The morphology of the plant and ear is a preliminary selection characteristic in breeding new varieties of maize.  As new maize cultivars were developed from the 1950s through the 2010s in China, most had changes in both plant and ear morphological characteristics that contributed substantially to maize yield gains.  Over the seven decades, plant and ear height fluctuated with a small increase from the 1950s to 2000s, and then a decrease in the 2010s, while the ear ratio and internodes length below the ear decreased significantly.  Leaf angles became significantly more upright, especially for the leaves above the ear, and the leaf area per plant improved markedly.  Leaf orientation increased from the 1950s to the 2000s then decreased in the 2010s.  Tassel size and the anthesis–silking interval were both reduced substantially.  Ear diameter, kernel number, and kernel weight increased from the 1950 to the 2000s, then decreased in the 2010s under the same cultivation conditions.  We found that modern maize hybrids have a lower plant height, ear height and ear ratio which increased lodging resistance, a more erect leaf which increased high-density planting tolerance, and smaller ears and kernels which facilitated rapid dehydration during late grain filling.  These morphological selection criteria, which are suitable for mechanized operations, are proposed as the focus for future maize breeding.

A high grain moisture content at harvest has been an important problem in the high latitude region of Northeast China, and it is closely related to the genotypes of varieties, local meteorological factors and planting management.  However, delayed harvest at a low temperature could not effectively reduce the grain moisture content.  In this study, we continuously observed the grain drying during the late stage of different maturing types of maize varieties in Daqing, Heilongjiang Province, China in 2016 and 2017.  A two-segment linear model was used to analyze the different stages of the drying processes: 1) Two-segment linear model fitting can divide the grain drying process of all varieties into two separate linear drying processes with different slopes.  2) During the rapid drying stage, the drying was faster at a higher temperature.  The rate of slow drying was influenced by air vapor pressure.  3) The moisture content and meteorological factors when the drying rate turns from one stage into the other were not consistent between varieties and years.  After entering the frost period, temperatures below 0°C will significantly reduce the rate of grain drying.  4) Due to the short growth period of early-maturing varieties, the drying time was prolonged, and the grain moisture content was lower than that of the mid-late maturing varieties.  Local meteorological conditions do not allow the drying of mid-late maturing varieties to achieve a lower moisture content.  When the temperature falls below 0°C, the drying rate of grain decreases markedly.  Therefore, one feasible way to solve the problem of high moisture content is to replace the early-maturing varieties and implement the corresponding cultivation techniques.

The rate of corn kernel breakage in the grain combine harvesters is a crucial factor affecting the quality of the grain shelled in the field.  The objective of the present study was to determine the susceptibility of corn kernels to breakage based on the kernel moisture content in order to determine the moisture content that corresponds to the lowest rate of breakage.  In addition, we evaluated the resistance to breakage of various corn cultivars.  A total of 17 different corn cultivars were planted at two different sowing dates at the Beibuchang Experiment Station, Beijing and the Xinxiang Experiment Station (Henan Province) of the Chinese Academy of Agricultural Sciences.  The corn kernel moisture content was systematically monitored and recorded over time, and the breakage rate was measured by using the grinding method.  The results for all grain samples from the two experimental stations revealed that the breakage rate y is quadratic in moisture content x, y=0.0796x2−3.3929x+78.779; R²=0.2646, n=512.  By fitting to the regression equation, a minimum corn kernel breakage rate of 42.62% was obtained, corresponding to a corn kernel moisture content of 21.31%.  Furthermore, in the 90% confidence interval, the corn kernel moisture ranging from 19.7 to 22.3% led to the lowest kernel breakage rate, which was consistent with the corn kernel moisture content allowing the lowest breakage rate of corn kernels shelled in the field with combine grain harvesters.  Using the lowest breakage rate as the critical point, the correlation between breakage rate and moisture content was significantly negative for low moisture content but positive for high moisture content.  The slope and correlation coefficient of the linear regression equation indicated that high moisture content led to greater sensitivity and correlation between grain breakage and moisture content.  At the Beibuchang Experiment Station, the corn cultivars resistant to breakage were Zhengdan 958 (ZD958) and Fengken 139 (FK139), and the corn cultivars non-resistant to breakage were Lianchuang 825 (LC825), Jidan 66 (JD66), Lidan 295 (LD295), and Jingnongke 728 (JNK728).  At the Xinxiang Experiment Station, the corn cultivars resistant to breakage were HT1, ZD958 and FK139, and the corn cultivars non-resistant to breakage were ZY8911, DK653 and JNK728.  Thus, the breakage classifications of the six corn cultivars were consistent between the two experimental stations.  In conclusion, the results suggested that the high stability of the grinding method allowed it to be used to determine the corn kernel breakage rates of different corn cultivars as a function of moisture content, thus facilitating the breeding and screening of breakage-resistant corn.

The uneven distribution of solar radiation is one of the main reasons for the variations in the yield gap between different regions in China and other countries of the world. In this study, different solar radiation levels were created by shading and the yield gaps induced by those levels were analyzed by measuring the aboveground and underground growth of maize. The experiments were conducted in Qitai, Xinjiang, China, in 2018 and 2019. The maize cultivars Xianyu 335 (XY335) and Zhengdan 958 (ZD958) were used with planting density of 12×10⁴ plants ha^–1 under either high solar radiation (HSR) or low solar radiation (LSR, 70% of HSR). The results showed that variation in the solar radiation resulted in a yield gap and different cultivars behaved differently. The yield gaps of XY335 and ZD958 were 8.9 and 5.8 t ha^–1 induced by the decreased total intercepted photosynthetically active radiation (TIPAR) of 323.1 and 403.9 MJ m^–2 from emergence to the maturity stage, respectively. The average yield of XY335 was higher than that of ZD958 under HSR, while the average yield of ZD958 was higher than that of XY335 under LSR. The light intercepted by the canopy and the photosynthetic rates both decreased with decreasing solar radiation. The aboveground dry matter decreased by 11.1% at silking and 21% at maturity, and the dry matter of vegetative organs and reproductive organs decreased by 9.8 and 20.9% at silking and by 12.1 and 25.5% at physiological maturity, respectively. Compared to the HSR, the root weights of XY335 and ZD958 decreased by 54.6 and 45.5%, respectively, in the 0–60 cm soil layer under LSR at silking stage. The aboveground and underground growth responses to different solar radiation levels explained the difference in yield gap. Selecting suitable cultivars can increase maize yield and reduce the yield gaps induced by variation of the solar radiation levels in different regions or under climate change.

Yield gap exists because the current attained actual grain yield cannot yet achieve the estimated yield potential. Chinese high yield maize belt has a wide span from east to west which results in different solar radiations between different regions and thus different grain yields. We used multi-site experimental data, surveyed farmer yield data, the highest recorded yield data in the literatures, and simulations with Hybrid-Maize Model to assess the yield gap and tried to reduce the yield gap by matching the solar radiation and plant density. The maize belt was divided into five regions from east to west according to distribution of accumulated solar radiation. The results showed that there were more than 5.8 Mg ha^–1 yield gaps between surveyed farmer yield and the yield potential in different regions of China from east to west, which just achieved less than 65% of the yield potential. By analyzing the multi-site density experimental data, we found that the accumulated solar radiation was significantly correlated to optimum plant density which is the density with the highest yield in the multi-site density experiment (y=0.09895x–32.49, P<0.01), according to which the optimum plant densities in different regions from east to west were calculated. It showed that the optimum plant density could be increased by 60.0, 55.2, 47.3, 84.8, and 59.6% compared to the actual density, the grain yield could be increased by 20.2, 18.3, 10.9, 18.1, and 15.3% through increasing plant density, which could reduce the yield gaps of 33.7, 23.0, 13.4, 17.3, and 10.4% in R (region)-1, R-2, R-3, R-4, and R-5, respectively. This study indicates that matching maize plant density and solar radiation is an effective approach to reduce yield gaps in different regions of China.

Increasing plant density is an effective way to enhance maize yield, but often increases lodging rate and severity, significantly elevating the risk and cost of maize production.  Therefore, lodging is a major factor restricting future increases in maize yield through high-density planting.  This paper reviewed previous research on the relationships between maize lodging rate and plant morphology, mechanical strength of stalks, anatomical and biochemical characteristics of stalks, root characteristics, damage from pests and diseases, environmental factors, and genomic characteristics.  The effects of planting density on these factors and explored possible ways to improve lodging resistance were also analyzed in this paper.  The results provide a basis for future research on increasing maize lodging resistance under high-density planting conditions and can be used to develop maize cultivation practices and lodging-resistant maize cultivars.

Competition is a common phenomenon in agriculture production.  Research on the relationship between competitive ability and crop yield is extensive, but the results have been inconsistent.  Few studies have focused on the relationship between population competitive intensity and yield of maize (Zea mays L.) cultivars.  The main objective of this study was to determine if a consistent relationship exists between maize yield and competitive ability.  A two-year field experiment was conducted, employing a de Wit replacement series design.  When two maize cultivars were grown in a mixture, yield was reduced for the modern cultivar and increased for the older cultivar.  In each replacement series, per plant level yield of each cultivar, and population level yield of the mixture, decreased with increasing proportion of the older cultivar.  Competitive ratio (CR) reflected differences in competitive ability of the three maize cultivars.  In each replacement series, population competition pressure (PCP) increased with increasing proportion of the older cultivar, indicating that the older cultivar was a strong competitor.  Biomass yield, grain yield, harvest index, thousand-kernel weight, and kernel number per plant, were negatively correlated with PCP.  Our results demonstrated that inter-cultivar competition affects maize productivity, and increasing PCP will decrease translocation of assimilates to grain and, ultimately, reduce yield.  Therefore, there is a negative correlation between population competitive intensity and yield performance in maize, breeders should develop a communal ideotype that would not perform well in competition in future. 

In 2010, Chinese maize yields increased from 961.5 kg ha-1 in 1949 to 5 453.8 kg ha-1. This increase is the result of genetic improvements, an increase in nitrogen application, and refinement of planting densities. The objective of this study was to provide a theoretical basis for maize production research by analyzing the maize yield gain characteristics. Six varieties of maize were selected for the study; each selection is representative of a typical or commonly used maize variety from a specific decade, beginning from the 1950s and continuing through each decade into the 2000s. The selections and their corresponding decade were as follows: Baihe, 1950s; Jidan 101, 1960s; Zhongdan 2, 1970s; Yedan 13, 1980s; Zhengdan 958, 1990s; and Xianyu 335, 2000s. Each variety was planted under four different densities (37 500, 52 500, 67 500, and 82 500 plants ha-1) and four different nitrogen applications (0, 150, 225, and 300 kg ha-1) to study the effects on yield gain characteristics. The obtained results demonstrated that there was a maize yield increase of 123.19% between the 1950s variety and the 2000s variety. Modern Chinese maize varieties had a higher yield advantage. They also displayed the additional potential to acquire higher yield under increased planting densities and nitrogen applications. At the present cultivation levels (planting at 67 500 plants ha-1 with 225 kg ha-1 nitrogen application), the contribution types and corresponding yield increase percentages were as follows: genetic improvement, 45.37%; agronomic-management improvement, 30.94%; and genotype× agronomicmanagement interaction, 23.69%. At high-yielding cultivation levels (planting at 82 500 plants ha-1 with 300 kg ha-1 nitrogen application), the contribution types and corresponding yield increase percentages were as follows: genetic improvement, 31.30%; agronomic-management improvement, 36.23%; and genotype × agronomic-management interaction, 32.47%. The contribution of agronomic-management and genotype × agronomic-management interaction to yield increase would be larger with the corresponding management improvement. To further increase maize grain yield in China, researchers should further examine the effects of agronomic-management on maize yield and the adaptation of variety to agronomic-management.

Experiment was conducted at the Gongzhuling Experimental Station of Institute of Crop Sciences, Chinese Academy of Agricultural Sciences, Jilin Province, China, during 2009-2010. Six representative varieties of maize (Baihe in the 1950s, Jidan 101 in the 1960s, Zhongdan 2 in the 1970s, Yedan 13 in the 1980s, Zhengdan 958 in the 1990s, and Xianyu 335 in the 2000s) were each planted under two different densities (52 500 and 82 500 plants ha-1) and two different nitrogen application levels (150 and 300 kg ha-1). Root characteristics and distribution among soil layers were studied by the field root digging method. The results showed that root mass increased with the process of the growth and development of the plant, and it peaked at kernel filling stage, and decreased at maturity due to the root senesces. Root mass of different maize varieties from the 1950s to 1980s had a trend of increase, while it decreased for the modern varieties. Root length and root surface areas had the similar changing trend. The study suggested that early maize varieties may have root redundancy, and reducing root redundancy may be a direction for variety improvement for high yield. Root characteristics were affected by nitrogen application level and density; modern varieties were more suitable for higher fertilizer application level and density conditions. Root characteristics distribution among soil layers decreased by an exponent equation, but the regression coefficients of different varieties were different. Though the root length density (RLD) of every soil layer of different varieties also decreased by an exponent equation, there were large variations of RLD in every part of a layer.

To evaluate the effects of different conservation tillage modes on crop yield in various ecological regions, we interviewed peasant households in the Northeast China Plain, North China Plain, Chengdu Plain, and Northwest China Oasis, China,to analyze the influencing factors of the tillage methods on crop yield. An index set was determined from seven yield parameters based on expert consultation and relevant literature. A comprehensive evaluation of conservation tillage methods in the various ecological regions was carried out using fuzzy theory. The approaches with the highest scores were identified for each region: wide and narrow rotation planting with high stubble standing retention in the Northeast China Plain; seeding after rotary tilled of wheat under corn straw mulching and no-tillage seeding of corn and other crops under wheat straw mulching in the North China Plain; no tillage with high stubble retention in the Northwest China Oasis;and no-tillage seeding of wheat and other crops under rice straw mulching in Chengdu Plain. These research data provide a useful guide for the selection of conservation tillage methods for optimum yields in different regions of China.