The study was conducted in the Sudan Savannah Ecological Zone in the Upper East Region in Ghana, at CSIR-SARI station at Manga and the Presbyterian Agricultural Station (PAS), at Garu. The study was carried out at both locations from July to October, 2021 for the first year and from July to October, 2022 for the second year. Binduri District is located approximately between latitudes 11˚11′S and 10˚40′N and longitudes 0˚18′W and 0˚6′E in the North-Eastern corner of the Upper East Region. The second location, PAS, Garu District, is located in the north-eastern part of the Upper East Region between Latitude 10˚1′N and Longitude 0˚1′W.

Initial soil (0 cm - 30 cm depth) at Garu indicated soil chemical properties of pH 5.7. It contained major nutrients of N (0.02 mg/kg), P (1.5 mg/kg) and K (22.2 mg/kg). The minor nutrients were Ca²⁺ (0.8 cmol/kg) and Mg²⁺ (0.7 cmol/kg) with OC (0.5). The loamy sand contained sand (80.2%), clay (9.6%) and silt (10.2%) (Table 1). Similarly, initial soil analysis at Manga indicated soil chemical properties of pH 5.6. It also contained major nutrients of N (0.03 mg/kg), P (1.4 mg/kg) and K (24 mg/kg). The minor nutrients possessed were Ca²⁺ (0.9 cmol/kg) and Mg²⁺ (0.7 cmol/kg) with OC (0.5 mg/kg). The loamy sand contained sand (83.9%), clay (6.4%) and silt (9.7%) (Table 2).

Generally, the initial soil analysis for both Garu and Manga showed the soils were slightly acidic and that the nutrient level were low phosphorus, nitrogen and manganese. However, the initial soil analysis indicated that soil from both locations had medium organic carbon and potassium (CSIR-SARI, 2007). The soil belongs to the Varempare Series, which are mainly sandy loams associated with hornblende and granites, while the PAS soils belong to the Tafali Series, which is also suitable for the cultivation legumes, cereals and vegetables. The climate of both locations is characterised by a unimodal rainy season from May /June to October and a dry harmattan season from November to April. The average amount of rainfall during the period is between 800 - 860 mm per annum. The vegetation is mainly the Sudan Savannah Ecology.

The experimental design used for the study was a 3 × 3 factorial arranged in a Randomized Complete Block Design (RCBD) with 4 replications. The treatments consisted (A) row arrangements [(i) 1 row maize altenating with 1 row cowpea (1M:1C), (ii) 2 rows maize altenating with 2 rows cowpea (2M:2C) and (iii) 3 rows maize altenating with 3 rows cowpea (3M:3C)] and (B) relative times of planting [(i) Simultaneous planting of maize and cowpea (SIM), (ii) maize planted 2 weeks before cowpea (M2WBC) and (iii) cowpea planted 2 weeks before maize (C2WBM)]. Sole maize and sole cowpea crops were added for comparison, bringing the total number of treatments to eleven (11). Maize seeds were planted at 0.75 m × 0.4 m, while the cowpea seeds were planted 0.75 m × 0.2 m. Each plot consisted of 4 sets of the intercrops for the 1M:1C and 3 sets of intercrops for the 2M:2C and 3M:3C. The central two sets of the intercrops for 1M:1C and 1 set of intercrops for 2M:2C and 3M:3C served as the harvestable areas for the final yield and yield components assessment. The sole maize and cowpea plots were planted with 6 rows per plot. Row length was 5 m. The 2 central rows of the sole maize and cowpea served as harvestable rows. Improved medium-maturing maize variety (Wangdataa) and improved medium-maturing cowpea variety (Wangkae) were sourced from Manga.

The experimental fields at both locations were ploughed with a tractor and an animal-drawn mould board plough used in ridging. Ridges formed measured 0.75 m apart. Planting of maize and cowpea was done with hoes. The crops were planted 3 seeds per hill and later thinned to two plants per hill when plants were established. The sole maize and cowpea were planted when component crops were planted simultaneously. Manual weeding was done when necessary, till component crops were harvested. Basal application of Nitrogen Phosphorus Potassium (NPK) was applied to the maize crop at the rate of 68 kg N, 38 kg P₂O₅ and 38 K₂O at 14 DAP and top-dressed with urea at 65.2 kg ha⁻¹ at 50 DAP.

Inoxacarb (30 g/L and Acetamiprid (16 g/L), a systemic insecticide against armyworm and a wide spectrum of pests on maize and vegetables were used. The Viper 46 EC (0.4 L/Acre), equivalent to 40 mL/15L of water and (Inoxacarb, 30 g/L and Acetamiprid, 16 g/L) were sprayed. The spraying was carried out on both maize and cowpea plots across both locations and seasons. Generally, high farm hygiene was maintained to ensure maximum productivity.

Figure 1. Plant height of maize as affected by row arrangement and relative times of planting in a maize + cowpea intercrop at Garu and Manga in 2021 and 2022 growing seasons.

Generally, plant height, total dry matter, leave area index, photosynthetically active radiation and chlorophyll content were significantly (p < 0.05) affected by row arrangement, relative times of planting, location and year with significant interactions. Positive interaction of the simultaneously planted component crops also promoted the effective utilization of growth resources that affected its growth and development. According to [9], increased plant height provides more green area for increased photosynthetic activities, absorption and translocation of assimilates needed for effective grain filling to obtain higher yields. According to [10], maize is very sensitive to the spatial arrangement of the intercropping of component crops. The row arrangement automatically spaced the maize plants, hence less competition for radiation was experienced resulting in homogeneous maize height at early stages [11]. Significant differences and interactions were recorded due to available radiation [12][13]. Maize-cowpea intercropping influenced the maize photosynthetic rate and chlorophyll accumulation due to the complementarity effect of ecosystem services from cowpea to maize. Photosynthetic variables indicated high performance indicating that intercropping can significantly improve the photosynthetic rate and chlorophyll accumulation of maize plants [3][14]-[16]. Dry matter (DM) accumulation and its allocation to kernels are key factors that determine the final maize grain yield [17] (Figure 1Figures 1-5).

Figure 2. Leaf area index of maize as affected by row arrangement and relative times of planting in a maize + cowpea intercrop at Garu and Manga in 2021 and 2022 growing seasons.

Figure 3. Absorbed Photosynthetically Active Radiation by maize as affected by row arrangement and relative times of planting in a maize + cowpea intercrop at Garu and Manga in 2021 and 2022 growing seasons.

Figure 4. Chlorophyll content of maize leaves as affected by row arrangement and relative times of planting in a maize + cowpea intercrop at Garu and Manga in 2021 and 2022 growing seasons.

Figure 5. Total Dry Matter of maize as affected by row arrangement and relative times of planting in a maize + cowpea intercrop at Garu and Manga in 2021 and 2022 growing.

The combined analysis of maize across the two locations and years indicated that relative times of planting (RTP) and row arrangement (RA) significantly influenced the phenological development of maize. A higher plant establishment, early tasseling and silking are determined good maize yield. These phenological parameters were obtained due to favourable environmental conditions. However, maize tasselling significantly differed (p < 0.05) between the seasons in the first year with maize tasseling earlier (52 days) than the second season (53 days). This was due to the different environmental conditions experienced at both experimental sites. Favourable climatic conditions experienced at both locations during the study influenced phenological development [13][18][19] (Table 1 and Table 2).

Table 1. Days to 50% tasselling of maize as affected by row arrangement and relative times of planting at Garu and Manga in 2021 and 2022.

Note: †: 1M:1C = 1 row maize alternating with 1 row cowpea; 2M:2C = 2 rows maize alternating with 2 rows cowpea; 3M:3C = 3 rows maize alternating with 3 rows cowpea; #: M2WBC = Maize planted 2 weeks before cowpea; C2WBM = Cowpea planted 2 weeks before maize.

Table 2. Days to 50% silking of maize as affected by row arrangement and relative times of planting at Garu and Manga in 2021 and 2022.

Note: †: 1M:1C = 1 row maize alternating with 1 row cowpea; 2M:2C = 2 rows maize alternating with 2 rows cowpea; 3M:3C = 3 rows maize alternating with 3 rows cowpea; #: M2WBC = Maize planted 2 weeks before cowpea; C2WBM = Cowpea planted 2 weeks before maize.

There were significant effects and interactions of RA, RTP and Year on total grain yield of maize for both locations and seasons. Maize planted 2 weeks before cowpea and maize planted simultaneously produced the highest grain yield than yields from maize planted 2 weeks after cowpea for the two seasons and experimental sites. The intercrop plots generally produced maize yields that were higher than the sole crops indicating higher crop productivity and efficient land use cropping system. The average total yield per hectare of 8.18 t/ha for Garu and 5.52 t/ha for Manga is far above the average maize in the Sudan ecological zone of 3.5 t/ha. The dominant performance of maize for both locations and seasons were maize planted 2 weeks before cowpea and maize planted simultaneously. This was due to early establishment, high tasselling, silking and other growth parameters obtained that gave them competitive advantage over the late crops. There was also effective use of growth resources to promote phenological and vegetative growth and finally yield and yield components of component crops. Similarly, planting maize 2 weeks before cowpea gave the maize a head start before the late crop. The higher maize grain yield obtained from the study in both locations and years could also be attributed to adequate and stable distribution of rainfall during the vegetative, tasselling, silking and grain filling stages of the maize plant because these are the growth stages that require adequate water for vigorous plant growth and efficient grain filling as reported by [19]-[21]. Maize yields obtained suggest that the overall productivity of the intercrop system was superior to the sole crop. Earlier studies confirm these findings [2][8] (Table 3).

Table 3. Total Grain Yield of Maize as affected by row arrangement and relative times of planting at Garu and Manga in 2021 and 2022.

Note: †: 1M:1C = 1 row maize alternating with 1 row cowpea; 2M:2C = 2 rows maize alternating with 2 rows cowpea; 3M:3C = 3 rows maize alternating with 3 rows cowpea; #: M2WBC = Maize planted 2 weeks before cowpea; C2WBM = Cowpea planted 2 weeks before.

Plant height of cowpea was significantly affected by row arrangement and relative times of planting at Garu and Manga in the 2021 and 2022 growing seasons. The intercrop system had no influence on plant height until plants attained 41 DAP where plant height was influenced largely, leading to significant variations among RA, RTP and Year. Plant height is genetically inclined than treatment combinations [22][23]; however, creating a favourable environment can enhance the performance and growth variables such as plant height due to genetic × environment interaction. Cowpea planted simultaneously with maize and sole cowpea produced the tallest plants from 41 DAP onwards throughout the data sampling period due to early establishment, as they were planted at the same time, and the advantage of a head start over the cowpea planted late. Cowpea planted simultaneously grew taller than all the other treatments due to the benefit of ecosystem services from maize and minimal competition. Cowpea height started slowly and increased steadily to a height of 35 cm for the sole cowpea. This finding is in line with [4]. Taller cowpea plants have their pods above the ground, preventing pod soil contact at maturity, thus reducing post-harvest yield losses. Pods hanging above the ground help in effective harvesting to reduce post-harvest losses and contaminated cowpea grain. Total dry matter of cowpea recorded significant differences for both Garu and Manga in both 2021 and 2022 cropping seasons. Total dry matter accumulation indicated that cowpea planted 2 weeks before maize and sole cowpea grew vigorously and accumulated more total dry matter than those planted simultaneously and those planted 2 weeks after maize throughout the experimental period. The dominance of cowpea planted 2 weeks before maize and sole cowpea was due to early planting, the head start advantage, coupled with minimal competition for growth resources. The cowpea that were grown simultaneously and those planted after maize faced negative intra and inter competition from maize. This clearly indicates that the maize-cowpea intercrop productivity and stability study had a high impact on the amount of dry matter accumulated, as large biomass would normally produce high total dry matter [22]. Absorbed photosynthetically active radiation did not show any significant differences at 41 and 51 DAP of cowpea, but showed significant differences at 61 DAP for both Garu and Manga in both 2021 and 2022 cropping seasons. The initial absorbed photosynthetically active radiation was similar at 41 DAP, declined at 51 DAP, but increased at 61 DAP when cowpea plants were approaching physiological maturity at Garu in 2021 and 2022. Absorbed Photosynthetically Active Radiation by cowpea at manga in 2022 was more diverse with 3M:3C+M2WBC and 3M:3C+SIM producing higher Absorbed Photosynthetically Active Radiation than all the other treatment combinations. This could be attributed to the wide leave architecture of the 3M:3C row arrangements [10] and the architecture of the row arrangement, coupled with planting both component crops simultaneously. Absorbed photosynthetically active radiation measurement is very important to evaluate agricultural land use efficiency and crop productivity. Efficiently absorbed photosynthetically active radiation is a prerequisite for productive farmland use measurement. Photosynthesis rate can be measured using absorbed photosynthetically active radiation measurement to quantify its contribution to total crop yield [13]. Absorbed photosynthetically active radiation is a determinant of crop yield. After podding, the pods and the developing kernels become sinks and therefore need a lot of assimilates or photosynthates from the sources for proper kernel development and maturity. Chlorophyll content of cowpea registered significant differences at 41 and 51 DAP but showed no significant differences at 61 DAP for both Garu and Manga in both 2021 and 2022. Production of chlorophyll was largely homogeneous for cowpea at both locations and seasons. The differences that occurred at the initial stages of growth could be attributed to the available radiation with less shade from the maize from 41 - 51 DAP. The level of chlorophyll content in cowpea is attributed to differences in treatment combinations and environmental variables [10][13]. This was also supported by [13] Leaf area index (LAI) of cowpea varieties is influenced by the plant architecture, spreading or prostrate, erect or semi-erect leave area index of cowpea did not record any significant differences at 41, 51, and 61 DAS for both Garu and Manga in both 2021 and 2022 cropping seasons. About 72% of plots recorded leaf area index more than 1, thus enhancing flowering, podding and increased yield of cowpea. According to [20][21], a leaf area index between 1 and 2 m² is required after flowering to obtain optimum yield of cowpea (Figure 3Figures 3-10).

Figure 6. Plant height of cowpea as affected by row arrangement and relative times of planting in a maize + cowpea intercrop at Garu and Manga in 2021 and 2022 growing seasons.

Figure 7. Total Dry Matter of cowpea as affected by row arrangement and relative times of planting in a maize + cowpea intercrop at Garu and Manga in 2021 and 2022 growing seasons.

Figure 8. Absorbed Photosynthetically Active Radiation by cowpea as affected by row arrangement and relative times of planting in a maize + cowpea intercrop at Garu and Manga in 2021 and 2022 growing seasons.

Figure 9. Chlorophyll content of cowpea leaves as affected by row arrangement and relative times of planting in a maize + cowpea intercrop at Garu and Manga in 2021 and 2022 growing seasons.

Figure 10. Leaf area index of cowpea as affected by row arrangement and relative times of planting in a maize + cowpea intercrop at Garu and Manga in 2021 and 2022 growing seasons.

Percentage plant establishment at both experimental sites recorded an average of 97% for Garu and 96% for Manga during the experimental period. At 38 DAP, cowpea flowered and at 43 DAP 50% podding was archived. Early flowering normally enhances crop yield. Conditions that enhanced the phenological development during the study included adequate rainfall, temperature and day length. Plant establishment registered significant differences and interaction for Year and RTP. According to [23], temperature, water availability, light quality, photoperiod, altitude, and mineral nutrition determine effective and adequate percentage plant establishment. Days to 50% flowering of cowpea were significantly affected by RTP and RTP × Y interaction at both locations and seasons. Early flowering at 38 DAP at Garu was enhanced by the adequate rainfall experienced throughout the study period [22][23]. This explains why row arrangement and relative times of planting had limited interactions and variations for days to 50% flowering of cowpea. There was no significant effect of RA on days to 50% to podding of cowpea, but there were significant and interaction effects of RTP and Year on days to 50% to podding for both seasons at Garu and Manga. Podding comes after flowering sets in. The earliest flowering occurred at 38 DAP, while the earliest podding started at 43 DAP by 2M:2C + C2WBM and 3M:3C + M2WBC at Garu in the 2022 cropping season (Table 5). A maximum of 5 days came between flowering and pod formation. Effective podding was influenced by early flowering, as the same treatment flowered at 38 DAP. However, podding of cowpea is reported to vary genetically, but not due to environmental influences [22][23]. (Table 4 and Table 5).

Table 4. Days to 50% flowering of cowpea as affected by row arrangement and relative times of planting at Garu and Manga in 2021 and 2022.

Note: †: 1M:1C = 1 row maize alternating with 1 row cowpea; 2M:2C = 2 rows maize alternating with 2 rows cowpea; 3M:3C = 3 rows maize alternating with 3 rows cowpea; #: M2WBC = Maize planted 2 weeks before cowpea; C2WBM = Cowpea planted 2 weeks before maize.

Table 5. Days to 50% to podding of cowpea as affected by row arrangement and relative times of planting at Garu and Manga in 2021 and 2022.

Note: †: 1M:1C = 1 row maize alternating with 1 row cowpea; 2M:2C = 2 rows maize alternating with 2 rows cowpea; 3M:3C = 3 rows maize alternating with 3 rows cowpea; #: M2WBC = Maize planted 2 weeks before cowpea; C2WBM = Cowpea planted 2 weeks before maize.

There were highly significant effects of row arrangement, relative times of planting and year, but no significant interactions on total grain yield of cowpea during the data collection period. The treatment that dominated in the cowpea yield is row arrangement, when cowpea was planted 2 weeks before maize, followed by cowpea planted simultaneously, which was significantly higher than cowpea planted 2 weeks after maize. Cowpea planted before maize and cowpea planted simultaneously with maize tend to start growth early, spread their leaves widely and attract more radiation for high photosynthates or assimilate production, hence effective phenological and vegetative growth and consequently higher yields. Cowpea planted 2 weeks before maize produced the highest cowpea grain yield of 3.22 t/ha at Garu, less than the cowpea grain yield of 3.79 t/ha obtained at Manga. These results were due to the different environmental conditions experienced at both experimental sites. The early cowpea also had a head start for cowpea planted 2 WBM, allowing component crops to effectively use the available growth resources. The yields of the intercrop cowpea were higher than the sole cowpea because of the interaction effect of component crops, which the sole cowpea did not experience. These findings contradicted Several previous studies that reported lower intercrop yields for cereal-legume trials [10][24][25]. According to [14], competition among component crops for growth factors such as light, available water and nutrients, and root space is among the crucial factors that determine the variations in their productivity levels. [26][27] also stated that crop yield is strongly tied to competition between intercrops. This means the competitiveness is negative, crop yield would reduce, but minimal competition may not affect crop yield (Table 6).

Table 6. Total Grain yield of cowpea as affected by row arrangement and relative times of planting at Garu and Manga in 2021 and 2022.

Note: †: 1M:1C = 1 row maize alternating with 1 row cowpea; 2M:2C = 2 rows maize alternating with 2 rows cowpea; 3M:3C = 3 rows maize alternating with 3 rows cowpea; #: M2WBC = Maize planted 2 weeks before cowpea; C2WBM = Cowpea planted 2 weeks before maize.