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.
Adapted ; Exotic ; Drought ; Maize germplasm ; Zea mays L.
Maize (Zea mays L.) is a staple food crop that plays a major role in the diet of millions of African people  . 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  . Despite the rising profile of maize, both as food and economic crop in West and Central Africa over the last few decades  and  , it is still prone to many production constraints such as drought  .
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  . 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  . 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  and  . 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  .
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.
Ninety-six (96) newly developed single-cross maize hybrids of IITA-bred adapted and CIMMYT-bred exotic inbred lines  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  . 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).
|1||EXL01 × ADL34||35||ADL37 × EXL02||68||ADL39 × ADL27|
|2||EXL04 × ADL34||36||ADL38 × EXL02||69||ADL34 × ADL32|
|3||EXL05 × ADL34||37||ADL27 × EXL03||70||ADL35 × ADL32|
|4||EXL24 × ADL 34||38||ADL32 × EXL03||71||ADL36 × ADL32|
|5||EXL01 × ADL35||39||ADL37 × EXL03||72||ADL39 × ADL32|
|6||EXL04 × ADL35||40||ADL38 × EXL03||73||ADL34 × ADL37|
|7||EXL05 × ADL35||41||ADL27 × EXL06||74||ADL35 × ADL37|
|8||EXL24 × ADL35||42||ADL32 × EXL06||75||ADL36 × ADL37|
|9||EXL01 × ADL36||43||ADL37 × EXL06||76||ADL39 × ADL37|
|10||EXL04 × ADL36||44||ADL38 × EXL06||77||ADL34 × ADL38|
|11||EXL05 × ADL36||45||ADL27 × EXL07||78||ADL35 × ADL38|
|12||EXL24 × ADL36||46||ADL32 × EXL07||79||ADL36 × ADL38|
|13||EXL01 × ADL39||47||ADL37 × EXL07||80||ADL39 × ADL38|
|14||EXL04 × ADL39||48||ADL38 × EXL07||81||EXL02 × ADL31|
|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|
|31||ADL33 × EXL17||65||ADL34 × ADL27||98||M1026-8 – Check|
|32||ADL47 × EXL17||66||ADL35 × ADL27||99||OBA SUPER 1 – Check|
|33||ADL27 × EXL02||67||ADL36 × ADL27||100||OBA 98 – Check|
|34||ADL32 × EXL02|
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.  .
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.  .
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.
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  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:
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 ).
|Source of Variation||Df||GY||DTS||PLHT||EASP||EPP||LFDTH|
|Rep × Y||4||0.8||1.7***||0.6||2.1**||1.8*||–|
|Genotype × Y||99||10.7||10.3**||11.8||12.6*||13.3||–|
|Drought stress environment|
|Rep × Y||4||1.8**||1.3*||0.9*||1.9**||1.5*||1.9***|
|Genotype × Y||99||11.6||11.8||2.9||10.4||12||7.8**|
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 ).
|Traits||All source combinations|
|Checks||Adapted × exotic||Exotic × exotic||Exotic × adapted||Adapted × adapted|
|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 ).
|Well-watered environment||Drought stress environment|
|Top 10||Top 10|
|Bottom 10||Bottom 10|
|Hybrid checks||Hybrid Checks|
|OBA SUPER 1||5131||82.2||OBA SUPER 1||911||82.2|
|OBA 98||5068||82.7||OBA 98||876||82.7|
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.
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  . 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  . 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  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  .
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.