Performance evaluation and selection of new maize hybrids under sole and inter crop production systems

Screening of maize genotypes under different cropping systems (sole and inter cropping) is very important to understand the genotypes response for different abiotic and a biotic stress. Nine maize genotypes including the standard check (BH-543) were planted and evaluated at research and farmers’ fields in the 2011 and 2012 cropping seasons. Farmers were invited to evaluate the genotypes based on their criteria of selection. Hawassa-Dume common bean variety was used for intercropping purpose in 2012. The genotypes substantially varied for yield and other traits both under sole and intercropping systems. When combined across seasons, the high yielding genotypes, genotype-1 and genotype-5 showed 38 and 37% yield advantage over the standard check. Besides, genotypes markedly varied for their compatibility for intercropping system with land equivalent ratio (LER) <1 for most of genotypes. However, genotype-4 and genotype-8 had LER >1 highlighting the need to evaluate genotypes for intercropping system at early stage of breeding.


INTRODUCTION
Maize (Zea mays L.) is the second most widely cultivated crop grown by smallholder farmers under rainfed condition in Ethiopia. Maize yield in Ethiopia vary considerably across seasons and locations making smallholders livelihoods vulnerable to climate variability. Maize and common bean are two of the leading crops in their respective category of cereals and pulses in southern Ethiopia. Accordingly, maize and common bean occupy 36 and 44% of the area devoted to cereals and pulses, respectively (CSA, 2017).
Intercropping systems play an important role in subsistence and food production in developing countries (Tsubo and Walker, 2002). It is most widely practiced in countries where arable land is scarce where it contributes to biodiversity and food security (Mushagalusa et al., 2008). Land scarcity is one of the constraints facing small farmers in Ethiopia. In the southern Ethiopia, 40% of farmers have an average land holding of 0.1 to 0.5 ha with a further 30% having 0.51 to 1 ha (CSA, 2017). This led farmers to use multiple cropping mainly intercropping to increase yield per unit area and reduce the risk from crop failure due to climate change.
Maize-common bean intercropping is an integral part of the cropping system in small-holder farmers expecting better yield and weed suppression (Getahun and Tenaw, 1990), and provides balanced diet compared to the *Corresponding author. E-mail: goshime.muluneh@yahoo.com Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License predominant cereal monoculture and gives high total productivity compared to sole crops of bean and maize (Walelign, 2014;Workayehu, 2014). However, all varieties released so far in the country were evaluated under monocropping system and has not been tested for intercropping system at early stage of breeding. Selection of genotypes both under sole and intercropping systems is of paramount importance to enhance yield and varietal adoption in the region. Therefore, the objective of this study was to identify best performed genotypes under sole and intercropping systems.

MATERIALS AND METHODS
The experiment was conducted under rain-fed condition at Hawassa research station (07°03ˈ71̎ N, 38°30ˈ88 ̎ E, 1689 masl elevation) and on-farms (farm1; 07° 79ˈ43̎ N, 37° 04ˈ 31̎ E, 1696 masl elevation and farm2; 07° 78ˈ 28 ̎ N, 37° 04ˈ 31 ̎ E , 1692 masl ) in 2011 and 2012 main rainy seasons in Ethiopia. This area is characterized by bimodal rainfall between March and September with mean annual maximum and minimum temperatures of 27.3 and 12.6°C, respectively. Nine hybrid maize genotypes were planted in 2011 and 2012 cropping seasons. The hybrids were planted both under sole and inter cropping systems at research station in 2012. These genotypes were also planted only under sole-cropping on two farmers' field in 2011 and 2012 without replication at each farmer's field but for analysis farmers were used as replication. Similarly, in 2011, these genotypes were planted at research field without replication. For analysis of the data collected in 2011, farmers and research station were used as replication because the trial was not replicated both at farmers' and research field. The recently released hybrid variety (BH-543) was included as check in 2011 and 2012 cropping seasons. The genotypes were planted using randomized complete block design with three replications at research field in 2012. Each genotype was planted on two rows 7.65 m 2 area at research station and three rows 11.48 m 2 area at farmers' field. Maize genotypes were planted in 75 and 30 cm spacing between consecutive maize rows and plant, respectively. Common bean genotype named Hawassa-Dume was planted between two rows of maize in one to one ratio for intercropping. At research field, sole common bean was also planted for land equivalent ratio determination. Common bean was planted at the spacing of 40 cm between rows and 10 cm between seeds within a row. Grain yield and other important agronomic traits of component crops were recorded to evaluate the genotypes grown under the sole and inter-cropping systems. Plants from the whole plot were hand harvested at physiological maturity. Ears were shelled, grain weight and grain moisture content measured, and yield was adjusted for 12.5% grain moisture content. However, for common bean yield was adjusted to 10% grain moisture content. In both seasons, farmers were participated to set selection criteria and evaluate maize genotypes. Yield deviation due to inter cropping from the sole maize yield was calculated using the formula Deviation (%) = ((inter crop maize yield / sole maize yield) × 100) -100 (1) Land equivalent ratio calculated was computed as in Adu-Gyamfi et al. (1997).
where Y m and Y b were grain yields of intercropped maize and bean; Y sm and Y sb were grain yields of sole cropped maize and bean.
Grain yield, number of ears (NE), plant height (PH), ear height (EH), gray leaf spot (GLS), turcicum leaf blight (TLB) and common leaf rust (CLR) were analyzed as randomized complete block design in SAS statistical package (SAS, 2002) version 9.0. Performance and stability of genotypes were visualized graphically through an average environment coordination (AEC) view of GGE biplot based on genotype-focused SVP (that is, "SVP=1") (Yan and Rajcan, 2002). Graphs were developed using R software (Table 1).

Analysis of variance
Highly significant grain yield (GY) variation was observed among genotypes for sole cropping system in 2011 (p < 0.01) (  (Table 3). This could be due to high drought stress in 2012 cropping season resulting into low genotype variation. This is because variation among genotypes in the optimum condition is high than under stress environments leading to lower chance of genotypes to express their genetic potential under stressful conditions (Mohammadai et al., 2012). In 2011, significantly high variation (p< 0.01) was observed among genotypes for plant height (PH), ear height (EH), gray leaf spot (GLS), turcicum leaf blight (TLB), common leaf rust (CLR) and number of ears (NE). Genotype-6 was more tolerant to across the three major foliar diseases (GLS, CLR and TLB) compared with the other genotypes (Table 4). Genotype-5 was also showed relative tolerance to these major foliar diseases. Similarly, in 2012 cropping season, significant variation was observed among genotypes tested under sole system for EH and GLS (Table 5). Genotype variation for PH, EH, GLS and CLR under sole cropping system has been previously reported (Berhanu, 2009). However, in this study, genotypes tested under both cropping systems responded consistently excepting small variation. For instance, high GLS score was recorded for genotype 2 and 3 under sole cropping compared with under intercropping system (Tables 5 and 6). However, our study highlights for PH, EH and GLS, Kariuki et al. (2016) reported significant difference between single crosses treatments tested in Kalro experimental stations in Kiambu and Embu counties in 2012. In 2012 under sole crop, significant variation among genotypes was observed for EH and GLS. Genotype variation for GLS, TLB and CLR has been previously reported for this area

Pedigree Code Type Seed color
Genotype-9 (Check) CN White Where CN = Conventional normal maize. ns, *, ** indicate non-significant and significant at P < 0.05 and 0.01, respectively. (Berhanu, 2009). Similarly, Daniel et al. (2014) and Goshime (2019) also reported significant difference between treatments for PH and EH. In the combined analysis, the difference was significant for PH, EH, CLR and NE whereas the variance was non-significant for GLS, TLB and GY (Table 7). For yield, in contrast to the current finding for grain yield, O' Leary and Smith (1999) reported highly significant variation between three cropping systems (monoculture, maize-bean inter cropping and maize-clover inter cropping). For PH of maize, Zaeem et al. (2019) reported significant variance between cropping system with the overall higher value obtained for inter cropping with soybean in their study. The highest PH and EH was showed by genotype-6 and by genotype-5, respectively (Table 7).

Mean performance of the genotypes
Combined over seasons and cropping systems, mean maize grain yield performance of genotypes showed that the highest grain yield advantage was obtained from     (Table 3). In 2011, the highest yield (10.1 t/ha) and the lowest (6.3 t/ha) were observed for genotype-1 and genotype-9, respectively. Similarly, in 2012, under inter cropping, the highest grain yield advantage was obtained from genotype-2 with 27% over the check. The second and the third yield advantage was obtained from genoypte-4 and genotype-1, respectively. Under sole cropping in 2012, except for genotype 8, all genotypes showed yield advantage over the check with the highest grain yield advantage observed for genotype-5. The maize yield of genotype-1, genotype-2, and genotype-5 were consistent under both cropping systems in 2012 (Tables 4 to 6). Generally, in 2012 under both sole and intercropping and combined analysis genotypes had higher grain yield advantage over the check (BH-543) except for genotype-8 (Table 3).The overall mean performance was higher from sole cropping compared with the yield harvested from inter cropping with yield penalty of 0.45 t ha -1 due to inter cropping  without considering the bean harvest in 2012 (Table 2). Similar to this, O'Leary and Smith (1999) obtained higher maize grain yield from sole cropping than maize inter cropped with bean and clover. The result showed the existence yield penalty due to inter cropping when we see the overall effect but individual there were some genotypes which had higher yield under inter cropping compared with performance under sole cropping and in line with this finding, Rusinamhodzi et al. (2020) reported as intercropping had maize yields reduction effect due to increased competition. When genotypes were compared in terms of yield reduction/deviation due to intercropping, most the genotypes showed reduction/negative deviation except for genotype-4 and genotype-8 (Figure 1). The highest deviation to the negative side was observed for genotype-5 (-20%) and genotype-9 (-19%) while the least negative deviation was observed for genotype-2 (-1%). The highest deviation to the positive side was observed for genotype-4 (16%) followed by genotype-8 (3%). The Table 7. Mean grain yield (t ha-1), plant height (cm), ear height (cm), gray leaf spot (GLS), turcicum leaf blight (TLB), common leaf rust (CLR) and number of ears harvested (NE) of maize genotypes combined data across seasons (2011 and 2012) and cropping systems (sole and inter-cropping).  higher deviation to the negative side indicated that the genotypes were affected by common bean in intercropping whereas the genotypes which had yield deviation to the positive side indicated that the maize genotypes are suitable for inter cropping or not affected by intercropping (Figure 1). However, the common bean yields obtained from intercropped with genotype-4 and 8 were the least compared with common bean yields obtained from intercropping with other genotypes (Table  3). Higher common bean yield obtained from intercropped with genotype-1, genoype-5 and BH-543 could in part be due to good leaf structure/architecture of maize genotypes resulting to high radiation interception and hence higher common bean yields or common bean had a better competitive advantage over the genotypes (Table 3).

Land equivalent ratio
The overall LER was evaluated to derive land benefits associated with intercropping of maize genotypes and the bean variety Hawassa-Dume. The LER in intercrops ranged from 0.86 to 1.19. Only three genotypes, genotype-2, genotype-4, BH-543, and genotype-8 had LER of 1.02, 1.19, 1.04 and 1.06, respectively, which is greater than 1 ( Geno-1 Geno-2 Geno-3 Geno-4 Geno-5 Geno-6 Geno-7 Geno-8 Geno-9 % GY deviation Yield Deviation(%)2012  (Walelign, 2014;Tolera et al., 2005;Assefa et al., 2016) and with maize-soybean in Indonesia (Yuwariah et al., 2018). The LERs of intercrops between maize and common bean can save lands up to 48 and 55%, which would have required as additional land for monoculture of each crop (maize or common bean) if not intercropped (Nassary et al., 2019). However, most genotypes in this study had <1 LER indicating that the land productivity will be greater when genotypes are planted in monocropping than intercropping even if the difference was not that much high (Table 9). This is consistent with non-significant difference for cropping system x genotype interaction indicating that maize genotypes responded similarly for cropping systems (Table 7). This study highlights that varieties selected based on monocropping performance may not necessarily do well under intercropping system. Genotypes are grouped into two mega environments. SC2011 and IC2012 grouped together in one mega environment and SC2012 grouped in the other mega environment (Figure 2). Genotype-1 was the ideal genotype followed by genotype-5 (Figure 3). From ranking biplot graph, genotype-1 and genotype-5 showed better performance in yield and were highly responsive to cropping systems. Genotype-2, genotype-3 and genotype-6 were highly stable genotypes compared with other genotypes (Figure 3).

Conclusion
The results of this study showed significant variation among genotypes for yield and other traits. Genotypes used in this study were developed for monocropping system and hence most genotyes had lower LER indicating that they are not compatible to incropping system. However, genotype-4 and genotype-8 demonstrated higher compatability to the intercropping system providing an opportunity for famers to grow under both cropping systems. In regions with maize commonly grown as an intercrop, it is of paramount importance to evalate maize genotypes for their compatibilty to intercropping system at early stage of genotype evaluation. Some morphological traits such as canopy architecture and tolerance to high planting density could be considered for variatal selection. The results of this study highlights the need for particpatory varietal selection where farmers criteria could also be met for fast-track realease and better adoption of maize varieties.

CONFLICT OF INTERESTS
The authors have not declared any conflict of interests.