Maize occupies an important position in all kinds of human production and life, it is not only the main grain crops, but also the main source of starch in all kinds of production. Maize yield is closely related to its planting density. Increasing the number of field populations is still the main way to improve maize yield in the world and China (Ren et al., 2017). Whereas, with the higher density the aeration-euphotic ability would worse, and this would increase the risk of empty stalk and lodging of maize (Yang et al., 2003). Chemical regulation is widely used in agricultural production by regulating plant endogenous hormone levels, affecting crop growth and development process, achieving stable crop yield, improving quality, and enhancing stress resistance (Shekoofa and Emam, 2008). In maize cultivation, the use of chemical regulation can effectively reduce plant height and ear position, play a certain role in lodging resistance and prevention of empty stalk, so as to increase the yield of maize (Meng et al., 2014). However, studies have shown that chemical regulation not only effectively inhibits the elongation of maize vegetative organs, but also inhibits the development of reproductive organs (Li et al., 2016), resulting in grain abortion and ear reduction, thereby reducing the yield of maize per plant (Liu, 2012). And the application effect of chemical regulation is also closely related to maize variety (Wei et al., 2011), planting density (Grassini et al., 2011), use stage and other factors. The differences among the studies have brought many uncertainties to the application of chemical regulation. Whether the planting density of maize, the spraying concentration and spraying date will affect the application effect of chemical regulation. The answers to these questions are difficult to obtain from independent field experiments.

Based on the research data of chemical regulation of maize in China, this study introduced the Meta-analysis to quantitatively study the time effect, regional effect and affecting factors of chemical regulation on maize yield, aiming to reveal the influencing mechanism of chemical regulation on maize yield in China and provide the basis for the rational application and promotion of chemical regulation in maize production.

1 Results and Analysis

1.1 Comprehensive effect analysis of chemical regulation on maize yield

The comprehensive effect of chemical regulation of maize yield was calculated. The results showed that P_Q<0.01, and I²>80%, indicating that the data heterogeneity was particularly significant, so the random effect model was used for analysis. Overall, chemical regulation can significantly increase maize yield. According to the data weight, the yield can be increased by 570.489 kg/hm², with the 95% confidence interval of 413.595~727.382 kg/hm² (Table 1).

1.2 Meta regression analysis of chemical regulation on maize yield

Meta regression analysis was conducted on variables that may affect heterogeneity such as provinces/autonomous regions (Henan, Heilongjiang, Hunan, Guangxi and Guizhou), experimental time (2007~2010, 2011~2014, and 2015~2018), maize varieties (compact, semi-compact and flat type), planting density (low density(-∞, 5.25), medium density[5.25, 6), high density[6, 7.5), ultra high density[7.5, +∞)), chemical regulator type (growth retarders and growth promoters), spraying dose (lower concentration(-∞, 100) and higher concentration[100, +∞)), and spraying date (seedling and panicle stage) (Table 2).

Table 2 Meta regression analysis of chemical regulation on maize yield

Province/autonomous region(p=0.007), chemical regulator type(p=0.000), spraying dose(p=0.023), and spraying date(p=0.020) were the sources of heterogeneity. The above four variables were further introduced into the regression model, and the variance component between studies was reduced from 350000 to 230548, indicating that 34.13% of its heterogeneity source could be explained (Table 2).

1.3 Publication bias analysis of chemical regulation on maize yield

According to the inverted funnel plot by publication bias test, the sample data points are unevenly distributed on both sides of the funnel, indicating that there may be publication bias, which needs further analysis (Figure 1).

Regression analysis was used for publication bias test, Egger’s linear regression analysis p=0.111 can be obtained, indicating that there is no publication bias (Table 3).

1.4 Sensitivity analysis of chemical regulation on maize yield

The residual standard deviation analysis of all data showed that except sample 8, 38, 75 and 77, the residual standard deviation of other data is all distributed in the range of 2 times standard deviation (Figure 2).

The above four samples were removed for sensitivity analysis. After removal, the mean difference, 95% confidence interval and the significance of mean difference did not change significantly, and the heterogeneity was still obvious (Table 4). Therefore, sample 8, sample 38, sample 75 and sample 77 cannot be regarded as extreme values, and the results of Meta-analysis are reliable (Table 2).

1.5 Analysis of factors affecting maize yield under chemical regulation

Maize variety, planting density, chemical regulator type, spraying dose and spraying date all affect the yield effect of chemical regulation on maize to varying degrees (Figure 3).

In Heilongjiang and Guangxi, the average yield of maize increased by 803.773 kg/hm² (confidence interval of 574.16~1 033.39 kg/hm²) and 509.162 kg/hm² (confidence interval of 318.24~700.09 kg/hm²) using chemical regulation, respectively, indicating that chemical regulation had a positive effect on maize yield and could promote the increase of maize yield. In Henan, the average yield of maize reduced by 540.81 kg/hm² (confidence interval of -785.20~-296.42 kg/hm²), indicating that chemical regulation had a negative effect on maize yield and could not promote the increase of maize yield. In Hunan and Guizhou, the 95% confidence interval of chemical regulation for maize yield increase is 0, indicating that the effect of chemical regulation is not significant.

From the experimental time, the yield of maize using chemical regulation showed an overall increasing trend, and the average yield of maize increased by 297.95, 597.05 and 776.85 kg/hm², respectively, in 2007~2010, 2011~2014 and 2015~2018. However, there is a large range of changes from 2015 to 2018, with a 95% confidence interval of 188.54 to 1 365.17 kg/hm², which is not stable from 2007 to 2010 and 2011 to 2014.

The application of chemical regulation in maize with ultra high density had the largest increase in yield, with an average yield of 1 051.41 kg/hm² and 95% confidence interval of 694.63~1 408.19 kg/hm². The effect of chemical regulation was significant. The yield-increasing effect of chemical regulation had little difference between low density and medium density maize, but the yield-increasing effect of medium density (511.49 kg/hm², 95% confidence interval of 413.60~609.38 kg/hm²) was more stable than that of low density (522.295 kg/hm², 95% confidence interval of 269.67~774.92 kg/hm²).

The chemical regulation was applied to flat, semi-compact and compact maize, and the average yield was increased by 509.49, 728.69 and 519.70 kg/hm², respectively, and with the 95% confidence interval of 318.24~700.74 kg/hm², 99.43~1 357.95 kg/hm², and 300.63~738.77 kg/hm², respectively. Although the yield increase of semi-compact maize using chemical regulation is higher than that of flat maize and compact maize, the yield increase effect of the latter two maize is more stable.

The application of growth promoter to regulate the average increase of maize growth by 902.49 kg/hm² (95% confidence interval of 687.05~1 117.92 kg/hm²), indicating that the growth promoter has a positive effect on maize yield and can promote the increase of maize yield. However, the 95 % confidence interval of the average yield increase is 0 when the growth retarder was applied to regulate maize growth, indicating that the regulation effect was not significant.

The average yield of maize increased by 1 506.86 kg/hm² with lower concentration of chemical regulator, which was higher than that by 486.04 kg/hm² with higher concentration.

The maize average yield increased by 396.62 kg/hm² with the application of chemical regulation in seedling stage (95% confidence interval of -85.63~878.88 kg/hm²), indicating that the yield increase effect of chemical regulation in maize seedling stage was not significant. The maize average yield increased by 591.96 kg/hm² with the application of chemical regulation in panic stage (95% confidence interval of 427.15~756.76 kg/hm²), indicating that the yield increase effect of chemical regulation in panic stage was significant, and could promote the increase of maize yield.

2 Discussion

2.1 Yield-increasing effect of chemical regulation

In this study, the field experiment of chemical regulation maize in China was analyzed. The results showed that chemical regulation could significantly increase maize yield. Compared with water-sprayed field, average increasing yield being 570.489 kg/hm² with no publication bias and no extreme values (confidence interval of 413.595~727.382 kg/hm²). The main reasons for increasing maize yield by chemical regulation are as follows: (1) Increasing planting density is one of the main means to improve maize yield. With the increase of planting density, the plant height and ear position of maize will also increase, and the bending resistance and rind penetrometer resistance of stem will also decrease, resulting in the increase of empty stalk rate and lodging rate of maize (Tian et al., 2016). The use of chemical regulation could reduce the plant height and ear position of maize, shorten the length of basal internodes, and increase the stem diameter (Feng et al., 2017). It can also increase the diameter of aerial roots of maize, improve the number and depth of entering the soil, thereby reducing the lodging rate of maize, and improving the density tolerance of maize (Hao et al., 2017). (2) Using chemical regulation, on the one hand, could reduce the plant height of maize and energy loss, increase the permeability of maize population, and create favorable external conditions for the accumulation of maize photosynthetic products (Li et al., 2016a). On the other hand, by regulating endogenous hormones in maize, promoting leaf photosynthesis, more photosynthetic products are transported to grain, accelerating grain filling, and improving grain weight (Li et al., 2015).

2.2 Factors affecting yield effect of chemical regulation

2.2.1 Maize variety and planting density

The use of chemical regulation can effectively solve the problem of yield reduction caused by the increase of maize lodging rate due to the increase of planting density. When maize is planted, the appropriate planting density is generally selected according to the variety type of maize. For example, when compact maize is planted, because the leaf angle of this type is small, and the leaf is erect upward, it has good light transmittance, so the general planting density is relatively large (Huang et al., 2012). When planting flat maize, because of its large leaf angle and flat leaf surface, the smaller planting density was selected (Yu et al., 2018). Shi et al. (2014) found that spraying chemical regulator had significant effect on maize varieties suitable for dense planting but had no significant effect on maize varieties suitable for sparse planting. Sun et al. (2017) found that under low density treatment, the use of chemical regulator in maize had the risk of reducing yield, and when the density was high, the yield of maize could be increased.

Jiang et al. found that the rational use of chemical regulation can effectively reduce the growth redundancy of maize stem under high density conditions and improve maize harvest index. Wei et al. (2015) and Shi et al. found that with the increase of planting density, the yield of maize increased at first and then decreased with the use of chemical regulation. This study also found that the average yield increase of ultra-high density maize was higher than that of high density, medium density and low density. In the integration analysis, the average yield increase of high density is lower than that of other densities, which may be due to the difference of suitable planting density of different types of maize. In this density range, there are many effects caused by the superposition of average yield increase in the process of rising and falling. The above studies showed that there was an obvious interaction among maize varieties, planting density and chemical regulation. The use of chemical control agents could significantly improve the lodging resistance of individual plant in high density maize population. However, too high density may lead to the decrease of maize grain yield. Therefore, when using chemical regulation, planting maize cannot blindly pursue high density and increase production. According to the type of maize varieties, we should choose appropriate planting density, coordinate the production relationship between population and individual plant, and increase maize yield.

2.2.2 The use of chemical regulator

The use of chemical regulation can effectively control the plant height and ear position of maize, move down the maize center of gravity, and prevent maize lodging (Zhao et al., 2010). The analysis of this study found that the yield-increasing effect of chemical regulation applied in maize seedling stage was not significant, but in panic stage was significant. Chemical regulator had internode-cascaded and additive inhibition elongation effects on the maize plant. When chemical regulator was sprayed on the maize at the stage of “n” expanded leaves, which showed that the elongation-inhibited internodes were the several ones above the (n-2) nodes (Qi et al., 2014). At the same time, there was a significant elongation of the upper internode in the panicle position, and the earlier the chemical regulator was sprayed, the earlier the elongation occurred. And the later spraying, the higher the affected node, the stronger the control effect on plant height and panicle position (Meng et al., 2016).

In this study, the dosage of chemical regulator was analyzed. The results showed that the average yield of chemical regulation sprayed with lower concentration was 1506.86 kg/hm², which was higher than that of higher concentration by 486.04 kg/hm². Although spraying chemical regulator can effectively control the plant height and ear position of maize, and the higher the concentration of chemical regulator is, the more obvious the control effect is. However, if the concentration of chemical regulator is too high, it may inhibit the normal growth of maize stems and the development of reproductive organs, resulting in the reduction of maize yield (Li et al., 2016b). Therefore, in the use of chemical regulation, it is necessary to use appropriate concentrations of chemical regulator for spraying at the appropriate growth stage of maize according to certain standards to achieve maize yield increase.

2.3 Research limitations

In this study, more than 100 Chinese and English papers about the effect relationship between chemical regulation and maize yield were collected according to certain keywords. But some of the data samples were missing, and some of them did not repeat the experiment. After screening, 25 effective papers and 78 groups of data samples were obtained. Due to the limited data sample, this study only analyzed the method of spraying leaf and stem, failed to analyze the soaking method, and it was difficult to comprehensively analyze the effect factors between chemical regulation and maize yield. In addition to the relevant factors that have been considered in this study, some other factors, such as soil fertility, climate, fertilization, have not been analyzed.

3 Materials and Methods

3.1 Data sources

Using maize, yield, chemical regulation, chemical control technology, chemical regulator and plant growth regulators as keywords, the field trials on the effects of chemical regulation on maize yield were collected in CNKI, Wanfang, Web of Science databases, etc. Combined with the requirements of Meta-analysis method for data analysis, the research data were further screened based on the following standard: (1) Field experiments should be conducted in China. (2) Chemical regulation treatment and clean water control treatment must be included at the same time. (3) The data included the average yield of the experimental treatment and the control treatment, the number of test replicates, or the field test data of 2 years and above. (4) The conditions such as experimental time, experimental place, experimental maize variety and planting density, type of chemical control agent, spraying time and concentration were clear. After the above standard screening, 25 available papers were obtained, including 78 groups of sample data.

3.2 Data classification

Among the 78 groups of data collected include five province/autonomous region: Henan, Heilongjiang, Hunan, Guangxi, and Guizhou. Chemical regulation (experimental group) refers to the uniform spraying of different doses of chemical regulators on the stems and leaves of maize in different periods during maize growth to achieve chemical regulation of maize. Water (control group) refers to no spraying chemical regulator during maize growth, only spraying the same amount of water at the same time (Table 5).

3.3 Data analysis

(1) Standard deviation calculation: In the Meta-analysis of continuous data, standard deviation is an important parameter, which reflects the importance of each group of data in the whole analysis. In the data statistics, if the standard deviation of yield is directly given in the papers when analyzing the repeated test data, it can be directly adopted. If the papers only list the yield of each repeated experiment, it is necessary to calculate the corresponding standard deviation of the average yield of each experiment according to the given data, and the comparison of two years and more experiments is also seen as a repeated experiment to calculate the corresponding standard deviation (Li et al., 2017).

(2) Calculation of effects: The outcome variable involved in this study is maize yield, which is continuous data, and the collected data are finally converted into kg/hm². Therefore, the mean difference was used as the effect scale index to calculate the effect amount MD (average yield of maize, standard deviation of yield and repetition of experiment) of chemical regulation and water control in the data. The weight of mean difference in each study was determined by the accuracy of effect estimation (Bond et al., 2003).

Where, MD is effect-mean difference, Yield_T is the average yield of maize with chemical regulation, Yield_c is the average yield of clean water control maize, unit is kg/hm² (95% confidence interval of MD: if<0, it indicates that chemical regulation has a negative effect on maize yield and cannot promote the increase of maize yield. If>0, it indicates that chemical regulation has a positive effect on maize yield and can promote the increase of maize yield. If=0, it indicates that chemical regulation has no significant effect on maize yield.)

(3) Heterogeneity experiment: The results of the test are related to the selection of the effect model. If the heterogeneity is significant, the random effect model is selected. If the heterogeneity is not significant, the fixed effect model is selected (Wei and Dong, 2016).

(4) Meta regression analysis: The collected data were divided into groups and analyzed by the Meta-regression model to find the sources of data heterogeneity (Zhang et al., 2009).

(5) Publication bias test: The data collected in this study are from publicly published papers, and generally more attention is paid to the results of significant studies. Therefore, the results with significant effects are easier to publish than those without significant effects, which may lead to publication bias (Kepes and Thomas, 2018). In this study, the linear regression and funnel plot were used to test the publication bias of the collected data.

(6) Sensitivity analysis: Mainly used to find outliers in data. If an effect is an outlier, the residual effect is integrated after removing the effect. Observe the change degree of Meta-analysis results, if the change is large, it shows that the effect is an outlier (Yin et al., 2018).

Authors’ contributions

DQ, ZXH, and LTC were the designer and modification executor of this paper. WF, and WJ participated in the paper conception, data search and result analysis. All authors read and approved the final manuscript.

Acknowledgments

The study was supported by the National Natural Science Foundation of China (31801374), and Science and Technology Innovation Team Project of Anhui Academy of Agricultural Sciences (2020YL058).

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