Genetic variability for drought tolerance in early-maturing maize inbreds under contrasting environments

Drought severely constrains maize (Zea mays L) production in the savannas of West and Central Africa (WCA). Understanding the levels of drought tolerance in early-maturing maize inbreds is crucial for the development of drought-tolerant maize hybrids for the sub-region. A total of 156 inbred lines were evaluated under drought and well-watered conditions at Ikenne and Bagauda in Nigeria for 2 yr. The objectives were to (i) determine the levels of drought tolerance in early-maturing inbreds, (ii) assess the performance of the inbreds and relationship among traits under the contrasting environments, and (iii) identify the most stable inbreds across environments. Inbreds differed significantly (p < 0.01) in grain yield and other measured traits under drought and well-watered conditions. Grain yield of inbreds was significantly (p < 0.01) correlated with ears per plant (r = 0.50), anthesis-silking interval (r = -0.55), plant aspect (r = -0.57), ear aspect (r = -0.35) and stay-green characteristic (r = -0.28) under drought. Forty-eight percent of the lines were identified as drought tolerant with tolerance indices ranging from 0.17 to 15.31. Broad-sense heritability estimate was 43% for grain yield under drought and 47% under well-watered con- ditions. Drought reduced grain yield of the inbreds by 3-88%, averaging 52%. Biplot analysis identified inbreds TZEI 18, TZEI 56, TZEI 1, and TZEI 19 as the most stable across environments. The inbreds with high levels of drought tolerance could be utilised for the development of drought-tolerant hybrids and synthetic varieties as well as for introgression of tolerance genes into tropical maize breeding populations

Agronomic traits associated with genetic gains in maize yield during three breeding eras in West Africa

Studies on genetic gains in grain yield in maize (Zea mays L) is crucial to identify traits of potential value and the necessary modifications in breeding methodologies and strategies for increased progress in future breeding efforts. Fifty early-maturing maize cultivars developed during three breeding eras were evaluated for 2 yr in two field experiments involving 16 multiple stress (drought, Striga-infested, and low soil nitrogen) environments and 35 optimum environments to determine the changes in agronomic traits associated with the genetic gains in grain yield over three breeding eras. The average rate of increase in grain yield was 30 kg ha–1 yr-1 corresponding to 1.59% annual genetic gain across multiple stresses. Among the agronomic traits under stress, only ears per plant (0.32% year-1), ear aspect (-0.51% year-1), plant aspect (-0.24% year-1) and days to anthesis (0.11% year-1) changed significantly (

Selection of extra-early maize inbreds under low N and drought at flowering and grain-filling for hybrid production

Extra-early maize (Zea mays L) tolerant to low-N and drought could stabilize yields in sub-Saharan Africa. Studies were conducted under drought and low-N conditions in Nigeria for three years to determine the potential of 90 extra-early inbreds for hybrid production and evaluate the performance of 36 hybrids derived from the inbreds under drought at flowering and grain-filling periods and well-watered conditions. Results showed significant ge¬notypic mean squares for grain yield and most other traits of the inbreds under drought and/or low-N conditions. TZEEI6, TZEEI4, TZEEI36, and TZEEI38 were identified as ideal inbreds under drought. Under low-N, TZEEI19, TZEEI96 and TZEEI45 were top ranking with TZEEI19 as the ideal inbred. TZEEI19, TZEEI29, TZEEI56, TZEEI38, and TZEEI79 were tolerant to both stresses. Eighteen of the 36 hybrids produced above-average yields across environments with four hybrids identified as very stable. TZEEI29 x TZEEI21 was the closest to the ideal geno¬type because it combines large mean performance with high yield stability. Hybrid yield under drought had large positive correlation with grain yield under well-watered environments. Selection for inbred traits such as days to silking and anthesis-silking interval under drought predicted fairly accurately hybrid yield under well-watered envi¬ronments. It was concluded that extra-early inbreds and hybrids are not only drought escaping but also possess drought and low-N tolerant genes

Yield Stability and Inter-Traits Relationships of Maize Hybrids Under Low- and Optimum-Nitrogen Conditions

The menace of low soil nitrogen poses a significant challenge to maize production in sub-Saharan Africa, necessitating the development of high-yielding hybrids with increased nitrogen (N) tolerance. Two hundred and forty (240) maize hybrids were evaluated across eight environments at Zaria and Mokwa in Nigeria during the 2020 and 2021 growing seasons, encompassing both low- and optimum-N conditions. The primary objective of the study was to identify high-yielding, stable, and low-N-tolerant hybrids for potential adoption in Nigeria. The study results underscored the significant impact of nitrogen availability on various agronomic traits, causing a substantial 53% reduction in grain yield. Significant genetic variation was observed among hybrids for most measured traits. SMLW146 × IITA1878, SMLW147 × SAM50M and SMLW147 × IITA1878 were the highest-yielding hybrids across the test environments with a yield advantage of 48.2%, 46.6% and 43.1%, respectively over the best check. GGE biplot analysis identified Mokwa Low-N 2021 (MLN21) and Mokwa optimum-N 2021 (MOP21) as discriminating and representative environments for selecting superior hybrids. SMLW147 × SAM50M emerged as the most stable low-N tolerant hybrid with the highest yield performance. Conversely, hybrid SMLW147 × IITA1878 produced a high yield across environments but was unstable. The study concludes by recommending SMLW147 × SAM50M for cultivation across both low- and optimum-N conditions, while SMLW147 × IITA1878 is specifically recommended for cultivation in optimum-N environments. Anthesis-silking interval, plant aspect, ear aspect, and ear height were identified as the most suitable secondary traits for selecting hybrids for high grain yield under low-N conditions

Approaches and progress in breeding drought‐tolerant maize hybrids for tropical lowlands in west and central Africa

Abstract Drought represents a significant production challenge to maize farmers in West and Central Africa, causing substantial economic losses. Breeders at the International Institute of Tropical Agriculture have therefore been developing drought‐tolerant maize varieties to attain high grain yields in rainfed maize production zones. The present review provides a historical overview of the approaches used and progress made in developing drought‐tolerant hybrids over the years. Breeders made a shift from a wide area testing approach, to the use of managed screening sites, to precisely control the intensity, and timing of drought stress for developing drought‐tolerant maize varieties. These sites coupled with the use of molecular markers allowed choosing suitable donors with drought‐adaptive alleles for integration into existing elite maize lines to generate new drought‐tolerant inbred lines. These elite maize inbred lines have then been used to develop hybrids with enhanced tolerance to drought. Genetic gains estimates were made using performance data of drought‐tolerant maize hybrids evaluated in regional trials for 11 years under managed drought stress, well‐watered conditions, and across diverse rainfed environments. The results found significant linear annual yield gains of 32.72 kg ha−1 under managed drought stress, 38.29 kg ha−1 under well‐watered conditions, and 66.57 kg ha−1 across multiple rainfed field environments. Promising hybrids that deliver high grain yields were also identified for areas affected by drought and variable rainfed growing conditions. The significant genetic correlations found among the three growing conditions highlight the potential to exploit the available genetic resources and modern tools to further enhance tolerance to drought in hybrids

Table2_Estimating genetic gains for tolerance to stress combinations in tropical maize hybrids.DOCX

Maize is a strategic food crop in sub-Saharan Africa. However, most maize growing tropical savannas particularly in West and Central African experience the occurrence of frequent droughts and Striga infestation, resulting in 30–100% yield losses. This production zones need maize cultivars that combine tolerance to the two stresses. IITA in collaboration with national partners has thus employed a sequential selection scheme to incorporate both drought tolerance and Striga resistance in topical maize hybrids using reliable screening protocols. The main objective of the present study was therefore to use grain yield and other agronomic traits recorded in regional collaborative hybrid trials conducted for 8 years under manged stressful and non-stressful conditions and across rainfed field environments to estimate genetic gains in grain yields using mixed model analyses. The results showed significant (p −1 under manged drought stress (MDS) and 86.60 kg ha−1 under Striga infestation (STRIN) with concomitant yield increases of 62.65 kg ha−1 under full irrigation (WW), 102.44 kg ha−1 under Striga non-infested (STRNO) conditions and 53.11 kg ha−1 across rainfed field environments. Grain yield displayed significant but not strong genetic correlation of 0.41 ± 0.07 between MDS and STRIN, indicating that gene expression was not consistent across the two stress conditions. Furthermore, grain yield recorded in MET had significant moderate genetic correlations of 0.58 ± 0.06 and 0.44 ± 0.07It with MDS and STRIN, respectively. These results emphasize the need to screen inbred linens under both stress conditions to further enhance the rate of genetic gain in grain yield in hybrids for areas where the two stresses co-occur. Nonetheless, this study demonstrated that the sequential selection scheme has been successful in generating hybrids with dependable yields that can reduce chronic food deficits in rural communities experiencing simultaneous presence of drought and S. hermonthica infestation in their production fields.</p

Table1_Estimating genetic gains for tolerance to stress combinations in tropical maize hybrids.DOC

Maize is a strategic food crop in sub-Saharan Africa. However, most maize growing tropical savannas particularly in West and Central African experience the occurrence of frequent droughts and Striga infestation, resulting in 30–100% yield losses. This production zones need maize cultivars that combine tolerance to the two stresses. IITA in collaboration with national partners has thus employed a sequential selection scheme to incorporate both drought tolerance and Striga resistance in topical maize hybrids using reliable screening protocols. The main objective of the present study was therefore to use grain yield and other agronomic traits recorded in regional collaborative hybrid trials conducted for 8 years under manged stressful and non-stressful conditions and across rainfed field environments to estimate genetic gains in grain yields using mixed model analyses. The results showed significant (p −1 under manged drought stress (MDS) and 86.60 kg ha−1 under Striga infestation (STRIN) with concomitant yield increases of 62.65 kg ha−1 under full irrigation (WW), 102.44 kg ha−1 under Striga non-infested (STRNO) conditions and 53.11 kg ha−1 across rainfed field environments. Grain yield displayed significant but not strong genetic correlation of 0.41 ± 0.07 between MDS and STRIN, indicating that gene expression was not consistent across the two stress conditions. Furthermore, grain yield recorded in MET had significant moderate genetic correlations of 0.58 ± 0.06 and 0.44 ± 0.07It with MDS and STRIN, respectively. These results emphasize the need to screen inbred linens under both stress conditions to further enhance the rate of genetic gain in grain yield in hybrids for areas where the two stresses co-occur. Nonetheless, this study demonstrated that the sequential selection scheme has been successful in generating hybrids with dependable yields that can reduce chronic food deficits in rural communities experiencing simultaneous presence of drought and S. hermonthica infestation in their production fields.</p