Zhu WANG， Jizhi YANG， Wenyu YANG*

1． Chengdu Vocational College of Agricultural Science and Technology, Chengdu 611130, China;

2． College of Agronomy, Sichuan Agricultural University, Ya’an 625014, China

The wheat/maize/soybean relaycropping system,characterized by simplicity, high-efficiency and sustainability,is developed on basis of wheat/maize/sweet potato system to adapt to the south of China with extensive dry lands and abundant heats, in order to further improve soybean yield and relieve soybean shortage in domestic market[1-2].Hence,the sowing term of preceding crop(maize)has effects on intergrowth stage of succeeding crop (soybean) and maize growth with different densities affects competition with soybean in terms of water, fertilizer and sunlight[3-4]. Therefore, to guarantee strong seedlings growing in order by deployment of maize density and adjustment of sowing term lays foundation for seedling growth in late period and it becomes crucial to control maize intergrowth with soybeans. In the test, Lin’ao No.4,a local major relay-cropping variety, was taken as materials to explore the effects of sowing term and density on stem and leaf, as well as yield of soybeans, in order to further improve wheat/maize/soybean relaycropping system, providing theory and technology supports for production.

Materials and Methods

Test materials

Chuannong No.19, a wheat variety, was sown on November 2 and harvested on May 21; Nandou 02-4, a soybean variety,was sown on June 13 and harvested on October 20; Lin’ao No. 4 was a maize variety used in the test. The test soils were purple soils,containing organic matter at 29.32 g/kg, total N at 1.67 g/kg, total P at 0.83 g/kg, total K at 29.455 g/kg,available N at 163.438 mg/kg, and available P at 27.451 mg/kg and available K at 104.269 mg/kg.

Test design

The test used two-factor split plotdesign. Specifically, maize sowing term (A) was a main-treatment, five treatments were arranged, including A1 on March 21, A2 on March 28, A3 on April 4, A4 on April 11and A5 on April 18 and maize density (B) was a sub-treatment, with three treatments,involving B1 of 37 500 plant/hm2,B2 of 52 500 plant/hm2, and B3 of 67 500 plant/hm2. The test included 3 repetitions, and a compartment at 2 m, with maize and wheat at 1 m, respectively,and the test area was 5 m×2 m. It is notable that different treatments included two planting zones. Furthermore, the wheat density was 1.3 million basic seedlings per hectare, and base fertilizer contained organic fertilizer at 6 000 kg/hm2, urea at 280 kg/hm2, and potassium chloride at 60 kg/hm2. Soybean density maintained at 247 500 plant/hm2, with linewidth at 33 cm and soybean-bush distance at 18.5 cm,and the soybean was sown 3 lines per compartment. The base fertilizers contained urea at 60 kg per hectare, superphosphate at 450 kg,and potassium chloride at 60 kg, and additional fertilizer was urea at 60 kg/hm2after rainfall in early flowering stage. As for maize, the linewidth was 50 cm,and maize-bush distances were 53.4 cm (B1),38.1 cm (B2)and 29.6 cm (B3).The base fertilizers included 330 kg urea, 750 kg superphosphate 150 kg potassium chloride,as per hole application. After the fertilization, 450 loads of liquid dung were applied per hectare and additional urea fertilizers at 330 kg were applied per hectare in bell stage. Specifically, urea was dissolved in 450 loads of liquid dung and harvested on July 24 (A1), July 28(A2), August 9 (A3), August 14 (A4)and August 16 (A5), respectively. The other management was the same as local methods, and water maintained the same during the whole growth stage.

Table 1 Classification of soybean development stages

Measurement items and methods

Growth stage and intergrowth survey As per Water R. Fehr method(Table 1), soybean growth stages and the intergrowth term with maize were recorded.

Measurement of stem and leaf

During the periods of V3 (compound leaf),R1,R2,R3,R4,R5,R6,and R7,plant height and stem diameter of randomly selected 8 soybean plants were measured,of which 5 plants were chosen to measure leaf area as per digging and weighing method. Subsequently, leaves of the 5 plants were bagged,and killed out at 105 ℃for 30 min, followed by drying at 80 ℃to a constant weight. Finally, specific SLW(SLW =leaf dry weight/leaf area) can be computed.

Measurement of maize and soybean yields After maize and soybean grew maturely, yields were measured in different regions to analyze comprehensive profits.

Results and Analysis

Effects on growth stage structure

As shown in Table 2, with early sowing at a high density,the germination period reduced of soybean, but the differences were not so significant.In terms of growth stage of treated soybeans, early sowing of maize would guarantee harvesting before soybean flowering or during early blooming season, in order to reduce intergrowth period of soybean and maize and avoid shading of soybean in growth stage. According to analysis of variance, during the periods of A1 and A2, soybean growth and intergrowth(with maize) stages differed insignificantly, but both were smaller compared with A3, and extremely smaller compared with A4 and A5. For the effects on intergrowth stage of soybean and maize,the effects of maize sowing term have achieved extremely significant level (F=341.179**), and the maize density and interplay between sowing stage and density have little effects on intergrowth stage. With postponing of maize sowing term, intergrowth stage grew dramatically and late-sowing(A5)was 23 d more compared with early sowing (A1). In the whole growth and development stage of soybean, the shrinkage of intergrowth stage with maize actually increased self-growth time.

Effects on soybean stem and leaf shapes

Effects on plant height As shown in Fig.1, except of V3, height of soybean was increasing as sowing term of maize postponed and the increase rate proved higher by late sowing,compared with early sowing. The height showed insignificant differences in A1 and A2,which were below compared with A3, A4 and A5. In R7, the height of A1 was just 72.6 cm, shorter compared with A5 at 31.27 cm. These indicated that early sowing of maize is conductive to soybean growth.

On the other hand, maize densities also have insignificant effects on soybean height, but soybean height was growing as maize density increased before R3, when plant height increase rates were B3＞B＞2B1.In late growth stage of soybean, however,after maize harvest, the effects of maize density on soybean height proved insignificant.

Effects on stem diameter As shown in Fig.2, as sowing term of maize postponed, stem diameter of soybean was declining, early sowing of maize is conductive to lateral growth. During the period of R7, the stem diameter of A1 proved thicker compared with A2, A3, A4 and A5 by 0.58%, 25.1%, 26.58% and 36.16%.Among different treatments, the in-crease of soybean stem diameter was the most during R2-R3, and maintained higher of A1 and A2 during R5-R6. With different maize densities,stem diameter of soybean was decreasing as maize density grew before R2,which was not well maintained after maize harvesting; the stem diameter differences of soybean showed large differences; stem diameter proved the largest during R2-R3, which coincided with maize sowing period.

Table 2 Components of growth period of soybean intercropping

Effects on leaf area index As shown in Fig.3, as growth stage advanced, leaf area index was growing,and reached the peak in R4. It is notable that leaf area index was the highest during V3-R1. The increase rate of soybean leaf area index showed insignificant differences during maize sowing period,but declined stably by early sowing and substantially by late sowing. In A1 and A2R7 phases,leaf area index reduced by 10.6%and 12.7%compared with R4,and reduced by 49.5%, 67.9% and 71.3% in A3, A4 and A5 stages. it can be concluded that early sowing of maize reduces leaf area index in late growth phase of soybean, which is beneficial for maintenance of photosynthesis.With different maize densities, before R2,leaf area index of soybean was increasing upon the density,but the differences were not so significant; LAI values showed B1＞B2＞B3 during V3-R1 or R4-R7 periods.

Effects on specific leaf weight It can be concluded from Fig.4 the specific leaf weight changed from increasing to decreasing of soybean, and reached the peak in R5. With different maize sowing terms, the specific leaf weight was decreasing as sowing term postponed. Except of V3, the specific leaf weight of A1 kept higher compared with A4 and A5. In R5, the specific leaf weight of A1 was 1.1,1.2,1.4 and 1.5 times as high as A2, A3, A4 and A5. With maize densities, specific leaf weight of soybean showed insignificant differences, but the decrease in late B1 was larger compared with B2 and B3.

Table 3 Yield of soybean and maize

Yield and output value of soybean and maize

According to analysis of variance,the effects on soybean yield were as follows: maize sowing term (F =444.304**)＞maize density(F=42.948**)＞sowing term×density (F=3.96ns) and the effects on maize yield were maize sowing term (F=146.712**)＞maize density (F=45.358**)＞sowing term×density (F =2.276ns). As shown in Table 3, soybean yield declined as maize sowing term postponed, and soybean yield by early sowing was extremely significantly higher compared with rest sowing terms. For example,the yield was 6.76%, 23.94%, 33.06%and 38.69%higher compared with A2,A3,A4 and A5.As for maize yield, the earlier sowing term,the high yield. For instance, maize yields in A1 and A2 were extremely significantly highercompared with A3,A4 and A5.It is obvious that early sowing of maize would improve yields of both soybean and maize.

It can be concluded from Table 4 that by early sowing of maize,the output values of soybean and maize were both extremely significantly higher compared with late sowing.Under different maize densities, for example,the output value differences of soybean at low and moderate densities were not so significant, which were both far higher compared with high density, and the of maize showed insignificant differences with moderate or high densities, although both were higher compared with low density.Therefore, with consideration of both maize and soybean output values,A1B2, namely early sowing of maize with density at 52 500 plant/hm2,should be chosen to get high yield of a year.

Discussions and Conclusions

For the wheat/maize/soybean relay-cropping system, with maize sown earlier,the damages of ground insects would reduce and the growth term would extend, creating suitable temperature and humidity for soybean seedling germination, and relieving competition with soybean. What’s more, maize can be harvested ahead of time,reducing intergrowth term with soybean, and promoting soybean growth. On basis of correlation analysis, the intergrowth period of soybean and maize is of negative correlation with soybean yield (R=-0.948 5**)and intergrowth term of positive correlation with soybean yield(R=0.942 1**).It is clear that the effects of maize density on soybean stem and leaf can be observed before soybean flowering, which disappeared after maize harvesting.

Researches of relay intercropping available just concentration on the effects of the same crop with different sowing term on self-growth[5]and less attention is paid to the effects of sowing term of preceding crop on succeeding crop growth[6-7]. The test indicated that early sowing of maize,seedling of soybean would germinate earlier in order;the growth rate of plant height is slow during V3-R1; maize can be harvested before soybean flowering,for the compensation capacity in late period performs the strongest.Because early sowing of maize provides a growth environment for soybean growth and metabolism. In late period, soybean yield is far higher compared with that by late sowing.With consideration of both soybean and maize, the best group should be early sowing of maize ( on March 21)with density at 52 500 plant/hm2.

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