## Abstract

Introduced maize (Zea mays L.) germplasm can serve as important sources of favorable alleles for enhancing the performance of new maize varieties and hybrids under drought stress conditions. Ninety-six elite maize hybrids alongside four hybrid checks were evaluated for grain yield and other agronomic traits under managed stress conditions over two seasons at Ikenne, Nigeria. Hybrids differed significantly for grain yield and other measured traits under both drought stress and well-watered conditions. Grain yield varied from 444 to 3022 kg ha−1 under drought stress, and from 3827 to 8887 kg ha−1 under full irrigation. Drought stress reduced grain yield by 70%. Each of the top 10 yielders under drought stress produced >2500 kg ha−1 and had a yield advantage of >10% over the best check. Three hybrids namely; ADL47 × EXL15, ADL41 × EXL15 and EXL02 × ADL47, produced competitive yields under both irrigation treatments.

## Keywords

Adapted ; Exotic ; Drought ; Maize germplasm ; Zea mays L.

## 1. Introduction

Maize (Zea mays L.) is a staple food crop that plays a major role in the diet of millions of African people [7] . It remains the economic mainstay of more than 300 millions of Africas most vulnerable people, providing half of the calorific intakes of peoples in southern Africa, 30% in eastern Africa and 15% in West and Central Africa [12] . Despite the rising profile of maize, both as food and economic crop in West and Central Africa over the last few decades [6]  and [8] , it is still prone to many production constraints such as drought [2] .

Drought is a major abiotic stress limiting maize production and productivity in sub-Saharan Africa (SSA), contributing about 15% and 17% average annual yield reductions in West and Central Africa and the tropics, respectively [5] . Today, many places in the guinea savannas that are arguably the current maize belt of Nigeria now experience yearly drought that often coincides with flowering period of maize crops and consequently leads to poor grain yields or total crop failure. It is being speculated that the frequency and intensity of drought would intensify in the years ahead in response to climate change [10] . Therefore, the survival of resource-poor, small-scale maize growers in Nigeria and other places in sub-Saharan Africa who cultivate drought-susceptible maize varieties with little or no access to irrigation facilities has become a great challenge [4]  and [1] . The most economically viable and sustainable option for salvaging the situation is breeding and releasing improved drought tolerant and high yielding maize cultivars for the farmers in order to guarantee profitable yields even in years of drought. Introduced maize germplasm can serve as important source of novel alleles for improvement of adapted germplasm for drought tolerance and high productivity [2] .

A set of new single-cross hybrids developed from adapted and exotic drought tolerant maize inbred lines bred at IITA and CIMMYT, respectively, were evaluated under well-watered and drought stress conditions in order to assess their grain yield potentials and identify superior high yielding and drought tolerant hybrids.

## 2. Materials and methods

### 2.1. Field experiment

Ninety-six (96) newly developed single-cross maize hybrids of IITA-bred adapted and CIMMYT-bred exotic inbred lines [2] plus four (4) hybrid checks (Table 1 ) were evaluated under drought stress and full irrigation conditions at Ikenne (latitude 6°54′N, longitude 3°42′E, altitude 60 masl), in Nigeria during the dry seasons of 2010 and 2011. Ikenne receives little rainfall from November to March of the year, making the site suitable for conducting drought tolerance experiments because maize crops planted at this period must be supported with irrigation. The soil at this site is eutricnitrosol (FAO classification). The experimental fields are flat and reasonably uniform, with high water-holding capacity [9] . Two out of the four checks are commercial hybrid maize varieties being marketed in Nigeria (Oba Super 1 and Oba 98) while the remaining two are drought tolerant synthetic hybrids developed at IITA (M1026-7 and M1026-8).

Table 1. The 96 single-cross hybrids and 4 hybrid checks evaluated under managed stress conditions in the dry seasons of 2010 and 2011 at Ikenne in Nigeria.
Entry Hybrid Entry Hybrid Entry Hybrid
15 EXL05 × ADL39 49 EXL10 × EXL01 82 EXL03 × ADL31
16 EXL24 × ADL39 50 EXL15 × EXL01 83 EXL06 × ADL31
17 ADL31 × EXL10 51 EXL16 × EXL01 84 EXL07 × ADL31
18 ADL41 × EXL10 52 EXL17 × EXL01 85 EXL02 × ADL41
19 ADL33 × EXL10 53 EXL10 × EXL04 86 EXL03 × ADL41
20 ADL47 × EXL10 54 EXL15 × EXL04 87 EXL06 × ADL41
21 ADL31 × EXL15 55 EXL16 × EXL04 88 EXL07 × ADL41
22 ADL41 × EXL15 56 EXL17 × EXL04 89 EXL02 × ADL33
23 ADL33 × EXL15 57 EXL10 × EXL05 90 EXL03 × ADL33
24 ADL47 × EXL15 58 EXL15 × EXL05 91 EXL06 × ADL33
25 ADL31 × EXL16 59 EXL16 × EXL05 92 EXL07 × ADL33
26 ADL41 × EXL16 60 EXL17 × EXL05 93 EXL02 × ADL47
27 ADL33 × EXL16 61 EXL10 × EXL24 94 EXL03 × ADL47
28 ADL47 × EXL16 62 EXL15 × EXL24 95 EXL06 × ADL47
29 ADL31 × EXL17 63 EXL16 × EXL24 96 EXL07 × ADL47
30 ADL41 × EXL17 64 EXL17 × EXL24 97 M1026-7 – Check

Experiments were planted in two adjacent blocks that received different irrigation treatments. The first block (Block 1) received irrigation throughout the life cycle of the crop whereas the second block (Bock 2) received irrigation for only 28 days which is approximately two to three weeks to anthesis so that water stress can coincide with the time of flowering. The blocks were separated by four ranges, each 4.25 m wide, to restrict lateral movement of water from the fully irrigated block to the drought stress block. Irrigation water was supplied with an overhead sprinkler irrigation system that dispenses 12 mm of water per week. Except for the different irrigation treatments, all field management practices were uniform for both the well-watered and water-stressed experiments.

Experimental hybrids were laid out in a 10 × 10 triple-lattice design in each block in single-row plots, 4 m long with spacing of 0.75 m between rows and 0.50 m spacing between plants within a row. Three seeds were sown per hill and later thinned to two plants per hill two weeks after planting (2WAP) to attain a population density of 53,333 plants ha−1 . Standard cultural practices were applied in field maintenance.

PVC access tubes were installed in December 2010 and 2011 in both well-watered and moisture stressed blocks to monitor volumetric soil moisture content during the growing cycles of the crops, particularly during the critical periods of moisture stress in Block 2. Details of the installation can be found in Ref. [2] .

### 2.2. Data collection

Volumetric soil moisture content was monitored each year with a portable soil moisture monitoring device known as Diviner 2000, starting from 35 days after planting (DAP). Details of the procedures were stated in Ref. [2] .

Soil moisture data were recorded first on weekly basis and later on daily basis when the impact of water stress became very critical in each year. Data were downloaded from the Diviner 2000 display unit on a desktop computer.

Data were also recorded on several physiological and agronomic traits but only those of days to 50% silking (DTS), plant height (PLHT), ear aspect (EASP), number of ears per plant (EPP), and grain yield (GY) are presented in this report. DTS was recorded as the number of days from planting to when 50% of plants in a plot had emerged silks. PLHT was measured in centimeters (cm) as the distance from the base of the plant to the height of the first tassel branch. Ear aspect (EASP) was visually rated on a scale of 1–5, where 1 = clean, uniform, large, and well-filled ears and 5 = rotten, variable, small, and partially filled ears. EPP was computed as the proportion of total number of ears divided by the number of plants harvested. All ears harvested from each plot were shelled and weighed to determine grain weight and a representative sample was taken to determine percent moisture. Grain yield (GY), measured in kg ha−1 adjusted to 15% moisture content was calculated from grain weight and percent moisture.

### 2.3. Data analysis

Separate analyses of variance (ANOVAs) were performed on the data collected in 2010 and 2011 for each environment (drought stress and well-watered) to generate entry means adjusted for block effects according to an alpha lattice design. Replications, years and incomplete blocks were considered as random effects while experimental hybrids were considered fixed effects. Hybrids were then analyzed as a randomized complete block design (RCBD) combined over the two years because the lattice design did not have significant advantage over RCBD. All analyses were performed with PROC GLM in SAS [11] using a RANDOM statement with TEST option. Persons correlation coefficients between grain yield and other traits under both irrigation treatments were calculated using procedures in SAS. Drought tolerance index (DTI) was computed as a percentage of grain yield loss due to drought stress on the yield realized under full irrigation as:

 ${\displaystyle DTI\quad (\%)=[({\mbox{Yield}}\quad {\mbox{under}}\quad {\mbox{well}}-{\mbox{watered}}-{\mbox{yield}}\quad {\mbox{under}}\quad {\mbox{drought}})/{\mbox{Yield}}\quad {\mbox{under}}\quad {\mbox{well}}-{\mbox{watered}}]{_{\ast }}100}$

## 3. Results

Results of ANOVA combined over the years revealed significant year effect for all measured traits except number of ears per plant under well-watered condition (Table 2 ). Genotype × year interaction was significant only for days to silking and ear aspect under well-watered and leaf death score under drought stress (Table 2 ). Hybrids differed significantly in grain yield performance and for all other measured traits under both irrigation treatments (Table 2 ).

Table 2. Mean squares from analyses of variance for the traits of 96 single-crosses and 4 hybrid checks generated in 6 sets and evaluated under well-watered and drought stress conditions over 2 years at Ikenne in Nigeria.
Source of Variation Df GY DTS PLHT EASP EPP LFDTH
Well-watered environment
Year (Y) 1 0.7* 15.8*** 2.7*** 1.1** 0.3
Rep × Y 4 0.8 1.7*** 0.6 2.1** 1.8*
Genotype 99 46.7*** 43.8*** 44.3*** 47.7*** 26.7***
Genotype × Y 99 10.7 10.3** 11.8 12.6* 13.3
Error 396 41.3 28.3 40.5 36.5 57.9
Drought stress environment
Year (Y) 1 13.6*** 0.5* 37.2*** 9.0*** 4.6*** 43.9***
Rep × Y 4 1.8** 1.3* 0.9* 1.9** 1.5* 1.9***
Genotype 99 29.8*** 44.1*** 27.3*** 32.2*** 28.7*** 28.6***
Genotype × Y 99 11.6 11.8 2.9 10.4 12 7.8**
Error 396 43.1 42.3 28.6 46.5 53.3 17.9
• ,**,*** Mean squares significant at p < 0.05, 0.01, and 0.0001, respectively.

The means and statistics of grain yield (GY) of the top 10 and bottom 10 hybrids and the checks under well-watered and drought stress conditions are presented in Table 3 . Under drought stress, GY ranged between 444 and 3022 kg ha−1 whereas under full irrigation it varied from 3827 to 8887 kg ha−1 (Table 1 ). The trial mean yield of 1868 kg ha−1 under drought represented only 23% of the trial mean yield (6119 kg ha−1 ) under well-watered conditions. Hence, the drought tolerance index (DTI), which is an indicator of hybrid yield loss due to drought stress, ranged between 54 and 90% with an average of about 70% (Table 3 ).

Table 3. Means of grain yield and other traits and their standard errors for all combinations of exotic and adapted DT inbred lines averaged over two years under drought stress (DS) and well-watered (WW) conditions at Ikenne in Nigeria.
Traits All source combinations
DS WW DS WW DS WW DS WW DS WW
Grain yield (kg ha−1 ) 1392 ± 177 5871 ± 346 2166 ± 71 6703 ± 105 2229 ± 105 6143 ± 142 1693 ± 67 6040 ± 106 1379 ± 89 5147 ± 129
Silking dates (d) 64 ± 0.70 58 ± 0.43 60 ± 0.22 56 ± 0.13 60 ± 0.39 55 ± 0.19 62 ± 0.23 57 ± 0.13 63 ± 0.42 57 ± 0.21
Ear aspect (1–5) 3.5 ± 0.09 2.9 ± 0.10 3.0 ± 0.04 2.8 ± 0.04 3.0 ± 0.06 3.0 ± 0.04 3.3 ± 0.04 2.9 ± 0.04 3.3 ± 0.06 3.2 ± 0.05
Ears per plant (no) 0.6 ± 0.05 0.9 ± 0.02 0.8 ± 0.01 1.0 ± 0.01 0.8 ± 0.02 1.0 ± 0.01 0.7 ± 0.01 1.0 ± 0.01 0.6 ± 0.02 0.9 ± 0.01
Leaf death score (1–9) 6.4 ± 0.42 6.7 ± 0.13 6.0 ± 0.20 6.5 ± 0.14 7.4 ± 0.17

The yield rank of hybrids from the four source combinations under both irrigation treatment conditions did not follow a similar trend (Table 3 ). Under drought stress, the exotic × exotic sets of inbreds and adapted × exotic sets of inbreds produced hybrids with higher mean grain yield than exotic × adapted and adapted × adapted hybrids. Adapted × exotic hybrid combinations were less variable in comparison with their counterparts from exotic × exotic crosses (Table 3 ). The adapted × exotic hybrids had a yield advantage of 28% over exotic × adapted hybrids. Under well-watered condition, the adapted × exotic hybrids produced the highest mean yield of 6703 kg ha−1 , and had yield advantages of 9% over the exotic × exotic hybrids and 14% over the hybrid checks (Table 3 ). The adapted × adapted crosses produced the lowest average yield under both irrigation treatments (Table 3 ). Under drought stress, the adapted × exotic and exotic × exotic hybrids silked 2 days earlier than other sets of hybrids. Both adapted × exotic and exotic × exotic hybrids had an average score of 3 for ear aspect and recorded an average of 0.8 for number of ears per plant, ratings that are better than other hybrid combinations. Other measured traits followed similar patterns under well-watered condition (Table 3 ).

Grain yield under drought stress condition had significant and positive (r = 0.5; P < 0.0001) correlation with yield under well-watered condition. Under both irrigation conditions, grain yield also had significant and positive associations with plant height and number of ears per plant, but had negative and significant association with days to 50% silking (data not shown).

The top 10 hybrids under each irrigation condition outclassed the best hybrid check (Table 4 ). Three hybrids involving adapted and exotic lines as parents (ADL47 × EXL15, ADL41 × EXL15, and EXL02 × ADL47) were found among the top 10 yielders under both irrigation treatments. ADL47 × EXL15, ADL41 × EXL15 and EXL02 × ADL47 had yield advantages of 32, 27 and 25% over the best check under drought stress, respectively. These three hybrids also out-yielded the best check by 6, 12 and 7%, respectively, under full irrigation (Table 4 ).

Table 4. Mean GYs and DTI of 10 highest yielding and 10 lowest yielding hybrids, and 4 hybrid checks under well-watered (WW) drought stress (DS) conditions.
Well-watered environment Drought stress environment
Hybrids GY DTI Hybrids GY DTI
Top 10 Top 10
EXL06xADL47 8887 87.3 EXL10xEXL04 3023 55.2
EXL15xEXL05 7588 71.9 EXL17xEXL05 2627 57.4
EXL02xADL47 7488 65.5 EXL10xEXL01 2594 53.9
Bottom 10 Bottom 10
Hybrid checks Hybrid Checks
M1026-7 7029 75.7 M1026-8 2075 66.9
M1026-8 6257 66.9 M1026-7 1707 75.7
OBA SUPER 1 5131 82.2 OBA SUPER 1 911 82.2
OBA 98 5068 82.7 OBA 98 876 82.7
Statistics Statistics
Mean 6119 69.5 Mean 1868 69.5
LSD0.05 1359 LSD0.05 918

GY = Grain yield measured in kg ha−1 , DTI = Drought tolerance Index expressed in %, LSD0.05  = Least significant Difference at 5% probability level.

Hybrids in bold exhibited comparatively high yield performance under both drought and well-watered conditions.

## 4. Discussion

The level of drought stress imposed on the trials in the two seasons was monitored in order to ensure that it was sufficient enough to elicit differential reactions of hybrids to the treatment. The average grain yield for experimental hybrids under drought in this study was 23% of that under well-watered conditions. This is within the range of 20–30% suggested as severe drought stress [3] . The non-existence of significant hybrid × year interaction effects, suggesting that hybrids had consistent performance over the two years, was consistent with the result of other authors [4] . Since hybrids were consistent in their performance over years, superior genotypes with enhanced drought tolerance and high yield performance can be selected under both irrigation treatments. The mean grain yield of above 2.5 t ha−1 produced by the top ten highest-yielding hybrids under drought stress in this study was higher than the 1.0–2.0 t ha−1 benchmark suggested by previous authors [3] for selecting drought tolerant hybrids in tropical maize.

There is sufficient genetic variability for drought tolerance in tropical maize germplasm, hence breeding for drought tolerance should be encouraged as part of the holistic approach to solving food insecurity particularly among the most vulnerable and resource poor farmers of sub-Saharan Africa. Also introduced germplasm can serve as sources of new novel alleles for germplasm improvement for higher yield performance as demonstrated by this study. All the top yielding hybrids were mostly those of exotic × adapted line combinations.

Three hybrids, namely ADL47 × EXL15, ADL41 × EXL15 and EXL02 × ADL47, had great potential for further testing and release to farmers as drought tolerant and high yielding hybrid varieties. These hybrids produced competitive yields under both irrigation treatments and out-classed the best hybrid check, thereby showing relatively little yield penalty due to the severe stress imposed. They also maintained good performance in three diverse locations in Nigeria [1] .

## Acknowledgments

This report is a part of Ph.D. thesis research fully funded by the Alliance for a Green Revolution in Africa (AGRA) at West Africa Centre for Crop Improvement (WACCI), University of Ghana, Legon , and the International Institute of Tropical Agriculture . The lead author is immensely grateful for the funding. All the staff members of the Maize Improvement Unit at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria, are appreciated for providing technical supports during field trials.

## References

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