No description available.

Maize (Zea mays ssp. mays), a crop of great importance worldwide, is characterised by its great adaptability and its extraordinary variability both at the molecular and the phenotypic level. Behind these remarkable features lies a complex evolutionary history. The main aim of this research is to contribute to a better understanding of it by answering questions about the origin, phylogenetic relationships, and initial dispersion of maize, using South America as a case study. To achieve this aim, archaeological specimens and primitive landraces were genotyped at 20 microsatellite loci. Sequences of Adh2-UM, a locus informative of the routes of initial dispersion of maize in South America, were also obtained. The data gathered were analysed using tree-building and network methods, and a Bayesian model-based clustering approach to infer population structure. Our results are consistent with a model of initial dispersion by which after its introduction, maize spread along the Andes and through eastern Bolivia and northwest Argentina towards the east of the continent to the area of Paraguay and southern Brazil, and gave rise to the maize distributed to the east of the Andean mountains. Analysis of population structure indicate that variation among the races analysed is more phenotypic than genetic, and that considerable amounts of gene flow has been occurring at a macro-regional level. We believe that cultural practices and cultural contacts played a fundamental role in making maize on these areas more genetically similar, while at the same time keeping maize phenotypically different. Further studies of ancient DNA, coupled with archaeological and ethnographical knowledge of cultural trajectories, have the huge potential of contributing to a better understanding of how and since when these macro-regional areas of gene flow have been in place.

Selenium (Se) is an essential element for humans, which is derived primarily from dietary sources. Habitual suboptimal dietary Se intake is associated with reduced Se status and adverse health outcomes including cardiovascular disorders, impaired immune functions and some cancers. The global extent of suboptimal dietary Se intake is difficult to estimate, but is likely to be widespread where food choices are narrow, for example, in subsistence agricultural contexts. This study aimed to: (1) characterise the likely contribution of maize grain to dietary Se intake in rural Malawi; (2) test the dependency of maize grain Se concentrations on soil factors; and (3) identify agronomic methods to improve Se concentration in maize grain. 88 field sites across Malawi were sampled across Malawi in 2009 and 2010 before determining maize grain, total soil and KH2PO4-extractable soil Se concentrations by inductively coupled plasma-mass spectrometry (ICP-MS). Dietary Se intakes from other food sources were estimated from the literature. The median maize grain Se concentration in Malawi was 0.019 mg Se kg-1 (range 0.005-0.533), representing a median intake of 6.7 µg Se person-1 d-1 from maize. Suboptimal (<30 µg d-1) dietary Se intake is therefore likely to affect most of the rural population in Malawi. Maize grain Se concentration was c. 10-fold higher in crops grown on high pH (>6.5) soils (Vertisols), probably because the dominant species of Se at high soil pH Se(VI) is more available to crops than Se(IV), as evidenced by the KH2PO4-extractable soil concentrations recorded. Total soil Se concentration ranged between 0.0521 and 0.6195 mg kg-1 but provided a poor index of Se availability. The results showed that KH2PO4-extractable Se concentrations >0.01 mg kg-1 and soil pH values >6.5 produced grain Se concentrations exceeding 0.15 mg Se kg-1, a value above which rural populations in Malawi would attain adequate Se intake. Field experiments in which three Se application methods (Na2SeO4 (aq), granular compound (NPK+Se) and granular calcium ammonium nitrate (CAN+Se) were applied were conducted at up to six sites in 2008/09 and 2009/10. Application of Se significantly increased grain and stover Se concentrations and the response was approximately linear for all sites and application methods in both years (R2 >0.90). The results showed that application of Se at 5 g Se ha-1 to maize would deliver adequate intakes for much of the population in Malawi. As total plant recovery of Se ranged from 3-45%, further work is required to identify and address the sources of this variation. In more detailed experiments, the fate of applied Se was investigated at two sites using the stable 74Se isotope. Recovery of applied Se was 0.65 and 1.08 g Se ha-1 at the Chitedze and Mbawa, sites respectively, representing 6.5 and 10.8% of the applied 10 g Se ha-1 by the maize crop; 0.2 g Se ha-1 of native soil Se was also absorbed, leaving 9.35 and 8.92 g Se ha-1 unaccounted. Of the total soil and applied fertiliser Se, fertiliser-derived Se (74Se-labelled) comprised 71 and 82% of plant-Se recovery at Chitedze and Mbawa, respectively. The residual effects of Se application on grain Se in maize crops grown in the subsequent cropping season were 0.3025 and 0.5858 µg kg-1 g-1 applied Se at Chitedze and Mbawa respectively. Residual Se detected as KH2PO4-extractable Se ranged from 0.0029 to 0.106 µg kg-1 g-1 applied Se between sites. Further studies are required to quantify the amount of Se immobilised in the soil pool or lost due to leaching or volatilisation. A further experiment examined how traditional processing procedures for maize grain affected Se concentration in maize flour. At Se fertilisation levels which would increase dietary Se intake to appropriate levels, there was no evidence that traditional milling produced any significant loss of Se from maize flour. Assessment of the contribution of maize to the dietary supply of other nutrients showed that calcium concentration, and hence intake from maize, were very low. Maize grain was low also in K, Cu and Zn but provided a good source of Fe, Mg, Mn and Mo. There is a need to monitor the concentrations of trace metals such as Cd, Co, Ni and Cr as these might exceed the daily allowance and pose a risk to human health.

Thesis (MSc. (Agricultural Economics)) -- University of Limpopo, 2014 / Maize is the most important crop in South Africa, being both the major feed grain for livestock and the primary staple food crop for the majority of the South African population. Furthermore, the maize industry contributes substantially to employment, manufacturing, foreign exchange and food security. The importance of maize in contributing to national growth is critical; this makes it meaningful to investigate the nature of maize farmers’ production decisions. This study quantifies the supply response of maize farmers to price and non-price factors in South Africa using econometric techniques. The non-price factors considered in this study are rainfall, technology and market policy. A modified Nerlovian partial adjustment model was applied on historical time series data spanning from 1980-2012 to estimate the supply response of maize in South Africa. To deal with the expected problems associated with time series data the study adopted several diagnostic tests. Results indicate a short-run supply elasticity of 0.49 and a long-run supply elasticity of 0.65, signifying that maize farmers are less sensitive to price changes. The results confirm that non-price factors seem to have more effect on maize supply in South Africa. These findings coincide with those obtained in supply response studies for field crops conducted in other developing African countries. The study also showed that non-price factors such as, rainfall, technology and market policies have a positive impact on maize production. Given the findings, the study recommends policies that focus more on non-price factors as a means of stabilising maize production. The study also recommends that Industry stakeholders and policymakers should find means to integrate the significant relationships between non-price factors and production output into future decisions and marketing policies to safeguard a healthy, growing and sustainable maize industry in South Africa. Key words: Maize supply response, Nerlovian partial adjustment model, price factors, non-price factors.

Thesis (M.Sc. (Agronomy))--University of Limpopo, 2010 / Refer to document.

Breeding maize (Zea mays L.) for tolerance to acidic soils could improve maize yields. The current study aims to identify maize genotypes with tolerance to acidic soils, as well as identifying secondary traits associated with the tolerance to soil acidity. Ten maize varieties were screened for tolerance to aluminium (Al) toxicity under glasshouse, laboratory and field conditions. In the glasshouse, two soil acidity levels (limed and unlimed soil) were used and the experiment was set up in a complete randomised design (CRD) with three replications. The experiment lasted for 10 days and measurements were taken on plant height (PH), leaf area, stem diameter and dry matter. In the laboratory, a haematoxylin staining (HS) experiment was conducted to determine the response of 10 maize varieties to Al toxicity. Two Al concentrations (0 and 222 μM) were used and the experiment was set up in a completely randomized design with three replications. After 7 days, shoot length, was recorded. Five stress tolerance indices were estimated to determine the resilience of each genotype. A root growth stress tolerance index was also computed for both experimental procedures. In the field, two trials were established at two sites, namely Mbinja and Mpumaze. Limed and unlimed plots were used, and the trial was set up in a randomized complete block design with three replications. Maize kernel yield and other standard field parameters were recorded. Selection of tolerant genotypes from the field screening was also done using three indices, namely harmonic mean (HM), stress tolerance index (STI) and stress susceptibility index (SSI). Both the glasshouse and laboratory assays identified similar genotypes of maize as being tolerant. These tolerant genotypes were Ngoyi, PANBG3492 BT, PAN 6Q408 and PHB 3442 based on the root growth stress tolerance index (RGSTI). It was therefore demonstrated that these two assays produced the same level of efficiency in identifying tolerant genotypes using this index. Based on ranking of seedling vigour index under soil acidity stress, the top three genotypes at Mpumaze were PHB32W71, PAN6616 and Sahara while at Mbinja, the top three were PAN6616, PAN6Q408 CB and PAN6P110. The genotypes PANBG3492 BT, PAN6Q408 and PHB3442 were also found to be tolerant to acidic soils at seedling stage. These genotypes are recommended for further evaluation in more sites to confirm their tolerance and yield potential under acidic soils. The study also revealed that plant height, leaf area and stem diameter could be used for indirect selection for tolerance to Al toxicity under glasshouse conditions. The seedling vigour index was also effective in identifying tolerant genotypes under glasshouse conditions. On the other hand, shoot length stress tolerance index and the haematoxylin score were useful for indirect selection for tolerance to Al toxicity in the laboratory. In the field, it was observed that ear length, leaf area and ear diameter can be useful in identifying genotypes that are tolerant to soil acidity. They can therefore be useful as indirect selection criteria under field conditions. Additionally, the best selection indices for identifying soil acidity tolerant genotypes under field conditions were the HM and the STI. It is recommended that varieties that were identified as tolerant be further evaluated in several soil acidity hot spots to confirm their tolerance and stability of performance under field conditions. / Agriculture and  Animal Health / M. Sc. (Agriculture)