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

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.

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

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 < 0.05) annual yield gains of 11.89 kg ha-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.

Genetic Gains in Yield and Yield Related Traits under Drought Stress and Favorable Environments in a Maize Population Improved Using Marker Assisted Recurrent Selection.

The objective of marker assisted recurrent selection (MARS) is to increase the frequency of favorable marker alleles in a population before inbred line extraction. This approach was used to improve drought tolerance and grain yield (GY) in a biparental cross of two elite drought tolerant lines. The testcrosses of randomly selected 50 S1 lines from each of the three selection cycles (C0, C1, C2) of the MARS population, parental testcrosses and the cross between the two parents (F1) were evaluated under drought stress (DS) and well watered (WW) well as under rainfed conditions to determine genetic gains in GY and other agronomic traits. Also, the S1 lines derived from each selection types were genotyped with single nucleotide polymorphism (SNP) markers. Testcrosses derived from C2 produced significantly higher grain field under DS than those derived from C0 with a relative genetic gain of 7% per cycle. Also, the testcrosses of S1 lines from C2 showed an average genetic gain of 1% per cycle under WW condition and 3% per cycle under rainfed condition. Molecular analysis revealed that the frequency of favorable marker alleles increased from 0.510 at C0 to 0.515 at C2, while the effective number of alleles (Ne) per locus decreased from C0 (1.93) to C2 (1.87). Our results underscore the effectiveness of MARS for improvement of GY under DS condition.