2.1 Experimental site and Season The experiment was conducted at Regional Agricultural Research Station, Bangladesh Agricultural Research Institute (BARI), Ishurdi, Pabna during two consecutive years, 2014 and 2015 (Kharif-1 season). The experimental site was situated at approximately 24° 07‘ N and 89° 04‘ E with an altitude of 13.72 m above mean sea level and it belongs to the Agro-ecological Zone 11 (High Ganges River Flood plain) in Bangladesh (FRG, 2012). The initial soil sample (0–15 cm depth) was analyzed at the Soil Resources Development Institute (SRDI), Rajshahi, Bangladesh. The soil of the experimental site was medium-high and clay loam texture having 1.22% organic matter, pH 7.40, 0.07% total nitrogen (N), 0.36 meq 100 g-1 soil potassium (K), 10.6 ppm phosphorus (P), 7.2 ppm sulfur (S) and 1.13 ppm zinc (Zn).
The meteorological data were collected from Bangladesh Sugar Crops Research Institute (BSCRI) far away 400 m from our experimental field. The climate of the experimental site was subtropical. Daily rainfall, maximum temperature and minimum temperature during the study period (March–July 2015 and 2016). The weather of the experimental site is a hot sub-humid with total rainfall of 886 mm in 2015 and 636 mm in 2016 during crop season, frequent rainfall occurred from June to July in both years. The average daily minimum and maximum temperatures were 25 °C and 30 °C in the first year and 25 °C and 31 °C in the second year during the growing period. However, the differences in minimum and maximum temperatures were slightly less in the first season than in the second season.
2.2 Experimental design and treatments The experiment was laid out in a randomized complete block design with three replications to evaluate the performance of maize and spinach intercropping under different row spacing for higher profitability through economic assessment and competition functions. The unit plot size was 12 m2 (3 × 4 m). In the study, maize was the main crop and Indian spinach was intercrop. The experiment consisted of five treatments viz., T1: hybrid maize normal row (75 × 20 cm) along with one row of IS (plant to plant 25 cm), T2: hybrid maize paired row (37.5/150 × 25 cm) along with one row of Indian spinach (plant to plant 25 cm), T3: hybrid maize paired row (37.5/150 × 20 cm) along with two rows of Indian spinach (40 × 25 cm), T4: hybrid maize paired row (37.5/150 × 20 cm) along with three rows of spinach (40 × 25 cm) and T5: sole maize (75 × 20 cm). Indian spinach was intercropped between single and paired rows of maize. It is mentioned that between the maize paired kept 37.5 cm space, and space between the two pair maize were kept 75 cm space.
2.3 Crop management Maize (BARI Hybrid Bhutta-9) and Indian spinach (BARI Puishak-2) were selected as test crops. The variety of both crops are chosen as they are suitable and high yielding for growing in the summer season. Seeds were collected from Bangladesh Agricultural Research Institute, Bangladesh. The crops (both maize and Indian spinach) were sown on 20 March during 2015 and 2016 with North-South row orientation. The seed rate was used @ 25 kg ha-1 and 1 kg ha-1, respectively in maize and Indian spenich. Fertilizers were applied at the rate of 250–75– 110–40–4–1 kg ha-1 of N, P, K, S, Zn and B, respectively for sole maize, and extra 40 kg ha-1 N for intercrop as urea, triple superphosphate (TSP), muriate of potash (MoP), gypsum, zinc sulphate and boric acid (FRG, 2012). One-third of urea and all other fertilizers were applied as basal and the remaining of two-thirds of urea was top-dressed in two equal splits at 20 and 40 days after emergence. Two hand weeding was done before top-dressed urea. A post sowing irrigation was done for proper germination and seedling establishment, and thereafter, three irrigations were applied at 20, 40 and 60 days after emergence. The infestation of insects and disease was less in maize, so no pesticides were applied, but rovral 50 WP @ 2 g liter-1 of water was sprayed at 50 DAE to control Cercospora leaf spot in spinach. The vine of spinach was harvested for vegetable four times from 25 May to 05 July in both years. Maize was harvested on 8 July and 10 July 2015 and 2016 respectively.
2.4 Assessment of economic indices The economic analysis was done considering total variable costs (TVC) and gross returns (GR). The variable costs included human labour, machinery rent and production inputs (seed, fertilizer, pesticides). Gross returns were calculated by multiplying the economic yield of crops by the price at harvest time. Gross margin (GM) was calculated as the difference between GR and TVC (GM = GR - TVC). Various competition functions such as maize equivalent yield (MEY), land equivalent ratio (LER), land equivalent co-efficient (LEC), monetary advantage (MA), area-time equivalent ratio (ATER), income equivalent ratio (IER) and per day return (PDR) were worked out to find out the benefit and the effect of competition between the treatments. The competition functions were calculated by using the formula.
2.5 Data processing and analysis Data on yield and other parameters of both the crops were recorded at the time of harvest in manually. For maize, ten plants were randomly selected in each plot and tagged for the data collection on plant height, number of cobs per plant, number of grains per cob, cob length, grain yield and straw yield. The cobs were cleaned, dried and then it was opner by a maize hand opener for the separation of grains. The threshed grains of maize were dried under the sun and maintained 12% moisture content using frequent observation. In case of spinach, fresh weight (vegetable) was taken for calculating yield. Yields of both the crops were taken from the whole plot basis and converted into tons per hectare. Collected data were analyzed (combined analysis) statistically using ‘R’ software (R Core Team, 2016) and mean separation was done as per least significant difference (LSD) at 5% level of significance. M&M should be: Accurate, Brief, Clear to allow reproduction of the experiment.