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Maize (Zea mays L.) hybrids with enhanced provitamin A (proVA) content have been deployed in sub-Saharan Africa, where low soil nitrogen and drought stress are common. The objectives of this study were to assess: (i) the effects of drought and low-N stress on grain proVA content of hybrids with enhanced proVA content, and (ii) the inheritance of proVA carotenoids under these stress conditions. An 11-line diallel cross (55 F1 crosses) was evaluated for carotenoid content and grain yield under optimum conditions, drought, and low-N stress. Compared with the optimum treatment, mean proVA was lower under both stress treatments. The consistency of genetic effects across stress treatments suggested that hybrids with improved proVA content can be developed for a broad range of environments, provided they are sufficiently adapted. General combining ability (GCA) was significant (P 85% of the variation among hybrids, whereas specific combining ability (SCA) effects were generally weak (P < 0.05), accounting for 5 to 15% of hybrid sums of squares across the three treatments. These results indicated that the inheritance of proVA was not affected by stress treatments. A negative correlation between grain yield and proVA carotenoids was detected, but the data suggested that it was caused by the genetic background of the germplasm used rather than pleiotropy. Our results provide insights that may help breeders design effective breeding strategies to develop proVA-enriched cultivars for resource-limited farming systems.

Genotype, environmental temperature, and agronomic management of parents influence seed yield in three-way cross hybrid maize seed production. The objective of this research was to generate information on the seed production of six three-way cross hybrids and their progenitors, adapted to tropical lowlands. Data on days to—and duration of—flowering, distance to spike and stigmas, and seed yield of five female single crosses and five male inbred lines were recorded for different combinations of four planting densities and four sowing dates in Mexico. The effect of planting density was not significant. The male inbred line T10 was the earliest and highest seed yield and T31 the latest, occupying second place in yield. The single crosses T32/T10 and T13/T14 were the earliest and had the highest effective seed yield. At the earliest sowing date, the females were later in their flowering, accumulated fewer growing degree days (GDD), and obtained higher yields since the grain-filling period coincided with hot days and cool nights. To achieve greater floral synchronization and therefore greater production of hybrid seed, differential planting dates for parents are recommended based on information from the accumulated GDD of each parent. The three-way cross hybrids were classified according to the expected seed yield of the females and the complexity in the synchronization of flowering of their parents.

Genetic improvement of maize with elevated levels of zinc (Zn) can reduce Zn deficiency among populations who rely on maize as a staple. Inbred lines of quality protein maize (QPM) and non-QPM with elevated Zn levels in the kernel have been identified. However, information about the optimal strategy to utilize the germplasm in breeding for high-Zn concentration is lacking. As a preliminary step, this study was conducted to ascertain the potential of QPM, non-QPM, or a combination of QPM and non-QPM hybrids for attaining desirable Zn concentration. Twenty elite inbreds, 10 QPM and 10 non-QPM, were crossed according to a modified mating design to generate hybrids, which were evaluated in four environments in Mexico during 2015 and 2016 in order to evaluate their merits as parents of hybrids. The highest mean values of Zn were observed when high-Zn QPM lines were crossed with high-Zn non-QPM lines. Hybrids with high Zn and grain yield were identified. General combining ability (GCA) effects for Zn concentration were more preponderant than specific combining ability (SCA) effects, suggesting the importance of additive gene action for the inheritance of Zn.

Maize (Zea mays L.) inbred lines with expired U.S. Plant Variety Protection Act certificates (ex-PVPA) can be an important source of favorable alleles to improve grain yield, root and stalk lodging, drydown, plant architecture, and reduced time to maturity of tropical hybrids. Understanding the heterotic patterns and combining ability of ex-PVPA lines with tropical lines is necessary to design effective breeding strategies using this germplasm. This study evaluated 10 Iowa Stiff Stalk Synthetic (BSSS) and 11 Non-Stiff Stalk Synthetic (NSSS) inbred lines in hybrid combinations with tropical mid-altitude testers in five mid-altitude environments (900–2000 masl) in Mexico and identified six hybrids with grain yield equal or superior to that of the check hybrids. Two NSSS inbred lines: PHR03 and PHN82 had significant (P

Medium and small seed companies require information on maize parental seed production to make rational choices on what germplasm to introduce into their seed and breeding pipelines. The objective of this study was to generate public data on the appropriate hybrid seed production information of five female lines (T21, T26, T28, T33 and T38) and one line (T10) as a male hybrid parent. The effect of female and male sowing date and sowing density on hybrid seed production characteristics including flowering time, flowering duration, distance between tassel and stigmas and effective seed yield was determined. Recommendations to stagger male and female sowing to achieve flowering synchrony for the hybrid parents were complemented with data of seedling growth stages. The results were similar for T21, T26, T28, and T38, sowing the female and T10 on a row proportion of 4 females:1 male, and on a second date, when the coleoptiles of the female plants are halfway to emerge, sowing the missing male row to complete the 4:2 ratio. T33 is a late flowering line, therefore it is desirable to sow T33 first and T10 five days later, or when the T33 coleoptile begins to emerge. Plant densities did not cause differences in most plant characteristics. Line T21 showed good female parental traits. While this study provides a knowledge framework for seed production technology for these single cross hybrids, data specific to seed production regions need to be generated by seed companies to define the best regimes for hybrid production.

High throughput phenotyping technologies are lagging behind modern marker technology impairing the use of secondary traits to increase genetic gains in plant breeding. We aimed to assess whether the combined use of hyperspectral data with modern marker technology could be used to improve across location pre-harvest yield predictions using different statistical models. A maize bi-parental doubled haploid (DH) population derived from F1, which consisted of 97 lines was evaluated in testcross combination under heat stress as well as combined heat and drought stress during the 2014 and 2016 summer season in Ciudad Obregon, Sonora, Mexico (27°20” N, 109°54” W, 38 m asl). Full hyperspectral data, indicative of crop physiological processes at the canopy level, was repeatedly measured throughout the grain filling period and related to grain yield. Partial least squares regression (PLSR), random forest (RF), ridge regression (RR) and Bayesian ridge regression (BayesB) were used to assess prediction accuracies on grain yield within (two-fold cross-validation) and across environments (leave-one-environment-out-cross-validation) using molecular markers (M), hyperspectral data (H) and the combination of both (HM). Highest prediction accuracy for grain yield averaged across within and across location predictions (rGP) were obtained for BayesB followed by RR, RF and PLSR. The combined use of hyperspectral and molecular marker data as input factor on average had higher predictions for grain yield than hyperspectral data or molecular marker data alone. The highest prediction accuracy for grain yield across environments was measured for BayesB when molecular marker data and hyperspectral data were used as input factors, while the highest within environment prediction was obtained when BayesB was used in combination with hyperspectral data. It is discussed how the combined use of hyperspectral data with molecular marker technology could be used to introduce physiological genomic estimated breeding values (PGEBV) as a pre-harvest decision support tool to select genetically superior lines.

In light of anticipated climate change, we assessed the possibility to use an airborne platform to measure canopy temperature (CT) and the normalized differential vegetation index (NDVI) as well as the suitability of both traits for their use in breeding for tolerance to heat stress. We evaluated 71 subtropical maize (Zea maysL.) hybrids under heat stress and combined heat and drought stress in an environment with average temperatures of 29.8°C during the growing season and 31.2°C during the flowering period. Grain yield (GY) ranged from 0.33 to 4.19 Mg ha−1 under heat stress and from 0 to 1.37 Mg ha−1 under combined heat and drought stress, going along with increases in CT from 42.5°C to 49.5°C and decreases in NDVI from 0.54 to 0.48. The NDVI explained differences between and within treatments, while CT explained differences in GY among treatments and genotypes within the heat and drought stress treatment, as indicated by genetic correlations with GY. A principal component analysis was used to identify combinations of physiological characteristics associated with genotypic variation in GY. Results showed that selection gains for GY could be improved by 0.486 Mg ha−1 and 0.015 Mg ha−1 under heat and combined heat and drought stress, respectively, if selection is simultaneously carried out for GY, NDVI, and lower CT and shorter anthesis silking interval. We postulate that the use of selection indices, including CT and NDVI in conjunction with GY, will improve selection gains and increase cost efficiency of breeding programs. Fil: Neiff, Nicolás. Centro Internacional de Mejoramiento de Maíz y Trigo; México. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Dhliwayo, Thanda. Centro Internacional de Mejoramiento de Maíz y Trigo; México Fil: Suarez, Edgar A.. Centro Internacional de Mejoramiento de Maíz y Trigo; México Fil: Burgueno, Juan. Centro Internacional de Mejoramiento de Maíz y Trigo; México Fil: Trachsel, Samuel. Centro Internacional de Mejoramiento de Maíz y Trigo; México

Maize (Zea mays L.) improvement involves formation, evaluation, selection, and recombination of genetically variable families or inbred lines, and because cultivars must combine many desirable traits, the process can be complicated and lengthy. This study compared 12 experimental open-pollinated varieties (OPVs) developed from maize population Pool 9A by two approaches, full-sib (FS) selection alone and FS selection combined with inbreeding with selection for Maize streakvirus (MSV) resistance. The experimental OPVs were evaluated in 13 environments in Africa, including three sites with artificial MSV inoculation. Full-sib selection combined with inbreeding and selection for MSV resistance [with mild selection for gray leaf spot resistance (GLS, caused by Cercospora zeae maydis Tehon & Daniels)], reduced ear height and reduced days to flowering] resulted in OPVs with similar grain yield but superior MSV resistance (P < 0.01), fewer days to anthesis (P < 0.01), and lower ear height (P < 0.05) than OPVs developed by FS selection alone. Each generation (from So to S 3 ) of inbreeding with selection resulted in OPVs that were 16, 8, 2, and 1% improved for MSV and GLS resistance, ear height, and days to anthesis, respectively. Our results demonstrate improvement of a maize population for MSV resistance and other traits by selection during inbreeding (from So to S 3 ), without negative impact on gains for grain yield achieved by evaluation and selection among the progenitor FS families.

Maize weevil, Sitophilus zeamais Motschulsky, is an important pest of stored maize (Zea mays L.) in the tropics. We used 14 southern African maize inbred lines to assess (i) combining ability for resistance to maize weevil in F 2 and F 2 -Syn 1 grain, (ii) the importance of maternal effects for resistance of F 2 and F 2 -Syn 1 grain to maize weevil, and (iii) combining ability for grain yield of F 1 hybrids. Maize weevil resistance of F 2 grain was evaluated in 2000 for a 14-parent diallel, and in 2002 for F 2 and F 2 -Syn 1 grain of a 10-parent subset diallel. Fifty-gram grain samples of each hybrid were infested with 32 weevils for a 10-d oviposition period, after which the samples were incubated in a laboratory. Grain yield was evaluated for the 14-parent diallel during the summer of 1999-2000 at four locations in Zimbabwe. General combining ability (GCA), specific combining ability (SCA), and reciprocal effects were highly significant (P < 0.01) for number of F 1 weevils emerged from F 2 grain samples for the combined 2000 and 2002 analysis for the 10-parent subset diallel. For weevil resistance of F 2 -Syn 1 grain, GCA and SCA effects were significant (P < 0.01), while reciprocal effects were not important. The GCA was more important than SCA for grain yield, indicating that yield was controlled mainly by additive gene action among these lines. There was no significant relationship between grain yield and weevil resistance. Although inheritance of weevil resistance was complex, our results suggest using inbred lines with good GCA for weevil resistance as female parents for hybrids or as components of synthetic (open-pollinated) cultivars for regions where farmers often store maize grain without chemical protection against weevils.

Maize weevil (Sitophilus zeamais Motschulsky) is an important pest of maize (Zea mays L.) in the tropics, causing serious losses for many resource-poor farmers who store grain on-farm for use as food and seed. This study evaluated whether weevil resistance of six maize populations could be divergently changed by S 1 and S 2 selection, and assessed the importance of replicating grain samples when screening for resistance. Weevil resistance was evaluated for unreplicated S 1 and for replicated and unreplicated S 2 lines by infesting 50-g grain samples with 32 young adult weevils, and then incubating the samples in a controlled temperature and relative humidity (CTH) laboratory. Divergent synthetics were formed by recombining the most resistant 10% and the most susceptible 10% of at least 100 lines screened for weevil resistance for each maize population. Replicated S 2 selection was successful for both populations where it was applied, resulting in an average of 16% (P < 0.01), 49% (P < 0.05), and 20% (P < 0.01) difference between divergent synthetics for weevil progeny emerged, grain weight loss, and the Dobie index of susceptibility, respectively. S 1 unreplicated selection was successful for two of the six populations, while S 2 unreplicated selection was not successful. Reciprocal effects were significant (P < 0.01) for weevil resistance of F 1 varietal crosses among the divergently selected synthetics, indicating the influence of maternal effects. Nevertheless, the most resistant crosses were those among the most resistant synthetics, confirming that additive gene action for weevil resistance was important. Our results provide practical insights regarding methodologies and demonstrate that it is possible to improve weevil resistance of maize populations.