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Chengdu Academy of Agriculture and Forestry Sciences, Chengdu 611130, China

1 Introduction

The topography of the southwest ecological zone is very complicated, with the area of mountainous hills accounting for 80%-90% of the total area, and the dry land and sloping farmland accounting for a large proportion[1]. There is difference in the mechanized maize production between the north and south of China, while the mechanized sowing maize in hilly areas of Sichuan still remains at the starting stage, the existing machines are mainly small and medium-sized sowing machines[2]. Because of the serious shortage of rural labor force, screening appropriate sowing machines is an important measure to ensure the grain production and the income of farmers. As a key factor in mechanized sowing of maize, the sowing depth determines the germination of seed, sprouting, and survival after sprouting, growth after survival, and content of nutrients[3]. Too deep or too shallow sowing will cause weak seedlings or drop of the germination rate[4-6]. Appropriate sowing depth can improve the uniformity of maize population and is favorable for increase of maize yield. Therefore, we studied the performance of small and medium-sized sowing machines in the yield and related traits of maize at different sowing depths, in the hope of providing guidance for mechanized maize production in southwest ecological zone.

2 Materials and methods

2.1ExperimentalmaterialsWe took maize variety Zhongyu 3 as the experimental materials. The sowing machine is tractor-driven medium-sized sowing machine ZHSB-10 and micro cultivator driven small sowing machine 2B-2 manufactured by Zefeng Agricultural Machinery Manufacturing Co., Ltd. of Zhongjiang County.

2.2ExperimentdesignWe carried out the experiment in Yangma Science and Technology Experimental Park in Chongzhou City of Sichuan Province in 2016. We set a total of four treatments (Table 1) and made randomized block arrangement with three repetitions. The plot was 40 m long and 4 m wide. The cultivation density was 67 500 plants/ha, compound fertilizer 300 kg/ha (N∶P∶K=15∶15∶15) was applied as base fertilizer. Field management was the same as the field production. We sowed the maize seeds on March 31 and harvested the maize on July 30.

Table1Combinedtreatmentofmaizesowingdepthandsowingequipment

TreatmentSowingdepth∥cmSowingequipment15ZHSB?1252B?238ZHSB?10482B?2

2.3Surveyitemsandmethods

2.3.1Germination rate and agronomic traits. At the three-leaf stage, we surveyed the germination rate, measured the plant height, stem diameter, leaf area index (LAI), leaf length and plant dry weight, and calculated the plant height uniformity; at the jointing stage, we surveyed the plant height, steam diameter, LAI, leaf length, and plant dry weight. After silking and pollen-dispersing stages, we surveyed the plant height, ear height, steam diameter, and plant dry weight. After the germination of maize, we selected three points with uniform growth in each treatment, each point was 15 m2, surveyed the quantity of germination, and calculated the germination rate. The survey of agronomic traits at each stage: we selected three points with uniform growth in each treatment, each point was 15 m2, measured all plants in each point, calculated the average value, calculated the plant height uniformity using reciprocal value of variation coefficient (1/cv)[7], and measured the LAI using the length-width coefficient method.

2.3.2Yield calculation and variety test. During harvesting, we took the middle two lines of each plot (15 m2) to calculate the yield, and converted the yield into the yield with standard moisture content (14%). Besides, we made records of the number of plants, number of ears, lodging rate, lodging and stem broken rate, empty stalk rate, and double ear rate; weighed all ears, selected 20 ears to conduct variety test, and surveyed the ear length, bald ear length, ear diameter, axis diameter, rows per ear, kernels per row, and 1 000-grain weight.

2.4DatastatisticsandanalysisWe carried out calculation and analysis with the aid of Excel software.

3 Results and analyses

3.1Germinationrateandplantheightuniformityofmaize

From Table 2, it can be seen that the germination rate and plant height uniformity of maize with sowing depth of 5 cm were higher than that with sowing depth of 8 cm, increasing by 1.5% and 2.0% respectively. Compared with the small sowing machine 2B-2, the medium-sized sowing machine ZHSB-10 had lower germination rate, reducing about 0.03%, and the plant height uniformity increased by 0.44%. The results showed that the sowing depth exerted great effects on the germination rate and plant height uniformity of mechanized sowing maize; the germination rate and plant height uniformity of maize with sowing depth of 5 cm were better; different sowing equipment had little effect on the germination rate and plant height uniformity of mechanized sowing maize.

Table2Germinationrateandplantheightuniformityofmaizebydifferenttreatments

TreatmentSowingdepth∥cmSowingequipmentGerminationrate∥%Plantheightuniformity15ZHSB?1091．819．18252B?292．089．3738ZHSB?1090．689．23482B?290．478．96

3.2AgronomictraitsofmaizeAccording to Table 3, (i) at the three-leaf stage, the plant height, steam diameter, leaf length, LAI, and plant dry weight of maize with sowing depth of 5 cm increased by 3.0%, 2.8%, 2.9%, 3.3% and 1.5% compared with the maize with sowing depth of 8 cm; the leaf length of the medium-sized sowing machine ZHSB-10 was 0.5% greater than that of the small sowing machine 2B-2, the plant height and LAI were 0.4% and 0.7% lower, while the stem diameter and plant dry weight had basically the same performance. The results showed that the sowing depth exerted great effects on agronomic traits of maize plant, and the agronomic traits of maize with the sowing depth of 5 cm were much better than that with the sowing depth of 8 cm, while different sowing equipment had little effect on agronomic traits of mechanized sowing maize at the three-leaf stage.

Table3Agronomictraitsofmaizeatdifferentgrowthstages

Sowingdepth∥cmSowingequipmentPlantheight∥cmSteamdiameter∥mmLeaflength∥cmLAIPlantdryweight∥gThree?leafstage5ZHSB?1012．10．558．90．0783．4852B?211．90．548．60．0773．428ZHSB?1011．50．538．40．0743．3782B?211．80．528．60．0763．43Jointingstage5ZHSB?1057．351．03118．10．5115．1152B?256．231．02717．60．5215．238ZHSB?1056．671．02417．10．5015．0982B?256．121．01417．60．5115．18Tasselingstage5ZHSB?10205．6106．51．9165．565．852B?2207．4105．41．8866．466．48ZHSB?10204．8108．31．8666．267．582B?2206．5106．11．8766．766．7

(ii) At the jointing stage, the plant height, steam diameter, leaf length, LAI, and plant dry weight of maize with sowing depth of 5 cm increased by 0.7%, 1.0%, 2.9%, 2.0% and 0.2% compared with the maize with sowing depth of 8 cm; the plant height and steam diameter of the medium-sized sowing machine ZHSB-10 were 1.5% and 0.7% higher than that of the small sowing machine 2B-2, and LAI and plant dry weight reduced by 1.9% and 0.7% respectively. These indicated that the sowing depth at jointing stage still had certain effects on the agronomic traits of maize. However, sowing equipment had little effect on the agronomic traits of maize plants at the jointing stage. (iii) After tasseling, the plant height and stem diameter of maize with the sowing depth of 5 cm were 0.4% and 1.6% greater than that with the sowing depth of 8 cm, while the ear height, leaf length, and plant dry weight were 1.2%, 0.8%, and 1.5% lower, and the amplitude of increase and decrease was small.

Compared with the small sowing machine 2B-2, the medium-sized sowing machine ZHSB-10 had the ear height, stem diameter, and plant dry weight increased by 1.6%, 0.5%, and 0.2% respectively, and the plant height and leaf length reduced 0.9% and 1.1%, and the amplitude of increase and decrease was small. The results showed that with the growth of maize plant, the effect of sowing depth on the agronomic traits of maize gradually declined. However, the sowing equipment had little effect on the agronomic traits of the maize plants from the three-leaf stage, jointing stage, and late period of growth.

3.3Emptystalkrate,doubleearrate,lodgingrate,andlodgingandstembrokenrateofmaizeAccording to Table 4, the empty stalk rate, double ear rate, lodging rate, and lodging and stem broken rate of maize with the sowing depth of 5 cm were 3.1%, 4.2%, 4.15%, and 4.2% on average, while that of the maize with the sowing depth of 8 cm were 3.15%, 4.3%, 3.65%, and 3.7% respectively, showing that the sowing depth had little effect on the empty stalk rate, double ear rate, lodging rate, and lodging and stem broken rate. The empty stalk rate, double ear rate, lodging rate, and lodging and stem broken rate of maize sown by the medium-sized sowing machine ZHSB-10 were 3.05%, 4.35%, 3.65%, and 3.8% respectively, that of the small sowing machine 2B-2 were 3.2%, 4.15%, 4.15%, and 4.1% respectively, indicating that the sowing equipment had little effects on the empty stalk rate, double ear rate, lodging rate, and lodging and stem broken rate of mechanized sowing maize.

3.4MaizeyieldandcomponentsofyieldFrom Table 5, it can be seen that the theoretical yield of maize with the sowing depth of 5 cm was 2.14% higher than that with the sowing depth of 8 cm, and the ear length, ear diameter, rows per ear, kernels per row, and 1 000-grain weight of the maize with the sowing depth of 5 cm was 0.28%, 0.41%, 0.31%, 0.55%, and 0.99% higher than that maize with the sowing depth of 8 cm, and the bald ear length and axis diameter reduced by 2.8% and 0.33%, and the amplitude of increase in components of yield was small. These indicated that the sowing depth of 5 cm could slightly increase the yield of mechanized sowing maize and improve the most yield traits. The yield of maize sown by the medium-sized sowing machine ZHSB-10 was 0.32% higher than that sown by small sowing machine 2B-2, but both kinds of sowing machines had little effects on the maize yield and components of the yield.

Table4Emptystalkrate,doubleearrate,lodgingrate,andlodgingandstembrokenrateofmaizebydifferenttreatments

Sowingdepth∥cmSowingequipmentEmptystalkrate∥%Doubleearrate∥%Lodgingrate∥%Lodgingandstembrokenrate∥%5ZHSB?103．34．33．83．852B?22．94．14．54．68ZHSB?102．84．43．53．882B?23．54．23．83．6

Table5Yieldandtraitsofyieldofmaizebydifferenttreatments

Sowingdepth∥cmSowingequipmentEarlength∥cmBaldearlength∥cmEardiameter∥cmAxisdiameter∥cmRowsperear∥rowKernelsperrow∥kernel1000?grainweight∥gTheoreticalyield∥kg/667m25ZHSB?1017．80．684．883．0516．136．5304．5534．652B?218．20．714．863．0716．336．8301．7532．38ZHSB?1018．00．724．843．0916．236．2301．8522．882B?217．90．714．863．0516．136．7298．4521．7

4 Conclusions and discussions

The germination rate is an important index to measure the sowing depth[8]. Studies have shown that the sowing depth of 5 cm has higher germination rate and plant height uniformity than the sowing depth of 8 cm, which is consistent with research results of Cao Huiying[9]. It is also found that at the three-leaf stage, the agronomic traits of the sowing depth of 5 cm have much greater increase than the sowing depth of 8 cm. But with the growth of maize, the effects of sowing depth on agronomic traits of maize gradually become smaller. Besides, the sowing depth of 5 cm could slightly increase the yield of mechanized sowing maize and improve the most yield traits. Therefore, it is appropriate to select the sowing depth of 5 cm in the maize production.

Different sowing equipment has little effect on the germination rate and plant height uniformity of mechanized sowing maize, and it also has little effect on agronomic traits of maize at different growth stages. At late period of growth stage, different sowing equipment has little effect on the empty stalk rate, double ear rate, lodging rate, lodging and stem broken rate, yield and components of yield. In summary, it is recommended to select appropriate sowing equipment in accordance with actual situations, select small sowing machines in small fields and those with big slope and scattered land, and adopt medium-sized sowing machine in large flat fields with relatively centralized land.

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