Advances in Crop Science and Technology
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Inter-row; Intra-row; Seedling per hill; Spacing; Transplanting

Introduction

Rice (Oryza sativa L.) is a plant belonging to the family of Poaceae. It is one of the three major food crops of the world and forms the staple food for over half of the world’s population. It provides 27% of dietary energy and 20% of dietary protein in the developing world FAO, 2010 [1].

In Ethiopia, rice was initially planted in the early 1970s on the plains of Fogera and Gambella (Seyoum & Gebrekidan, 2005). The acreage and productivity of Ethiopia's rice crop have increased since 2006. The Ethiopian government acknowledged at the Coalition for African Rice Development's (CARD) third general meeting that rice may make a substantial difference in enhancing food security and reducing poverty (E. E. C. f. R. Development, 2012.). It is regarded as the "millennium crop" because it is one of the target commodities that has received the attention it deserves in the promotion of agricultural production and is therefore anticipated to help ensure food security in the nation (E. E. C. f. R. Development, 2012.; M. M. o. A. a. R. Development, 2010) [2,3].

Since the area planted to rice is growing annually, the crop in the irrigated area is becoming more and more important. The National Rice Research and Development Strategy states that over the first five years, the average annual increase in irrigated rice was 10.2 thousand ha, and during the second five years, it was around 18 thousand ha (M. M. o. A. a. R. Development, 2010) [4].

Keeping seedlings in a nursery and subsequently planting them in a different field is known as transplanting (Institute, 2000). In many regions of the world, this is the primary planting technique for rice (Uphoff, 2003). By using multiple sun energy sources, plants grown with the ideal distance between them develop healthily above and below ground [5].

Therefore, to improve the yield and quality of rice, seedlings need to be transplanted at their optimum spacing and seedling per hill. Keeping this in view, the present study was undertaken with the objective of assessing the effect of plant spacing and the number of seedlings per hill on yield components and the yield of transplanted rice under irrigation [6].

Materials and Methods

Description of the study area

A study was carried out under irrigation from June to November 2016 at the Werer Agricultural Research Center (WARC). Situated in the Rift Valley of Amibara Woreda in Melka Werer village, the site is situated in the Afar National Regional State, approximately 280 kilometers northeast of Addis Ababa. Situated at 740 meters above sea level, the coordinates of the location are 9° 60' N latitude and 40° 9' E longitude). As per Chekol and Mnalku (2012), the region is typified by irregular and infrequent rainfall, with an average yearly rainfall of 568.6 mm, falling short of the 2846.7 mm total evapotranspiration. Minimum temperature is 19.5 °C, highest temperature is 34.8 °C, and the average annual temperature is 27.14 °C [7,8].

Treatments and experimental design

The treatments consisted of factorial combinations of two inter-row spacings (20 cm and 30 cm), three intra-row spacings (10 cm, 15 cm, and 20 cm), and three seedlings (2, 3, and 4) per hill in a randomized complete block design with three replications. There were 18 and 12 rows in plots with 20 cm and 30 cm inter-row spacing, respectively. The gross plot size was 4 m length × 3.6 m width (18 rows × 4 m long and 12 rows × 4 m long, respectively, for 20 cm and 30 cm spacing). Two outermost rows of plots and 50 cm from both ends in each plot were considered as borders. Thus, the net plot size was 3 m length × 3 m width (10 rows × 3 m long) and 3.2 m length × 3 m width (16 rows × 3 m long), respectively, for 30 cm and 20 cm inter-row spacing [9-12].

Seedling management and transplanting

The nursery was grown on well-leveled seed beds that were 10 m by 1.5 m by 6 cm. Depending on the percentage of seed viability, rows of seeds spaced 10 cm apart were drilled using a 50 kg ha^-1 seed rate. To ensure constant hydration and prevent bird damage during germination, a thin layer of dried grass and dirt was placed over the seeds. As the seedlings began to show, the grass was cut down. Up until transplanting, the seed bed was regularly irrigated in the morning and the evening. following the seedlings reached the 4-5th leaf stage, or roughly 20-25 days following emergence, they were transplanted [13].

The plots were leveled to allow irrigation, and they were irrigated before seedling transplanting and every four-day interval after transplanting using the furrow irrigation method. Di-ammonium phosphate (18% N, 46% P2O5) was applied at transplanting at a rate of 50 kg ha^-1. The transplanting of seedlings in each plot was done as per the treatments. Urea (46% N) was applied in split form at a rate of 100 kg ha^-1. The first half was applied after a few days on all transplanted plots (just after the seedlings recovered from the transplanting shook), and the second half was applied at heading initiation. Gap filling was done 7-10 days after transplanting [14].

Data collection

Days to 50% heading; Days to physiological maturity; Plant height (cm), panicle length (cm), Number of total tillers Number of effective tillers, Number of kernels panicle-1, thousand kernels weight (g), Aboveground dry biomass yield (kg ha-1), grain yield (kg ha^-1), straw yield (kg ha^-1), and harvest index [15].

Data analysis

Data were analyzed statistically using the analysis of variance (ANOVA) technique using Genstat Version 18 (Goedhart & Thissen, 2005). When significant differences existed between treatments, comparisons of means were made using the least significant difference (LSD) test at 5% probability levels [16].

Results and Discussion

Crop phenology and growth parameters

Inter-row and intra-row spacing had a highly significant (P < 0.01) effect on days to 50% heading, while the main effect of the number of seedlings per hill showed a substantial (P < 0.05) effect. The three components' interaction effect was not significant, though. Table 1 show that the maximum days to 50% heading (93.78 days) came from two seedlings per hill, while the lowest days to 50% heading (92.83 days) came from four seedlings per hill. The two seedlings per hill may be the cause of the delayed heading time since there is less rivalry for resources and more room for healthy growth when there is reduced planting density. More planting density has also been shown to accelerate early growth, which is consistent with the current result [17].

Through analysis of variance, the number of seedlings per hill and intra-row spacing did not indicate a significant effect on days to physiological maturity, but the effect of inter-row spacing did (P < 0.01). In comparison to 20 cm inter-row spacing, 30 cm inter-row spacing resulted in noticeably more days to physiological maturity (123.78 days) [18].

The main effect of the number of seedlings per hill showed a significant (P < 0.05) effect, and inter-row and intra-row spacing showed a highly significant (P < 0.01) effect on plant height. On the other hand, the two- and three-way interaction effects of inter-row and intra-row spacing and the number of seedlings did not significantly influence plant height [19]. The tallest plant height (94.01 cm) was obtained from 3 seedlings per hill, and it was statistically at par with 4 seedings per hill, while the shortest plant height (90.28 cm) was recorded from 2 seedlings per hill (Table 2). In conformity with this result, Payman & Singh (2008) observed an increasing trend in rice plant height as the leeding rate increased from 40 to 80 kg ha^-1. In contrast with this result, M. Alam, Baki, Sultana, Ali, and Islam (2012) found that seedlings per hill gave the maximum plant height (111.9 cm) of rice, and the shortest height (108.13 cm) was recorded from 5 seedlings per hill. Similarly, Bhowmik (2012) found that two seedlings per hill gave the maximum plant height (93.98 cm) of rice, and the shortest height (91.27 cm) was recorded from five seedlings per hill. Likewise, Biswas (2015) obtained the highest plant height (120.60 cm) from 2 seedlings per hill and the shortest (111.52 cm) plant height from 8 seedlings per hill [20].

The findings showed that inter- and intra-row spacing had a highly significant (P < 0.01) impact on plant height. According to the results, plants grown at a 20 cm inter-row spacing produced a substantially higher plant height (94.26 cm) than at a 30 cm inter-row spacing (89.49 cm). This difference may have resulted from resource competition caused by overcrowding in closer spacing. In contrast to 30 cm inter-row spacing in rice, narrow spacing (20 cm) produced the tallest plants, according to findings published by Kandil, El-Kalla, Badawi, and El-Shayb (2010) [21].

Furthermore, plant height was significantly impacted by intra-row spacing. The plants with the greatest height (94.64 cm) were spaced 20 cm apart from one another in rows, while the smallest plant height (89.79 cm) was spaced 15 cm apart. Sultana, Rahman, and Rahman (2012) discovered that the shortest plant height (86.92 cm) came from 20 cm spacing and the maximum plant height (87.52 cm) from 25 cm intra-row spacing [22]. These results are consistent with this finding. Similar to this, Bhowmik (2012) found that the shortest plant height (92.13 cm) came from 10 cm spacing and the highest plant height (93.16 cm) from 15 cm intra-row spacing [23] (Table 1).

Table 1: Days to 50% heading, days to physiological maturity and plant height of transplanted rice as influenced by the main effects of inter and intra-row spacing and number of seedlings per hill

An analysis of variance revealed that the number of seedlings per hill and the interaction between the two factors had a highly significant (p < 0.01) impact on panicle length. The main effect of intra-row spacing was also found to be important. The length of the panicle was not significantly impacted by the primary effects of the number of seedlings per hill, inter-row spacing, or three-way interactions. The number of seedlings per hill and intra-row spacing interaction had a substantial (p < 0.05) impact on panicle length. Three seedlings spaced 20 cm apart within the row produced the longest panicle length (22.36 cm), while four seedlings spaced 10 cm apart produced the smallest panicle length (19.67 cm) [24,25] (Table 2).

Table 2: Panicle length of rice (cm) as affected by interaction of number of seedlings per hill and intra-row spacing

Yield components

Number of total tillers

The major effects of inter-row and intra-row spacing, number of seedlings per hill, and interaction between the two factors had a highly significant (p < 0.01) impact on the total number of tillers per hill. The crop transplanted with 3 seedlings and 20 cm intra-row spacing produced the highest total number of tillers per hill (18.95), whereas the combination of 4 seedlings and 10 cm intra-row spacing produced the lowest total number of tillers per hill (12.13) (Table 3). Two seedlings per hill produced a much higher number of tillers than three seedlings per hill, according to Shrirame, Rajgire, and Rajgire (2000), in contrast to this result [26].

Table 3: Number of total tillers per hill of rice as affected by interaction of number of seedlings per hill and intra-row spacing.

The main effect of inter-row spacing had a highly significant (P < 0.01) effect on the number of total tillers per hill. A significantly higher number of total tillers per hill (15.27) was obtained from 30 cm than from 20 cm (14.11). The production of more tillers in wider rows could be due to the absorption of more nutrients and moisture and also to the availability of more sunlight in comparison to densely transplanted plants. In agreement with this result, Biswas (2015) recorded the highest tillers per hill (13.58) from 30 cm inter-row spacing and the lowest tillers per hill (6.58) from 15 cm inter-row spacing in rice. Similar results were also reported by Haque (2002): wider spacing produced higher total tillers per hill in rice [27,28].

Number of effective tillers

Regarding the number of effective tillers per hill, the major effects of seedling number, inter-row spacing, and intra-row spacing demonstrated a highly significant (P < 0.01) influence. However, the number of effective tillers was not significantly impacted by the two- and three-way interaction effects of seedling quantity, intra-row spacing, and inter-row spacing. Because it varied highly substantially (p < 0.01), the number of effective tillers per hill depended on the number of seedlings per hill. The difference in the number of seedlings per hill was revealed to be the cause of the variation in effective tiller production. Two seedlings per hill produced the greatest number of effective tillers per hill (14.49), while four seedlings per hill produced the lowest number (12.22). This conclusion was in line with that of Islam et al. (2014), who found that 2 seedlings per hill produced the greatest number of effective tillers and 4 seedlings per hill produced the lowest number of effective tillers. However, Biswas (2015) found that, out of 4 seedlings per hill, the largest number of effective tillers per hill (10.07) was produced, while the lowest number (6.55) was produced out of 8 seedlings per hill [29].

The results revealed that the highest number of effective tillers per hill (14.26 and 16.07) were produced when the rice was transplanted at 30 cm and 20 cm inter-row and intra-row spacing, respectively, while the lowest numbers of effective tillers per hill of 13.07 and 11.41 were observed in narrow inter-row (20 cm) and intra-row (10 cm) spacing, respectively. The result was in conformity with those of Sohel, Siddique, Asaduzzaman, Alam, & Karim (2009) and Uddin, Hasan, Ahmed, & Hasan (2010) who reported the highest effective tillers at optimum spacing as compared to lower spacing. Similarly, Biswas (2015) reported that 30 cm × 20 cm spacing produced the highest number of effective tillers (10.43) in transplanted rice as compared to narrow spacing (15 cm × 20 cm). Many authors reported that the number of effective tillers per hill increased linearly with an increase in spacing, but the increase did not maintain the number of effective tillers per unit area due to a reduction in the initial plant population per m2 due to variation in plant geometry (Biswas, 2015, Nayak, Dalei, & Choudhury, 2003) [30] (Table 4).

Table 4: Number of effective tillers per hill and 1000 grain weight of rice as influenced by the main effects of inter-row spacing, intra row spacing and number of seedlings per hill

Number of grains per panicle

Number of grains per panicle was significantly (p<0.05) affected by two-way interaction of number of seedlings per hill and intra-row spacing, and inter-row and intra-row spacing. From the result it was observed that the combination of intra-row spacing of 20 cm and 3 seedlings per hill produced the highest number of grains per panicle (131.4) and the lowest grains per panicle (115.5) was recorded from the combination of 10 cm inter-row and × 2 seedlings per hill (Table 5). In agreement with this result, (Balock, 2002) reported that the increased plant spacing considerably resulted in vigorous plant growth and caused a significant increase in number of filled grains per panicle in rice. Similarly, (Mehrvar & Asadi, 2006) also reported that by increasing seed rate the number of grains per spike was reduced [31] (Table 5).

Table 5: Number of grains per panicle of rice as affected by interaction of intra-row spacing with number of seedings per hill and inter-row spacing

The interaction of inter-row and intra-row spacing showed that the spacing of 20 cm × 20 cm produced the highest number of grains per panicle (131.4), and the lowest number of grains (117.3) was obtained from the combination of 20 cm × 10 cm inter- and intra-row spacing. In line with this result, Biswas (2015) obtained the highest number of grains per panicle (129.44) from 20 cm × 20 cm spacing, and the lowest (90.58) was obtained from 15 cm × 20 cm spacing. On the other hand, K.K. (2009) reported the highest number of grains per panicle (147.5) from 25 cm × 25 cm and the lowest recorded (125.9) from 30 cm × 30 cm inter- and intra-row spacing in rice [32].

Thousand grains weight

Thousand grain weights are an important yield component that determines the yield per hectare. The main effects of inter-row and intra-row spacing were significant on grain weight, while the main effect of the number of seedlings per hill and the interactions were not significant. The results showed that with the increase in spacing, the grain weight also increased significantly. Significantly higher thousand grain weight (22.69 g) was produced when rice was transplanted at 30 cm inter-row spacing than closer spacing (20 cm). Similarly, the highest thousand grain weight (22.84 g) was produced when the crop was transplanted at 20 cm intra-row spacing, and the lowest (21.87 g) was observed at the closest spacing (10 cm). In line with this result, (M. S. Alam, Baki, M.A., Sultana, M.S., Ali, K.J., and Islam, M.S., 2012; Biswas, 2015) reported the highest thousand grain weight (26.40 g) in wider spacing of 30 cm inter-row and 20 cm intra-row spacing than closer spacing of 20 cm inter-row and 15 cm intra-row spacing. Similarly, M. A. Ali, Ali, L., Sattar, M., and Ali, M. (2010) obtained an increased grain weight at a wider (25 cm) row spacing in wheat than at a 15 cm spacing.

Yield Parameters

Aboveground dry biomass yield

Biomass yield was significantly (p < 0.01) affected by the main factors and interactions of seedling number, intra-row spacing, and inter- and intra-row spacing, but their three-way interaction was not significant. The highest dry biomass yield (13634 kg ha^-1) was obtained from 20 cm intra-row spacing and 3 seedlings per hill, while the lowest dry biomass yield (8253 kg ha^-1) was obtained from 10 cm intra-row spacing and 4 seedlings per hill. The plants grown with wider spacing have more area of land around them to draw nutrition from and more solar radiation to absorb for a better photosynthetic process, hence performing better as individual plants with a high total tiller production. In line with this result, Bozorgi et al. (2011) and Mohammadian R.N. (2011) reported the highest dry biomass yield from plant spacing of 20 cm along with 3 seedlings per hill as compared to narrow spacing and a high number of seedlings per hill in rice. In contrast to this result, Bhowmik (2012) reported the highest biomass yield (7.48 t ha^-1) from the interaction of narrow intra-row spacing (10 cm) and 4 seedlings per hill, while the lowest biomass yield (3.44 t ha^-1) was reported from the interaction of wider intra-row spacing (15 cm) and 2 seedlings per hill of rice.

Analysis of variance showed that the interaction effect of inter and intra-row spacing had highly significant (P < 0.01) effect on aboveground dry biomass yield of rice. The highest dry biomass yield (11584 kg ha^-1) was obtained from 20 cm inter-row and 20 cm intra-row spacing while the lowest dry biomass yield (7922 kg ha^-1) was obtained from 20 cm inter-row and 10 cm intra-row spacing. The increase in aboveground dry biomass in response to increasing (widening) of the row spacing might be due the better environment for growth and development of crop that might have resulted in better plant height, number of effective tillers and a greater number of grains per panicle. In agreement with this result, (M. A. A. B. S. a. A. H. Ali, 2011) found increased biomass yield with wider row spacing (25 cm) due to higher production of tillers in rice. In contrast to this result,(Bhowmik, 2012) found the highest above ground dry biomass yield (6.7 t ha^-1) from narrower inter and intra-row spacing (20 cm × 10 cm) and lowest biomass yield (4.39 t ha^-1) from wider (25 cm × 15 cm) inter and intra-row spacing in rice (Table 6).

Table 6:Biomass yield of rice (kg ha-1) as affected by interaction of intra-row spacing with number of seeding per hill and inter-row spacing

Grain yield

Grain yield was significantly impacted (p < 0.01) by the three main effects interaction between the number of seedlings and inter-row spacing, the number of seedlings and intra-row spacing, and the inter-row and intra-row spacing. However, there was no discernible impact of the three-way interaction on grain yield. The highest grain yield (4314 kg ha^-1) was obtained from 30 cm inter-row spacing and 3 seedlings per hill, and the lowest grain yield (3147 kg ha^-1) was obtained from 20 cm inter-row spacing and 2 seedlings per hill, in relation to the interaction of number of seedlings and inter-row spacing. It's possible that the reduced plant population made better use of available resources to boost grain yield. The quantity of grains in each panicle, the number of efficient tillers per hill, and the thousand grain weight were the determinants of the final yield. For most crops, using the best seed rates is crucial to increasing output (Hiltbrunner, Streit, & Liedgens, 2007). A higher seed rate will result in a larger population of plants and increased competition between them for sunshine, water, nutrients, and space, which will lower yield and quality. Lower seed rates result in fewer plants per unit area, which reduces yield (Bozorgi et al., 2011; Hameed, Shah, Shad, Bakht, & Muhammad, 2003). M. S. Alam, Baki, M.A., Sultana, M.S., Ali, K.J., and Islam, M.S. (2012) reported that the most significant factor influencing yield is the quantity of tillers per unit area, which is in line with this outcome. Thus, the more tillers there are, especially fruitful ones, the larger the yield. The findings of this study were consistent with those of M. A. Ali, Ali, L., Sattar, M., and Ali, M. (2010); Iqbal, Akbar, Ali, Sattar, & Ali (2010; Kamara, Ekeleme, Omoigui, & Chikoye (2011); Sultana et al. (2012), who reported that because plant populations are typically high in narrow spacing and most panicles are produced on the main culm rather than on tillers, so that tillering capacity affects grain yield.

Additionally, a highly significant (P < 0.01) interaction impact between the number of seedlings and intra-row spacing was revealed by the analysis of variance. The results indicate that 20 cm intra-row spacing and 3 seedlings per hill produced the largest grain yield (5079 kg ha^-1). On the other hand, 15 cm and 10 cm intra-row spacing with 2 seedlings per hill produced the statistically lowest grain yields (3029 kg ha^-1 and 3093 kg ha^-1). Less computation in wider plant spacing and fewer seedlings per hill may be the cause of the variation in intra-row spacing in response to the number of seedlings per hill.

The inter- and intra-row spacing interaction had a considerable impact on grain yield. The 20 cm × 20 cm inter and intra row spacing produced the maximum grain yield (4288 kg ha^-1) and the 20 cm × 10 cm inter and intra row spacing produced the lowest grain yield (3015 kg ha^-1). A higher grain yield with inter- and intra-row spacing of 20 cm x 20 cm may be the result of optimal spacing that generated a large number of productive tillers per hill, which determined the final grain yield. Thus, the yield increases with the number of tillers, particularly fruitful tillers.

The greater output of rice may be explained by the more effective use of growing resources, reduced intraspecies competition, and increased nutrient availability among the widely separated crop plants. Consistent with this outcome, the highest grain output was recorded by Bozorgi et al. (2011) and Mohammadian R.N. (2011) from plant spacing of 20 cm × 20 cm, respectively. In a similar vein, Biswas (2015) found that in rice, the interaction of 20 cm × 20 cm spacing produced the highest grain yield (5.22 t ha^-1) compared to the closest spacing (15 cm × 20 cm). Bhowmik (2012) found that the interplay of tight spacing (20 cm × 10 cm) resulted in the maximum rice grain yield (3.13 t ha^-1) and the lowest grain yield (1.85 t ha^-1) (Table 7 and Table 8).

Table 7: Grain yield (kg ha-1) of rice as affected by the interaction of number of seeding per hill with inter-row spacing with and intra-row spacing

Table 8: Grain yield (kg ha-1) of rice as affected by the interaction of inter and intra-row spacing

Straw yield

Straw yield was significantly (p < 0.05) impacted by the three primary effects of inter-row and intra-row spacing as well as their interactions. The other three- and two-way interactions, however, did not significantly affect the outcome. The 20 cm × 20 cm plant spacing produced the maximum straw production (7296 kg ha^-1), while the 20 cm × 10 cm spacing produced the lowest straw yield (4907 kg ha^-1). Sultana et al. (2012) showed a similar tendency in straw production, with larger straw yields achieved in rice rows spaced 20 cm apart as opposed to rice rows spaced narrowly. Against this outcome, Akanda (2007) and Biswas (2015) discovered that the tightest spacing (15 cm × 20 cm) produced the largest straw output (6.57 t ha^-1) and the lowest straw yield (5.29 t ha^-1) was recorded from wider spacing (30 cm × 20 cm).

The primary influence on straw yield was the number of seedlings per hill, which was found to be extremely significant (P < 0.01). The hills with 3 seedlings produced the maximum straw yield (7169 kg ha^-1), whereas the hills with 4 seedlings produced the lowest straw yield (5584 kg ha^-1). This difference in straw yield may be attributed to the fact that 3 seedlings produced a larger total tiller than 4 seedlings per hill. Similarly, Habib & Bhat (2013) discovered that a maximum straw production of 9280 kg ha1 was produced by 3 seedlings as opposed to 8570 kg ha1 by 5 seedlings per hill (Table 9 and Table 10) [33].

Table 9: Straw yield (kg ha-1) of rice as affected by interaction of inter and intra-row spacing
LSD = Least Significant Difference at 5% level; CV = Coefficient of Variation; Means within

Table 10: Straw yield and harvest index of rice as influenced by the main effects of number of seedlings per hill, inter-row spacing, and intra row spacing.

Summary and Conclusion

Ethiopia has a huge potential for both rain-fed and irrigated rice production, which is estimated at about thirty million ha; more than half of this area is found in the irrigated lowlands of the country. Rice production management plays a great role in increasing and sustaining the production of the crop. Transplanting is the major method of rice planting used in many parts of the world. Given the fact that rice is a recently cultivated crop in Ethiopia, there is a great research gap with regard to specific agronomic recommendations for the rice-producing areas of the country.

New Rice for Africa (NERICA-4) is a newly adapted rice variety suitable for irrigated conditions, but there is no sufficient information regarding optimum spacing and the number of seedlings per hill for achieving maximum yield of the variety. Therefore, this experiment was conducted to assess the effect of plant spacing and the number of seedlings per hill on yield components and the yield of transplanted rice under irrigation. Two levels of inter-row spacing (20 cm and 30 cm), three levels of intra-row spacing (10 cm, 15 cm, and 20 cm), and three levels of seedling (2, 3, and 4 seedlings) per hill were tested in factorial arrangement in three replications of Randomized Complete Block Design (RCBD).

Results revealed that the main effect due to the number of seedlings per hill was significantly (P < 0.01) affected by all phenological, yield components and yield except days to physiological maturity, harvest index, and 1000 kernels weight (g). The highest plant height (94.01 cm) was produced when the crop was transplanted at 3 seedlings per hill. While the highest number of effective tillers per hill (14.49) was recorded from 2 seedlings per hill.

Similarly, the main effect of inter- and intra-row spacing affects significantly (P < 0.01) all recorded parameters except harvest index. The highest number of effective tillers per hill (14.26, 16.07), straw yield (6517 kg ha^-1, 7009 kg ha^-1), and 1000 kernel weight (22.69 g, 22.84 g) were produced when the crop was transplanted at 30 cm inter-row and 20 cm intra-row spacing, respectively.

Panicle length, number of total tillers per hill, number of grains per panicle, aboveground dry biomass yield, and grain yield were significantly affected (P < 0.01) by the interaction of the number of seedlings and intra-row spacing. The highest panicle length (22.36 cm), number of total tillers per hill (18.95), number of grains per panicle (131.6), aboveground dry biomass yield (13634 kg ha^-1), and grain yield (5079 kg ha^-1) were obtained from 3 seedlings per hill and 20 cm intra-row spacing.

The number of grains per panicle, aboveground dry biomass yield, grain yield, and straw yield were significantly (p < 0.01) affected by the interaction of inter- and intra-row spacing. The highest number of grains per panicle (131.4), aboveground dry biomass yield (11584 kg ha^-1), grain yield (4288 kg ha^-1), and straw yield (7296 kg ha^-1) were obtained from the interaction of 20 cm × 20 cm inter- and intra-row spacing.

In addition, grain yield was significantly (p < 0.05) affected by the interaction of the number of seedlings per hill and inter-row spacing. While the highest grain yields (4314 kg ha^-1) was recorded with the interaction of 3 seedlings and 30 cm inter-row spacing.

Therefore, from the results of the study, it can be concluded that transplanting 3 seedlings per hill with 30 cm inter-row spacing and 20 cm intra-row spacing is tentatively recommended to obtain the maximum grain yield of transplanted rice under irrigation.

Acknowledgments

The authors are grateful for the financial support of the Ethiopian Institute of Agricultural Research for the execution of the research. The authors also thank the staff members of the Werer Agricultural Research Center for their technical assistance during the time of conducting the experiment and Haramaya University.

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