Time of transplanting and spacing effects on growth and yield of rice (Oryza sativa L.) under rainfed in the Northern region of Ghana

Abstract

Rice productivity in the Guinea Savannah zone of Ghana is constrained by inappropriate transplanting age and plant spacing, which affect growth and yield formation. This study evaluated their combined effects on rice performance under rainfed conditions. A field experiment was conducted using a 4 × 5 factorial arrangement in a randomized complete block design with three replications. Treatments consisted of four planting spacings (20 × 20, 25 × 25, 30 × 30, and 35 × 35 cm) and five transplanting ages (14, 21, 28, 35, and 42 days after planting). Data were collected on growth traits, yield components, and grain yield. Significant interactions were observed between transplanting age and spacing. The highest biomass (2.16 kg mm⁻²) was recorded at 14 days after planting with 20 × 20 cm spacing. Panicle production peaked at 21 days after planting under 35 × 35 cm spacing. The maximum grain yield (7,810 kg ha⁻1) was obtained at 21 days after planting combined with 20 × 20 cm spacing. The findings indicate that rice yield is strongly influenced by the interaction between transplanting age and spacing. Early transplanting at 21 days combined with optimal closer spacing enhances grain yield, suggesting that integrated crop management practices are essential for improving rice productivity in the Guinea Savannah zone of Ghana.

1 Introduction

Rice production plays a pivotal role in sustaining livelihoods and reducing malnutrition globally (Fukagawa and Ziska, 2019). Approximately 155 million hectares of rice are cultivated worldwide across diverse ecological systems (United States Department of Agriculture, 2023), of which about 75% is under rainfed conditions (Ulzen et al., 2023). Similarly, in Ghana, rice cultivation is predominantly concentrated in lowland rainfed ecologies, which serve as the primary source of national production (Azumah and Adzawla, 2017). Global rice production continues to increase, primarily driven by productivity gains. At present, China and India are the world's leading producers of rice (United States Department of Agriculture, 2023). In Africa, rice production has also expanded significantly, largely in response to rising consumption; however, much of this growth is attributable to the expansion of cultivated land rather than improvements in yield (Yuan et al., 2024). Despite these efforts, the continent currently produces only about 60% of its total rice requirements, with average yields estimated to be approximately 49% lower than the global average. Yield differentials are largely influenced by the specific rice-growing ecology. Globally, average rice yield is estimated at 6 t ha⁻¹, whereas yields in upland systems average 4 t ha⁻¹ and those in irrigated systems can reach up to 8 t ha⁻¹ (Muthayya et al., 2014).

In 2017, Ghana produced approximately 721,610 tons of rice, while national consumption was estimated at 1.3 million tons (International Food Policy Research Institute (IFPRI), 2020) depicting a woeful gap. This production-consumption gap has compelled the Government of Ghana to rely heavily on rice imports, primarily from Thailand, Vietnam, and India, resulting in significant financial outflows (International Food Policy Research Institute (IFPRI), 2020). For instance, the value of rice imports was estimated at US$1.2 billion in 2015 (International Food Policy Research Institute (IFPRI), 2020). The average annual rice yield in Ghana was 2.96 t ha⁻¹ in 2018, which falls below the global average, with even lower yields reported for the Northern Region (2.6 t ha⁻¹) Ministry of Food and Agriculture (MoFA) (2019). The Northern Region nonetheless remains the country's principal rice-producing area, accounting for about 37% of national rice output in 2012 (Ragasa et al., 2013).

A substantial proportion of Ghana's land under rice cultivation is located in the Northern Region, covering an estimated 81,165 hectares (Brief G. R. M. P., 2020). However, according to (Ministry of Food and Agriculture (MoFA), 2019), no district within the Northern Region has consistently achieved yields above the national conditions. Plant density plays a critical role in determining rice growth and productivity. Excessive plant density beyond the optimal threshold reduces growth and grain yield due to heightened competition for soil nutrients and solar radiation (Dzomeku et al., 2016). Conversely, appropriate spacing facilitates efficient resource use, enabling plants to access sufficient nutrients and sunlight, thereby enhancing growth, development, and yield. Indeed, Xu et al. (2020) reported that optimal spacing significantly maximizes rice yield compared to other spacing arrangements. However, scholarly work examining rice planting technologies in the northern region of Ghana has explicitly compared transplanting with other crop establishment methods such as broadcasting and dibbling among smallholder farmers. These studies report a very low adoption rate of seedling transplanting, a situation that contributes to the persistently low rice yields recorded annually in the country (Amfo et al., 2023). Furthermore, (Yussif et al., 2024), in a study conducted in Northern Ghana, confirmed that broadcasting remains the dominant establishment method, largely due to tradition as well as labor and cost considerations, even though transplanting offers greater yield potential. This overreliance on traditional methods could be reduced through research that identifies and promotes optimal transplanting age and appropriate spacing, thereby providing farmers with practical, evidence-based recommendations to enhance productivity. The present study was designed to assess the influence of transplanting ages and spacing on rice growth and yield.

2 Materials and methods

2.1 Site description

The experiment was conducted during the 2021 cropping season under rainfed conditions at Sagnarigu- Kukuo in the Sagnarigu District. The site is located within the Guinea Savanna Agro-Ecological Zone of Ghana. Sagnarigu is characterized by a unimodal rainfall pattern, which occurs between April and November, with mean annual rainfall ranging from 1,000 to 1,200 mm and peaking in September. Temperature distribution is relatively uniform throughout the year, with average monthly minimum and maximum values of 21.9 °C and 34.1 °C, respectively. Relative humidity in the area ranges from 46 to 76.8% (Savannah Agricultural Research Institute, 2007). The vegetation of the experimental sites is predominantly composed of extensive grasslands interspersed with scattered trees. The landscape is characterized by drought-tolerant tree species such as Acacia, mango (Mangifera indica), baobab (Adansonia digitata), shea nut (Vitellaria paradoxa), dawadawa (Parkia biglobosa), and neem (Azadirachta indica) (Dugje et al., 2012). In 2021, between March and November, the study location recorded an average annual rainfall of 82.6 mm, with peak precipitation occurring in August. Temperature during this period ranged between 23.8 °C and 34.4 °C, while relative humidity fluctuated between 52.5 and 75% (Savannah Agricultural Research Institute (SARI), 2022). The soils of the area are sandy loams with a near-neutral to moderately acidic reaction (pH 5.6-6.0). They are pale in color, poorly drained, and range from shallow to moderately deep. Seasonal variability strongly influences their condition: they become waterlogged during the wet season and excessively dry in the dry season. Overall, the soils are inherently low in fertility, offering limited nutrient reserves for crop production.

2.2 Experimental design

The experiment was arranged in a 4 × 5 factorial design and replicated three times. The treatments consisted of four plant spacing levels (20 cm × 20 cm, 25 cm × 25 cm, 30 cm × 30 cm, and 35 cm × 35 cm) and five transplanting ages (14, 21, 28, 35, and 42 d after planting). The experimental field covered a total area of 1,260 m² (35 m × 36 m). Each block measured 105 m² (35 m × 3 m), while individual treatment plots measured 21 m² (7 m × 3 m). Alleys measuring 1 m separated adjacent treatment plots within a block, and 2 m alleys separated adjacent blocks. To minimize water movement between plots, bunds were constructed around each treatment plot. Treatments were assigned using a randomized complete block design (RCBD) with three replications. The test variety used for the experiment was Agra.

2.3 Data collection

Data was collected on plant height, number of tillers per plant, number of days to 50 % flowering, number of panicles per plant, number of spikeletes per panicle, straw yield and grain yield where grain weight was determined at 14% moisture content using a digital scale, and yield was subsequently expressed in kilograms per hectare (kg ha⁻¹) following the procedure described by Paudel (1995).

2.4 Data analysis

Data collected from each field experiment were subjected to analysis of variance (ANOVA) using GenStat statistical software, 18th edition. Treatment means were separated using the least significant difference (LSD) test at the 5% probability level.

3 Results

3.1 Plant height

Time of transplanting and spacing significantly (P < 0.05) influenced plant height at both 42 and 63 d after planting (DAP). At 42 DAP, mean plant height ranged from 28.4 cm to 46.3 cm. The tallest plants (46.3 cm) were recorded in plots spaced at 35 cm × 35 cm where seedlings were transplanted at 28 DAP (Table 1). In contrast, the shortest plants (28.4 cm) were obtained from the 30 cm × 30 cm spacing with transplanting at 42 DAP.

At 63 DAP, mean plant height varied from 59.8 cm to 109.8 cm. Similarly, the highest plant height (109.8 cm) was observed in plots spaced at 35 cm × 35 cm with seedlings transplanted at 28 DAP (Table 1). The lowest plant height (59.8 cm) occurred under the 25 cm × 25 cm spacing when seedlings were transplanted at 28 DAP.

3.2 Tiller count

There was no significant (P > 0.05) interactive effect between transplanting and spacing on tiller count per plant at 63 DAP. However, spacing as a main effect had a significant (P < 0.05) influence on tiller count per plant at 63 DAP. Plants from 35 cm x 35 cm plots produced the highest tiller count per plant with a value of 64.19 which was followed by plants from 30 cm x 30 cm plots with a value of 46.48 (Figure 1). The least tiller count per plant was recorded in plants from 20 cm × 20 cm plots with a value of 25.24.

Figure 1

Tiller number per plant at 84 d after planting (DAP) was significantly affected (P < 0.05) by both transplanting time and spacing (Table 2). Mean tiller count varied from 9.7 to 38.76. The highest tiller production (38.76) was observed in plants grown at a spacing of 35 cm × 35 cm with seedlings transplanted at 14 DAP, whereas the lowest tiller count (9.7) occurred in plants spaced at 20 cm × 20 cm and transplanted at 42 DAP.

3.3 Number of days to 50 % flowering

Transplanting age and spacing had a significant effect (P < 0.05) on the number of days to 50% flowering under rainfed conditions (Table 3). The mean number of days to 50% flowering ranged from 75 to 122 d under rainfed conditions and from 77 to 122 dunder irrigation. The longest duration to flowering (122 d) was observed in plants spaced at 35 cm × 35 cm and transplanted at 42 d after planting (DAP) under both water regimes. Conversely, the earliest flowering was recorded in plants spaced at 20 cm × 20 cm and transplanted at 14 DAP under rainfed conditions (76 d).

3.4 Number of panicles

Time of transplanting and spacing had a significant (P < 0.05) influence on panicle number per plant (Table 4). Plants from 35 cm x 35 cm spacing transplanted at 21 DAP produced the highest panicle number per plant with a value of 28.26 whilst 20 cm × 20 cm transplanted at 42 DAP produced the least (5.74).

3.5 Number of spikelet per panicle

Time of transplanting and spacing significantly (P < 0.05) affected the number of spikelets and infertile spikelets per panicle (Table 5). The highest number of spikelets per panicle (240.8) was recorded in plants grown at a spacing of 35 cm × 35 cm with seedlings transplanted at 21 DAP. In contrast, the lowest spikelet number (123.4) was obtained from plants spaced at 20 cm × 20 cm and transplanted at 35 DAP; this result was statistically similar to that of plants spaced at 25 cm × 25 cm and transplanted at 42 DAP, which recorded 144.4 spikelets per panicle.

With respect to infertile spikelets, the highest number (43.3) was observed in plants grown at 25 cm × 25 cm where seedlings were transplanted at 35 DAP, whereas the lowest number of infertile spikelets (19.2) occurred in plants spaced at 35 cm × 35 cm and transplanted at 42 DAP (Table 5).

3.6 Straw weight kg m⁻²

Time of transplanting and spacing significantly (P < 0.05) influenced straw weight (kg m⁻²). The highest straw weight (2.16 kg m⁻²) was obtained from plants grown at a spacing of 20 cm × 20 cm with seedlings transplanted at 14 d after planting (DAP) (Table 6). In contrast, the lowest straw weight (0.460 kg m⁻²) was recorded in plants spaced at 30 cm × 30 cm and transplanted at 35 DAP.

3.7 Grain yield

Transplanting time and plant spacing significantly (P < 0.05) influenced thousand-seed weight and grain yield (Table 7). Plants spaced at 35 cm × 35 cm and transplanted at 21 DAP produced the highest thousand-seed weight (39.69 g), whereas plants spaced at 25 cm × 25 cm and transplanted at 42 DAP recorded the lowest seed weight (28.39 g).

Grain yield ranged from 1,632 kg ha⁻¹ to 7,810 kg ha⁻¹ (Table 7). The highest grain yield (7,810 kg ha⁻¹) was obtained from crops spaced at 20 cm × 20 cm and transplanted at 21 DAP. This was followed by crops spaced at 25 cm × 25 cm and transplanted at 14 DAP, which yielded 6,455 kg ha⁻¹. The lowest grain yield (1,632 kg ha⁻¹) was recorded in crops spaced at 35 cm × 35 cm and transplanted at 42 DAP. This yield was not significantly different (P > 0.05) from those obtained at 25 cm × 25 cm transplanted at 42 DAP (2,536 kg ha⁻¹), 30 cm × 30 cm transplanted at 42 DAP (2,501 kg ha⁻¹), and 35 cm × 35 cm transplanted at 35 DAP (2,274 kg ha⁻¹).

4 Discussion

4.1 Plant height and tiller count

These results suggest that plants grown at the widest spacing (35 cm × 35 cm) likely had greater access to essential growth resources such as soil moisture, nutrients, and solar radiation compared with those grown at narrower spacings. This observation agrees with the findings of Sherif et al. (2020), who reported that wider plant spacing enhances the availability of moisture, nutrients, and light to individual plants. Similarly, Lee et al. (2021) noted that early rice growth following transplanting is strongly influenced by temperature and solar radiation.

At 63 d after planting (DAP), plants grown at 35 cm × 35 cm spacing and transplanted at 28 DAP under rainfed conditions recorded the tallest plant heights of 109.83 cm (Table 1). The wider spacing in these treatments likely promoted improved canopy development and reduced inter-plant competition, thereby enhancing light interception and photosynthetic efficiency, which ultimately resulted in superior vegetative growth. These findings are consistent with those of Sherif et al. (2020) and Dzomeku et al. (2016), who reported that wider plant spacing, in combination with appropriate soil management, leads to increased plant height.

Plants from 25 cm x 25 cm spacing transplanted 28 DAP produced the least plant height with value of 59.77 cm (Table 1). This was probably due to higher competition for sunlight and nutrient created by the narrow spacing of the rice plants. (Bissah et al., 2022) corroborates this statement by indicating that when rice plants are stressed, they grow at reduced height.

Plants grown at a spacing of 35 cm × 35 cm, particularly those transplanted at 28 d after planting (DAP), produced the highest tiller numbers per plant, with values of 64.19 and 59.58, respectively, under rainfed conditions (Figure 1; Table 2). The wider spacing likely enhanced access to essential growth resources such as nutrients, soil moisture, and solar radiation, thereby promoting vigorous growth and increased tiller production at the maximum tillering stage. Similar observations have been reported by Ahtisham (Tahir et al., 2018) and Ginigaddara and Ranamukhaarachchi (2011), who noted that wider plant spacing favors higher tiller production in rice.

In contrast, the lowest tiller count per plant (25.24) was recorded in plots spaced at 20 cm × 20 cm. This reduction in tiller number may be attributed to increased intra-specific competition for light and nutrients under closer spacing. This finding is consistent with the report of Dzomeku et al. (2016).

Furthermore, the highest number of effective tillers was observed in plants grown at 35 cm × 35 cm spacing and transplanted at 14 DAP. This response may be due to better seedling establishment and a more efficient utilization of available resources created by wider spacing, as well as the beneficial effects of soil amendments on nutrient availability. These results are in agreement with the findings of (Kashkool et al., 2020; Abdul Halim et al., 2018), and (Hasanuzzaman et al., 2009).

The lowest effective tiller count per plant (9.7), recorded in plots spaced at 20 cm × 20 cm and transplanted at 42 DAP, was likely due to intense competition for space and nutrients, coupled with transplanting shock associated with the use of older seedlings. Similar findings were reported by (Yussif, 2015) and Lee et al. (2021), who observed that increased competition for light and nutrients, as well as transplanting stress resulting from delayed transplanting, significantly reduced tiller production in rice plants.

4.2 Number of days to 50 % flowering, straw weight, number of panicles and number of spikelets per panicle

The longest days to flowering (122.43 d) was observed in plants spaced at 35 cm × 35 cm and transplanted at 42 d after planting (DAP) (Table 3). This may be attributed to greater availability of nutrients, moisture, and sunlight associated with wider spacing, while delayed transplanting at 42 DAP likely extended the vegetative growth phase. Similar observations have been reported by Marie-Noel et al. (2021) and Sherif et al. (2020). The combined effect of wider spacing and delayed transplanting appears to have promoted enhanced resource use and prolonged the vegetative stage, a conclusion supported by Dzomeku et al. (2016). Additionally, transplanting at 42 DAP may have induced transplanting shock, temporarily interrupting growth and further delaying flowering. This finding contrasts with Yun et al. (2023), who reported that earlier transplanting results in a longer time to flowering.

Plants grown at 20 cm × 20 cm spacing and transplanted at 14 d after planting (DAP) reached flowering in the shortest time. This may be attributed to early establishment and greater competition for nutrients, which shortened the vegetative growth stage. This finding is supported by Lee et al. (2021), who reported that transplanting younger seedlings leads to earlier heading or flowering compared with older seedlings.

The highest straw weight was observed in plants spaced at 20 cm × 20 cm and transplanted at 14 and 21 DAP. This may be due to the combination of closer spacing and early transplanting, which promoted greater straw production per unit area. Similarly, Ram et al. (2014) reported that closer spacing increased yield by approximately 12.3% compared with wider spacing, highlighting its potential effect on straw accumulation.

The lowest straw weight (0.460 kg m⁻²) was recorded in plants spaced at 30 cm × 30 cm and transplanted at 35 d after planting (DAP). This reduced straw production may be attributed to the combination of wider spacing and delayed transplanting, which limited biomass accumulation per unit area. Consequently, straw yield per unit area was lower. This finding is supported by Tadesse et al. (2019) and Asmamaw (2017), who reported that widely spaced rice plants produce less straw per unit area compared with closely spaced plants.

Plants grown at 35 cm × 35 cm spacing and transplanted at 21 d after planting (DAP) produced the highest number of panicles per plant. This is likely because seedlings transplanted at 14 and 21 DAP responded more effectively to the wider spacing, which enhanced the availability of nutrients particularly nitrogen, thereby supporting improved nutrition and higher dry matter accumulation. This observation is consistent with the findings of (Wang et al., 2022).

Conversely, the lowest panicle number was recorded in plants spaced at 20 cm × 20 cm and transplanted at 42 DAP. The combination of narrower spacing and delayed transplanting may have limited nutrient uptake and solar radiation availability, reducing panicle formation. (Li et al., 2020) similarly reported that delayed transplanting can impair spikelet and panicle differentiation, ultimately decreasing rice yield.

Plants spaced at 35 cm × 35 cm and transplanted at 21 d after planting (DAP) produced the highest number of spikelets per panicle and the greatest number of filled spikelets. This may be attributed to favorable temperatures, improved nitrogen availability, reduced competition from neighboring plants and weeds, and greater resistance to pests and diseases. (Chowdhury et al., 2016) supported this observation, noting that reduced competition ensures sufficient nitrogen availability, which enhances grain filling in rice.

In contrast, the lowest number of spikelets per panicle was recorded in plants spaced at 20 cm × 20 cm and transplanted at 35 DAP. This reduction is likely due to delayed transplanting of the seedlings, which can negatively affect panicle development. Liu et al. (2017) reported similar findings, indicating that older seedlings at transplanting produce fewer effective panicles per unit area and fewer spikelets per panicle, ultimately reducing grain yield and sink capacity per tiller.

Infertile spikelets have a significant negative effect on grain yield per hectare (Oladosu et al., 2018), whereas the number of spikelets per panicle is positively correlated with crop yield (Kumar et al., 2022). In this study, the lowest number of infertile spikelets per panicle was observed in plants spaced at 20 cm × 20 cm and transplanted at 14 d after planting (DAP) under (Table 5).

4.3 Thousand seed weight and total grain yield

Plants spaced at 20 cm × 20 cm and transplanted at 21 days after planting (DAP) produced the highest grain yield of 7,810 kg ha⁻¹, followed by plants at 25 cm × 25 cm transplanted at 14 DAP, which yielded 6,455 kg ha⁻¹ within the achievable yield range of 6–8 t ha⁻¹. In contrast, the lowest grain yield (1,632 kg ha⁻¹) was recorded in plants spaced at 35 cm × 35 cm and transplanted at 42 DAP, highlighting the interactive effect of transplanting time and spacing on yield. These results differ from those of Ginigaddara and Ranamukhaarachchi (2011) and Roshan et al. (2011), who reported that wider spacing combined with early transplanting promotes vigorous growth and the production of more and heavier grains. In this study, closer spacings of 20 cm × 20 cm and 25 cm × 25 cm responded well to early transplanting (14 and 21 DAP), resulting in higher grain yields.

5 Conclusion

Transplanting at a spacing of 20 cm × 20 cm at 14 d after planting (DAP) resulted in a biomass of 2.15974 kg m⁻². Panicle production was optimized when rice was transplanted at 21 DAP using a wider spacing of 35 cm × 35 cm.

Crops grown at 35 cm × 35 cm spacing and transplanted at 21 DAP recorded the highest number of spikelets per panicle under rainfed rice ecology. The heaviest thousand-grain weight (37.50 g) was obtained from crops transplanted at 14 DAP with a spacing of 35 cm × 35 cm.

The highest grain yield (7,810 kg ha⁻¹) was achieved with crops transplanted at 21 DAP at a spacing of 20 cm × 20 cm followed by plants at 25 cm × 25 cm transplanted at 14 DAP, which yielded 6,455 kg ha⁻¹. Conversely, the lowest grain yield (1,632 kg ha⁻¹) was recorded from crops transplanted at 42 DAP at a spacing of 35 cm × 35 cm. It was observed that, spacing of 20 cm × 20 cm transplanted 14 DAP with weights of 2.1597 4 kg m⁻² produced the most straw. Plant spacing 35 cm x 35 cm transplanted at 21 d after planting optimized panicle production. Crops from 35 cm x 35 cm plots transplanted at 21 DAP showed the highest number of spikelets per panicle under rainfed rice ecology and finally 35 cm by 35 cm transplanted 14 DAP produced the heaviest (37.50 g) thousand seed weights.

6 Recommendation

For enhanced productivity, it is recommended that farmers adopt plant spacing of either 20 cm × 20 cm or 25 cm × 25 cm in combination with transplanting at 14 or 21 d after planting (DAP), both within the study area and in similar rice-growing ecologies. Future research should focus on varying seedling nursery duration while maintaining a uniform transplanting time under the System of Rice Intensification (SRI). This approach is widely recognized for its potential to improve resource-use efficiency and maximize rice yields.

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