Strip-width determines competitive strengths and grain yields of intercrop species in relay intercropping system

The study addresses how strip width in maize/soybean relay intercropping (MSR) influences interspecific competition and grain yields. Intercropping is widely practiced in developing countries but faces mechanization challenges. Wider strips could facilitate machinery use, yet may reduce intercropping advantages. Cereal–legume intercropping offers environmental and input benefits, but in MSR soybean often suffers from maize shading, reducing its yield. The study aims to determine an optimal strip width that enhances facilitation while reducing competition, formulating three hypotheses: (i) soybean yields are lower in narrow strips; (ii) increasing inter-row spacing increases soybean yield without reducing maize yield; (iii) wider strips are more productive and beneficial than narrow strips.

Prior studies report mixed effects of strip width in intercropping. Some found higher LER with wider strips (e.g., 3–6 m), suggesting easier mechanization and management, while more recent work indicates an optimal strip width around 1 m, with benefits decreasing beyond this. In maize/common bean systems, LER decreased from 1.55 to 1.27 when row ratios widened (2:2 to 5:5). Cereal–legume intercropping is advantageous for reducing inputs and improving environmental outcomes. In MSR, maize yields are often equal or higher than sole maize, soil quality improves, and nutrient availability increases. However, soybean frequently experiences shading-induced morphological and physiological constraints. Literature emphasizes the need to balance strip width for complementarity (light, water, nutrients) and mechanization practicality.

Location: Research Farm, Sichuan Agricultural University, Ya'an, Sichuan, China (29°59′ N, 103°00′ E), 2012–2013; subtropical humid climate; average temperature 16.2 °C; ~1200 mm rainfall. Soil: purple clay loam, pH 6.7; SOC 30.6 g/kg; available N 65.5 mg/kg, P 18.9 mg/kg, K 97.6 mg/kg (0–30 cm). Design: Five treatments, three replicates: SM (sole maize, 70 cm rows), SS (sole soybean, 70 cm rows), and MSR with three strip-widths: T1 narrow (180 cm): soybean and maize row spacing 40 cm, soybean–maize inter-row 50 cm; T2 medium (200 cm): 40 cm rows, soybean–maize inter-row 60 cm; T3 wide (220 cm): 40 cm rows, soybean–maize inter-row 70 cm. In MSR, two soybean rows were relay-intercropped with two maize rows 60±5 days after maize sowing. Plot sizes: T1 7.2×6 m, T2 8×6 m, T3 8.8×6 m. Varieties and planting: Maize (Chuandan-418), soybean (Gongxuan-1). Plant densities: maize 60,000 plants/ha (SM and MSR); soybean 1,000,000 plants/ha (SS and MSR). Maize within-row spacing: 24 cm (SM), 19 cm (T1), 17 cm (T2), 15 cm (T3). Soybean within-row spacing: 14 cm (SS), 11 cm (T1), 10 cm (T2), 9 cm (T3). Sowing: maize on Mar 28, 2012 and Apr 4, 2013; soybean on Jun 13 both years. Harvest: maize Aug 8, 2012 and Aug 2, 2013; soybean Oct 29, 2012 and Oct 28, 2013. Total system period 210±5 days; co-growth 60±5 days. Fertilization: Maize basal N 135 kg/ha (urea), P 40 kg/ha (calcium superphosphate), K 10 kg/ha (potassium sulfate). Soybean basal N 75 kg/ha, P 40 kg/ha, K 4 kg/ha. Topdressing: at maize V stage and soybean R1: N 135 kg/ha to soybean and 75 kg/ha to maize. Weeds controlled manually. Measurements: PAR transmittance at soybean canopy at V5, V7, R1 using LI-191SA sensors and LI-1400 logger, 11:30–12:30 h on clear days; PAR transmittance (%) = PAR at soybean top / PAR at maize top ×100. Leaf area index (LAI) at V5, V7, R1 from 10 plants per treatment using leaf length×width×0.75 coefficient; dry matter by oven drying (80 °C for 1 h then 65 °C to constant weight). Photosynthesis (Pn, Tr, Gs, Ci) measured on three fully expanded trifoliates per plot with LI-6400 between 11:00–12:00 at 400 µmol mol−1 CO2. Yield components: from 40 soybean plants and 24 maize ears per plot; grain number/plant, 1000-grain weight (mg), grain yield (kg/ha); total MSR yield = maize + soybean yields. Land equivalent ratio (LER): pLERm = SYim/SYsm; pLERs = SYis/SYss; total LER = pLERm + pLERs. Competition ratio (CR): CRm = (LERm/LERs)×(Asr/Amr); CRs = (LERs/LERm)×(Amr/Asr). Economic analysis: Partial budgeting; total expenditures (land rent, seedbed, seed, fertilizers, labor for thinning/weeding, harvesting, threshing), gross income = yield × market price (2012–2013), net income = gross − expenditures. Statistics: Statistix 8.1; means compared by LSD at p≤0.05.

- PAR transmittance increased with greater inter-row spacing in MSR; SS had the highest PAR. At R1 (average over two years), PAR transmittance was highest in T3 (≈57%) and lowest in T1 (≈31%). Pattern: SS > T3 ≈ T2 > T1. - LAI and dry matter of soybean increased with inter-row spacing; SS > T2 > T3 > T1. At R1, T2 increased soybean dry matter by 131% vs T1 (averaged across years). - Photosynthetic traits of soybean improved with PAR in MSR. Among intercropped treatments, T2 had the highest Pn, Tr, and Gs at V5, V7, and R1; T1 had the lowest. Ci was highest in T1 and lowest in SS. - Yield components and yields: In MSR, soybean achieved 52% (T1), 77% (T2), and 57% (T3) of SS yield; maize achieved 98% (T1), 98% (T2), and 87% (T3) of SM yield. Soybean yield trend: SS > T2 > T3 > T1. Total MSR grain yield was highest in T2 and lowest in T3; reported averages across years: max ≈8429 kg/ha (T2), min ≈7307 kg/ha (T3). Pattern: T2 > T1 > T3. - Land use advantage: LER >1 in all MSR treatments. Across two years, LER was highest in T2 (up to ≈1.76) and lowest in T3 (≈1.44), indicating greatest land advantage at 200 cm strip width. - Competition: Maize had higher competition ratios than soybean in all MSR treatments, but soybean CR improved at medium spacing. Averages across years: maize CR ≈2.01 (T1), 1.29 (T2), 1.53 (T3); soybean CR ≈0.55 (T1), 0.81 (T2), 0.66 (T3), reflecting better complementarity at 200 cm. - Economics: Average net income (US$ ha−1) over 2012–2013 was highest in T2 (≈1132) compared to T1 (≈636) and T3 (≈727); SS and SM had lower averages (≈572 and ≈584).

Findings strongly support hypothesis (i): soybean yield was lowest in narrow strips (T1) due to severe shading and reduced PAR. Hypothesis (ii) was partially confirmed: increasing inter-row spacing to 200 cm (T2) increased soybean yield without reducing maize yield (maize ≈98% of SM), indicating improved complementarity and reduced light competition while maintaining maize competitiveness. Hypothesis (iii) was partially rejected: the widest strips (T3, 220 cm) reduced both soybean and maize yields relative to T2, lowered LER, and diminished economic benefits, likely due to increased intraspecific competition from reduced within-row spacing, increased mutual shading within soybean, and less temporal/spatial complementarity. Overall, an intermediate strip width (200 cm) optimizes resource sharing (light, nutrients, water), enhances soybean performance while sustaining maize, maximizes LER and total yield, and delivers the best economic returns. These results underscore that inter-row spacing is a key design lever for balancing interspecific interactions in MSR.

A 200 cm strip width (medium strips) maximized competitive balance between maize and soybean, increased soybean yield while maintaining maize yield, and produced the highest land equivalent ratio and system grain yield in maize/soybean relay intercropping. Wider strips (220 cm) reduced intercropping advantages (yield, LER, and income). Achieving intercropping benefits at optimal (narrow-to-medium) strip widths may require small or adapted machinery for practical adoption. Future research should further refine strip geometry and explore mechanization innovations to sustain intercropping benefits.

The study did not measure water use of intercrop species; water-related conclusions are referenced from previous work. Results are from a two-year, single-location experiment and may require validation across environments. Practical mechanization constraints with narrow-to-medium strip widths are noted as a challenge for large-scale adoption.