvest， the average HGW （in dry matter） for eight hybrids was 30.72 g， which increased to 31.51 g （of 2.56% increase） at the second harvest and 31.67 g （a further 0.50% of increase） at the third harvest. JNK728 showed the highest HGW at all three harvests （reached to 34.60 g at the third harvest） and YD9953 showed the lowest HGW at all three harvest timings （only was 28.73 g at the third harvest）. Grain weight of JNK728 was about 20% more than that of YD9953， demonstrating their genetic difference in grain physical status. Daily increase in HGW among all varieties was 0.07 g on average during the period of three harvests （0.02 g to 0.16 g of increase according to variety）. 　　Quality of grains mechanically harvested at different timings
　　Grain loss， percentages of foreign substance and damaged/broken grain during mechanical harvest were the indices for judging the quality of harvest.
　　The rate of broken grain （i.e. kernels damaged by the machine operation） was significantly different among varieties （P<0.05） （Fig. 2a）. The first harvest gave the highest rate of broken grains during mechanical harvest， varying from 5.15% in YD9953 to 12.64% in LD575 （on average of 7.96% for all varieties）. It was more than the national standard of ≤ 5% of broken rate permission for mechanical grain harvest in China[24]. The overall rate of broken grain dropped to 3.83% at the second harvest with only LD575 and ZY432 were still above the 5% of ‘acceptance’. Varieties FDC10 and LY35 both achieved around 2% of broken grain in the harvest. There was no constant decrease in the rate of broken grain from the second to the third harvest as the rate among varieties varied from 2.12% in FDC10 to 6.73% in LD575. The average rate of broken grains among varieties was 4.77% and was 24.4% higher than that of the second harvest.
　　Content of foreign substance in the harvest was not significantly different among varieties （P>0.05） （Fig. 2b）. With the delayed harvest by the combiner， the content of foreign substance decreased from the first harvest as 2.47% on average to 1.82% at the second harvest and 0.97% at the third harvest. There showed a 26.19% of decrease in the content of harvested foreign substance among the first and second harvests， and 46.67% of decrease among the second and the third harvests.
　　Varieties JNK728 and YD9953， in all their three harvests， gave below 1% of foreign substance， far less than the national standard of 3% of ‘acceptance’[11]， meaning the harvest was cleaner than others. The first and second harvests of FDC10 and LD575 gave above 3% of foreign substance， suggesting their concerns in mechanical grain harvest， especially when the harvest was taken early.
　　 Differences in the rate of grain loss during mechanical harvest were also noticeable among varieties and within harvest timings （Fig. 2c）. However， there gave no convincing tendencies among harvest， as the first harvest showed 1.18% of grain loss for eight hybrids on average （varying from 0.72% to 1.92%）， the second harvest was 1.62% of grain loss （varying from 0.64% to 3.44%） and the third harvest was 1.36% of grain loss （varying from 0.56% to 2.35%）. Most of the harvests showed less than 2% of grain loss which was far below the limit set by the national standard of 5%[11]. 　　Relationship between grain water content and the quality of mechanically harvested maize grains
　　Maize grain water content was affected by the timing of harvest （Fig. 1b）， and the actual grain water content influenced the quality of mechanically harvested maize grains （Fig. 3a， 3b， 3c）. Positive relationships among grain water content during harvest and the rates of broken grain and the foreign substance harvested were noticed， fitting the following equations：
　　y1=0.422x-2.984 （R2=0.445）（3）
　　y2=0.248x-3.245 （R2=0.698）（4）
　　A negative relationship between water content and grain loss was also observed， fitting the following equation：
　　y3=-0.052x+2.450 （R2=0.089）（5）
　　Wherein y1 is the percentage of broken grain; y2 is the percentage of foreign substance in the harvest; y3 is the percentage of grain loss during harvest; and x is the grain water content at harvest.
　　Discussion
　　Choosing suitable maize varieties
　　To make the mechanical grain harvest of maize feasible and acceptable， proper varieties are of high priority for farmers to adopt[12，25-27]. Most of the varieties currently used in Anhui are common maize hybrids. Though with high yield potentials， they are mostly with low dehydration rate. It is also possible that grain water content during harvest might be significantly and positively related to grain yield[26]. Consequently， when its time for farmers to harvest， there is high water content in grains （normally around 30%-40%）[6] which is unacceptable for directly harvesting grains by combiners， due to the increased risk of grain loss and damage[7，10，13，24，27，28].
　　The current study revealed a similar conclusion in the positive relationship among water content and broken grain and foreign substance. The rate of broken grain （y） during mechanical harvest was positively related to the grain water content （x） as y=0.422x-2.984 （R2=0.445）. It was comparable to another equation gained before[13] in the same province which was y=0.489x-5.911 （R2=0.097）. The relationship between grain water content （x） and grain loss （y） was also similar as y=-0.052x+2.450 （R2=0.089） in the study and y=-0.068x+4.574 （R2=0.240） in Wang et al.[12]. The negative relationship between water content and machine losses during harvest was also found by Liu et al.[9] showing as y=-0.010x+0.864 （R2=0.005）. Differences were found in some previous results. Chai et al.[28] reported a quadratic polynomial between grain moisture content （x） and harvest breakage （y） as y=0.037x2-1.483x+20.422 （R2=0.452）， under the condition of x>19.9%. Xie et al.[10] revealed ybroken rate=0.332xwater content-1.213 （R2=0.819） when x<26.9% and ybroken rate=1.490xwater content-32.092 （R2=0.915） when x>26.0%. Similarly， Li et al.[24] found ybroken rate=1.365xwater content-28.743 （R2=0.874） when x>27.1% and ybroken rate=0.398xwater content-3.006 （R2=0.720） when x<26.9%. 　　 Such differences might be due to the varieties used and/or seasonal variations. Genetic properties firstly determined the dehydration rate in grains after the crop was physiological mature[6，15-6，29]. From careful selection， maize varieties with fast dehydration rate and high yield performance have been obtained[30-31]. As shown in the current study， JNK728 （released in 2017）， ZY432 （released in 2018） and LY35 （released in 2019） were the varieties gave relatively higher yield than others. Those hybrid varieties were worthy of recommendation for the local farmers to use in their production， especially JNK728 and ZY432 were more suitable for mechanical grain harvest under Anhui productive conditions.
　　Furthermore， an optimal maize variety appealing to mechanical grain harvest also needs to be with improved disease resistance and lodging resistance[31-32]. It happened to be under a dry weather condition during the late stages of maize growth in the year of 2019 in Anhui which was unsuitable for the evaluation of diseases and lodging resistance. Therefore， more work is still needed on the comprehensive evaluation of the above-mentioned varieties and some other candidate varieties as well.
　　Adopting proper cultivation practices
　　Cultivation is another concern for mechanical grain harvest， not only to fit the machine operation per se but also to determine the physical conditions of plants at harvest. First thing needs to be considered is the planting density[3，6，7，33-35]. Varieties with rapid dehydration rate need to be early mature， which in turn will affect the grain weight and harvested yield. Only through high density of planting （e.g. increase from 60 000 to over 75 000 plants per hectare） could guarantee the optimal yield outcome[25，34]. Planting density increased from 60 000 to 90 000 per hectare might decrease the grain moisture content， broken rate and the rate of foreign substance during harvest[7]， but high density would increase the risks of fungal affection and lodging， which disadvantageously affected the crop yield and grain quality[7，33，35-38]. It had already been suggested that for variety ZD958， row spacing of 60 cm could give higher yield than that of 55 cm and 65 cm， and the machine efficiency was also the highest[39]. Hybrids DH661， ZD958 and LD981 yielded 5.0%， 10.2% and 12.5% higher at density of 82 500 plants per hectare than at density of 67 500 plants per hectare[40]. However， over densely planted JNK728 from 75 000 to 90 000 plants per hectare increased the rate of lodging from 3.5% to 12.5%， and yield decreased from 8 416.5  to 7 653.0 kg/hm2[34]. In the current study， planting density was designed according to the recommendations from the seed providers. Except the density for ZY432 was adjusted to 90 000 plants per hectare and LD575 was adjusted to 55 550 plants per hectare， other varieties were in the same density of 66 660 plants per hectare. More work， concerning with each individual variety， is still needed to establish its optimal planting density under the situation of mechanical grain harvest. 　　For mechanical grain harvest， farmers also need to be careful in the application of fertilizers. Higher nitrogen conditions exhibited greater yield potentials[41]， but over-fertilization of nitrogen always led to the increased risk of plant lodging which was disadvantageous for mechanical grain harvest[42]. Ren et al.[40] found no significant difference in yield between N treatment of 180 and 270 kg/hm2. Since it was a preliminary study taken in Anhui， further work relevant to variety- and location-specific fertilizing strategies is still needed before a comprehensive procedure is recommendable for farmers.
　　Sowing and harvest timings are also important as delayed sowing might affect the plant height and lodging[34]. Postponing the sowing date of summer maize in Xinxiang， Henan Province， from May 25 to June 27 obtained higher kernel weight and final yield[43]. Delayed harvest is required for grain drying and the proper temperature during grain filling stage could increase the filling at about 0.3 mg per day per degree[44]. Late harvest increased the yield and grain weight， also improved the grain quality[45-46]. It is also important to notice that maize harvest is not ‘the later the better’. Harvest quality was the best when grain moisture content dropped to around 20%[24]. Too delayed harvest might be associated with yield losses due to grain shrinkage and other biotic and abiotic constraints[47]. To coordinate the relationship between yield and grain moisture content， it was suggested that the late mature varieties needed to be sown earlier and the early mature varieties needed to be sown later[21]. We also revealed the advantages of delayed harvest in this current study. Though the yield increase rate differed among genotypes， an overall tendency of yield increase was realized. ZY432 and JNK728 gave the highest yield increase from harvest on Sept. 28 to Oct. 11， showing as 36.05% and 21.92%， respectively. However， the very last harvest might not guarantee a further improved grain quality as suggested from the results that the third harvest led to an increased rate of broken kernels over the second harvest. Such results agreed with many of the previous studies.
　　Integrating cultivation with mechanical operation
　　Significant differences had already been noticed among combiners used for harvest， especially in the rate of broken grain and grain loss[11，39]. Machine capacity （e.g. thresh capacity， separation and cleaning） and the skill of the operators， such as the control of advance speed， all affect the quality of harvest[4]. As revealed that at low feed rates， over 90% of machine losses occurred at the cornhead rather than in the threshing， separating and cleaning areas. Header losses occurred due to ear drop from late season harvest and negligible losses inside the machine when operated at 4.8 km per hour[3]. 　　As it was not popular yet for the locals to harvest maize grains by machines， there were not many combiners available nearby at the moment in Anhui. Therefore， the introduction and demonstration of the machine is in urgent need. Only with the vast willingness of adopting mechanical grain harvest is aroused， huge market needs will then lead the manufactories to develop or invent more relevant machines to satisfy the need of production. In this study， three harvests were taken by the same machine with no comparable results with other or more advanced machines. However， three different operators were hired to do the job which might lead to an operator-related variation due to their differences in skills. Qualification of operators is another issue needs to be improved facing the coming of a new era of mechanical maize grain harvest.
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