Structurée en trois principaux chapitres, cette étude présente une approche pluridisciplinaire en combinant desanalyses spatiales et temporelles de la variabilité interannuelle des saisons de pluies d’une base de données agroclimatique,des essais phénologiques mensuelles d’une sélection de variétés photopériodiques de mil et de sorgholocales et améliorées, et l’utilisation conjointe d’un modelé de culture calibre pour les effets de la latitude et d’unSystème d’Information Géographique (SIG). Elle permet de tirer des conclusions originales et d’identifier lesenvironnements cibles prometteurs pour la vulgarisation des variétés de mil et de sorgho photopériodiques enAfrique de l’Ouest. Elle met en évidence l’existence de relations significatives entre les réponses phénologiques,le degré de sensibilité à la photopériode des variétés et les facteurs environnementaux, dont les principales sont :‒ La variabilité interannuelle des pluies en Afrique de l’Ouest durant les cinquante dernières années (1950-2000) est marquée par une diminution globale de la pluviométrie annuelle caractérisée par une variabilitéaccrue des dates de début de saison des pluies dans les latitudes basses que dans les latitudes élevées etune variabilité interannuelles des dates de fin de saison plus notable sur les latitudes du Nord plus quedans les basses latitudes au sud.‒ La distribution spatiale des variétés de mil et de sorgho photopériodiques est étroitement liée à larépartition spatiale et temporelle de la variabilité des dates de débuts et de fin de la saison de pluies. Lesvariétés photopériodiques se localisent essentiellement dans les basses latitudes ou la variabilité des datesde début de saison est plus élevée que celle des dates de fin de saison plus stable.‒ La réponse phénologique des variétés de mil et de sorgho photopériodiques est strictement dépendantede la date de semis et de la latitude. Les effets de la latitude sur la phénologie et la réactionphotopériodique des variétés est proportionnelle à la distance entre la zone de culture d’une variété et salatitude d’origine.‒ Le modèle de développement des céréales photopériodiques « Impatience » calibré et corrigé à cet effet(effet de la latitude) prévois et défini avec plus de précision les zones d’adaptation variétale optimale pourles variétés de mil et de sorgho photopériodiques en Afrique de l’Ouest.‒ L’adéquation entre la durée du cycle phénologique calibrée par le modèle de culture corrigé de l’effet dela latitude et celle observée dans les systèmes agraires à base de mil et de sorgho photopériodiques dansles agro-systèmes villageois au Mali valide le modèle les cartes d’adaptation variétale optimale. / Structured in three main chapters; this study addresses multidisciplinary approach combining spatial and temporalanalysis of interannual variability of rainfall, multi-locations trials of a large and representative sample of photoperiodicvarieties of sorghum and millet combined with a photoperiod response crop model and Geographic Information System(GIS). It allowed identifying specific target environment to promote the vulgarization of photoperiodic varieties ofsorghum and millet in West Africa. This study depicted significant relationships between phenology, PP sensitivityresponses and environmental factors, where the mains components are listed below:‒ The interannual rainfall variability in West Africa over the past fifty years (1950-2000) was marked by ageneral decrease in annual rainfall characterized by high variability in the onset of growing period at lowerlatitudes and an increase in the interannual variability of the end of growing period in the northern latitudeswith least significant variability through climatic periods.‒ The spatial distribution of PP sensitivity varieties of millet and sorghum is closely related to the spatial andtemporal distribution of the interannual variability of the onset and end dates of growing period. Photoperiodsensitive varieties are located mainly at lower latitudes where the interannual variability in the onset of thegrowing period is much higher than the end of the growing period.‒ Phonological response and expression of photoperiod sensitivity are strictly dependent on sowing date andlatitude. Effects of latitude on the expression of photoperiod sensitivity and phenology are proportional tolatitude.‒ The "impatience" sub-crop model calibrated and corrected for latitudinal effect predicts and identifies moreaccurately optimal varietal adaptation areas for sorghum and millet varieties in West Africa.‒ The adequacy between calibrated duration of the phonological cycle by the crop model adjusted for latitudinaleffect and that observed in farming systems in agro-systems in Mali, allowed to validate the predicted optimalvarietal adaptation maps by defined the model.

Der Klimawandel fordert die Anbausysteme heraus, um das derzeitige Produktionsniveau zu verbessern oder sogar aufrechtzuerhalten. Es wird erwartet, dass zukünftige Trends bei Temperatur und Niederschlag die Ernteproduktivität beeinträchtigen. Es ist daher notwendig, möglicher Lösungen zur Anpassung der Anbausysteme an den Klimawandel zu untersuchen. Ziel dieser Arbeit ist es, das Wissen über die Anpassung von weltweit relevanten Getreidepflanzen an den Klimawandel zu erweitern. Die zentrale Fragestellung ist, ob globale Anbausysteme an den Klimawandel angepasst werden können, indem die Phänologie der Kulturpflanzen durch Anpassung von Wachstumsperioden und Sorten gesteuert wird. Die Phänologie und die Ertragsreaktionen sowohl auf den Temperaturanstieg als auch auf die Sortenselektion werden zunächst anhand eines Ensembles von “Global Gridded Crop Models” bewertet. Anschließend wird die Komplexität der Anpassung durch phänologisches Management analysiert, insbesondere unter Berücksichtigung der bestehenden großen Wissenslücken bei der Auswahl von Pflanzensorten. Das Ergebnis der Analyse ist ein regelbasierter Algorithmus, der phänologische Zyklen der Kulturpflanzen auswählt, um die Zeit für die Ertragsbildung zu maximieren und Temperatur- und Wasserbelastungen während der Wachstumszyklen der Kulturpflanzen zu minimieren. Die berechneten Aussaatdaten und Wachstumsperioden werden verwendet, um globale Muster von Sorten zu parametrisieren, die an aktuelle und zukünftige Klimaszenarien angepasst sind. Diese Arbeit zeigt, dass die Auswirkungen des Klimawandels auf die Pflanzenproduktivität erheblich variieren können, je nachdem, welche Annahmen für das agronomische Management getroffen werden. Änderungen im Management zu vernachlässigen, liefert die pessimistischste Prognose für die zukünftige Pflanzenproduktion. Relativ einfache Ansätze zur Berechnung angepasster Aussaatdaten und Sorten bieten eine Grundlage für die Berücksichtigung autonomer Anpassungsschemata als integraler Bestandteil globaler Modellierungsrahmen. / Climate change is challenging cropping systems to enhance or even maintain current production levels. Future trends in temperature and precipitation are expected to negatively impact crop productivity. It is therefore necessary to explore adaptation options of cropping systems to changing climate. The aim of this thesis is to advance knowledge on adaptation of world-wide relevant grain crops to climate change. The central research question is whether global cropping systems can be adapted to climate change by managing crop phenology through adjusting growing periods and cultivars. Phenology and yield responses to both temperature increase and cultivar selection are first assessed making use of an ensemble of Global Gridded Crop Models. Then, the complexity of adaptation through phenological management is analysed, particularly addressing the existing large knowledge gaps on crop cultivar choice. The outcome of the analysis is a rule-based algorithm that selects crop phenological cycles aiming at maximizing the time for yield formation and minimizing temperature and water stresses during the crop growth cycles. The computed sowing dates and growing periods are used to parametrize global patterns of cultivars adapted to present and future climate scenarios. This thesis demonstrates that the impacts of climate change on crop productivity can vary substantially depending on which assumptions are made on agronomic management. Neglecting any changes in management return the most pessimistic projection on future crop production. Relatively simple approaches to compute adapted sowing dates and cultivars provide a base for considering autonomous adaptation schemes as an integral component of global scale modelling frameworks.

Recurring drought conditions have always been an endemic feature of climate in South Africa, limiting maize development and production. However, recent projections of the future climate by the Intergovernmental Panel on Climate Change suggest that due to an increase of atmospheric greenhouse gases, the frequency and severity of droughts will increase in drought-prone areas, mostly in subtropical climates. This has raised major concern for the agricultural sector, particularly the vulnerable small-scale farmers who merely rely on rain for crop production. Farmers in the Luvuvhu River catchment are not an exception, as this area is considered economically poor, whereby a significant number of people are dependent on rain-fed farming for subsistence. This study was therefore conducted in order to improve agricultural productivity in the area and thus help in the development of measures to secure livelihoods of those vulnerable small-scale farmers. Two drought indices viz. Standardized Precipitation Evapotranspiration Index (SPEI) and Water Requirement Satisfaction Index (WRSI) were used to quantify drought. A 120-day maturing maize crop was considered and three consecutive planting dates were staggered based on the average start of the rainy season. Frequencies and probabilities during each growing stage of maize were calculated based on the results of the two indices. Temporal variations of drought severity from 1975 to 2015 were evaluated and trends were analyzed using the non-parametric Spearman’s Rank Correlation test at α (0.05) significance level. For assessing climate change impact on droughts, SPEI and WRSI were computed using an output from downscaled projections of CSIRO Mark3.5 under the SRES A2 emission scenario for the period 1980/81 – 2099/100. The frequency of drought was calculated and the difference of SPEI and WRSI means between future climate periods and the base period were assessed using the independent t-test at α (0.10) significance level in STATISTICA software. The study revealed that planting a 120-day maturing maize crop in December would pose a high risk of frequent severe-extreme droughts during the flowering to the grain-filling stage at Levubu, Lwamondo, Thohoyandou, and Tshiombo; while planting in October could place crops at a lower risk of reduced yield and even total crop failure. In contrast, stations located in the low-lying plains of the catchment (Punda Maria, Sigonde, and Pafuri) were exposed to frequent moderate droughts following planting in October, with favorable conditions noted following the December planting date. Further analysis on the performance of the crop under various drought conditions revealed that WRSI values corresponding to more intense drought conditions were detected during the December planting date for all stations. Moreover, at Punda Maria, Sigonde and Pafuri, it was observed that extreme drought (WRSI <50) occurred once in five seasons, regardless of the planting date. Temporal analysis on historical droughts in the area indicated that there had been eight agricultural seasons subjected to extreme widespread droughts resulting in total crop failure i.e. 1983/84, 1988/89, 1991/92, 1993/94, 2001/02, 2002/03, 2004/05 and 2014/15. Results of Spearman’s rank correlation test revealed weak increasing drought trends at Thohoyandou (ρ = of 0.5 for WRSI) and at Levubu and Lwamondo (ρ = of 0.4 for SPEI), with no significant trends at the other stations. The study further revealed that climate change would enhance the severity of drought across the catchment. This was statistically significant (at 10% significance level) for the near-future and intermediate-future climates, relative to the base period. Drought remains a threat to rain-fed maize production in the Luvuvhu River catchment area of South Africa. In order to mitigate the possible effects of droughts under climate change, optimal planting dates were recommended for each region. The use of seasonal forecasts during drought seasons would also be useful for local rain-fed maize growers especially in regions where moisture is available for a short period during the growing season. It was further recommended that the Government ensure proper support such as effective early warning systems and inputs to the farmers. Moreover, essential communication between scientists, decision makers, and the farmers can help in planning and decision making ahead of and during the occurrence of droughts. / Agriculture, Animal Health and Human Ecology / M. Sc. (Agriculture)