No description available.

No description available.

Pre-harvest aflatoxin contamination is a major limitation to maize production in Texas and the southern United States, causing major economic loss and severe health problems worldwide. Screening for resistance to aflatoxin accumulation is commonly conducted through inoculation with a highly concentrated solution of Aspergillus flavus FR: Link spores, a naturally occurring fungus which infects maize and produces a toxic metabolite (aflatoxin) to humans and animals consuming the grain. No commercial hybrids exist with full resistance to aflatoxin accumulation; however, sources exist to reduce susceptibility. These sources commonly lack good agronomic characteristics for use in commercial hybrids. Exotic germplasm with favorable traits for reduced aflatoxin accumulations are introgressed with temperate and locally adapted lines. This program utilizes only one isolate of A. flavus even though many isolates exist in the environment. The objectives of this thesis are i) to evaluate the progress of the Maize Breeding and Genetics Program’s accomplishments of breeding maize for the reduction in susceptibility of aflatoxin accumulation in yellow inbreds through analysis of hybrid and inbred per se trials and ii) to determine whether interaction exists between geneticallydifferent isolates of A. flavus and several genotypes of maize. Response to aflatoxin accumulation for hybrids and inbreds was measured at up to three environments across Texas. Significant differences were detected for most years and environments. Maize lines CML285, CML288, CML323, CML325, CML326, CML338, Tx601y and lines derived from Population 69 and from Tx772 crosses in hybrid combinations tended to accumulate less aflatoxin than commercial hybrid checks. Significant differences were detected at each environment aflatoxin accumulation was measured for inbred lines per se. Inbreds Tx772, Tx601y, CML289, CML294, CML323 and derived lines from Population 69 show reduced aflatoxin accumulations. Interaction between genetically different isolates of A. flavus and several genotypes of maize were not detected in hybrid or inbred trials at two or three environments, across locations and across years. Introgression of exotic germplasm into locally adapted germplasm has improved agronomic characteristics for use in the Southern U.S. and brought sources for decreased aflatoxin accumulation.

One major problem facing maize producers in the southern US is contamination with the mycotoxin aflatoxin, produced by Aspergillus flavus (Link:fr). Aflatoxin is a serious threat to human and animal health, with no resistant commercial hybrid available. Development of resistance to aflatoxin production has several major limitations. Aflatoxin is highly variable both across and within environments, even under inoculation, requiring several locations and replications for breeding. Additionally, there is no screening method that is reliable, rapid, inexpensive, and allows for high throughput. Several secondary traits, such as kernel texture, kernel integrity, husk cover, and visible ear rot, have previously shown to be related to aflatoxin accumulation. These traits are easily characterized in the field and are candidates for indirect selection if they are correlated to aflatoxin concentration. Root lodging, a plant’s inability to maintain upright stature, is another complex characteristic of root related traits that traditionally is selected for indirectly. It can greatly reduce harvestable yield. It is affected by morphological traits and environmental conditions, but its genetic components are little understood. This dissertation comprises three studies presented in chapters II, III, and IV. Chapter II involved white and yellow hybrid maize trials as well as quality protein maize trials from several years across Texas environments. Data was analyzed both per and across location to determine repeatability of response to aflatoxin. Additionally, aflatoxin levels were correlated to several secondary characteristics (female flowering, endosperm texture, husk cover, and ear rot ratings) to determine usefulness in indirect selection. Chapter III was a phenotypic evaluation of a recombinant inbred line (RIL) mapping population, which was derived from divergent parental inbreds Tx811 and CML176. The trials were conducted in two Texas locations, and phenotypic data for aflatoxin concentration, kernel integrity, endosperm texture, female flowering date, and root lodging was collected. Variance components for these traits and genetic and phenotypic correlations were determined. Chapter IV was a genotypic evaluation of the Tx811/CML176 mapping population using simple sequence repeat markers. Genotypic and phenotypic data were combined to identify quantitative trait loci (QTL) and epistatic interactions for response to aflatoxin and for root lodging.

No description available.

M.Sc. (Chemistry) / Please refer to full text to view abstract

Thesis (M.Agric. (Agricultural Economics)) -- University of Limpopo / Maize is the most important cereal crop grown in South Africa. This crop is produced throughout the country under diverse environments. The study only focuses on the technical efficiency because it is an important subject in developing agriculture where resources are limited, but high population growth is very common. Technical efficiency is the ability of a farmer to obtain output from a given set of physical inputs. Farmers have a tendency of under and/or overutilising the factors of production. The main aim of this study was to analyse the technical efficiency of small-scale maize producers in Ga-Mothiba rural community of Limpopo Province. The objective of the study was to determine the level of technical efficiency of smallscale maize producers and to identify the socio-economic characteristics that influence technical efficiency of small-scale maize producers in Ga-Mothiba. Purposive and Snowball sampling techniques were used to collect primary data from 120 small-scale farmers. Cobb-Douglas production function was used to determine the level of technical efficiency and Logistic regression model was used to analyse the variables that have influence the technical efficiency of maize production. Cobb-Douglas results reveal that small-scale farmers in Ga-Mothiba are experiencing technical inefficiency in maize production due to the decreasing return to scale, which means they are over-utilising factors of production. Logistic regression results indicate that out of 13 variables included in the analysis as socio-economic factors, 10 of them (level of education, income of the household on monthly basis, farmer`s farming experience, farm size, cost of tractor hours, fertiliser application, purchased hybrid maize seeds, membership to farmers` organisation, is maize profitable) were found to be significant and 3 (gender, age and hired labour) are non-significant. However, farm size was found to be the most significant variable at 99% level, showing a positive relationship to smallscale maize producer`s technical efficiency. Therefore, it is recommended that government should do the on-farm training since farmers mainly depend on trial and error and farmers` should have access to enough arable land and tractor services. However, farmers need to be trained on matters relating to fertiliser application, on the amount of seeds a farmer should apply per ha, and the importance of using hybrid seed.

Maize grain yield is often highly influenced by the genotype-by-environment (GxE) interactions which limit plant breeders’ ability to select for superior performance as variation in line performance exists across different environments. While studies have focused on the genetic components underlining GxE interactions, few have investigated the environmental components. The focus of this thesis was to investigate the role that soil moisture stress plays in the formation of GxE patterns using a unique set of hybrid recombinant inbred lines (RIL) that exhibit minimal phenological differences. With the use of environmental groups (EG, environments with similar GxE patterns), the field trials demonstrated that different EGs form when relative soil moisture stress is moderate to high; however, relative low soil moisture stress did not appear to influence the formation of GxE patterns. Seasonal corn heat unit (CHU) accumulation among EGs was the likely cause for EG differentiation among the low moisture stress EGs. It appears that CHUs were the primary cause of different GxE patterns forming when they did not exceed the recommended CHU for a population and when CHU differed greatly from one environment to another. When CHU were not limiting different GxE patterns formed based on the relatively moderate to high soil moisture stress conditions within the growing season. / Ontario Research Fund, National Science and Engineering Research Council, and The Ontario Ministry of Agriculture, Food and Rural Affairs.

Master of Science / Department of Agronomy / Charles W. Rice / Maize (Zea mays L.) is a major staple crop whose wild ancestor was domesticated about 9,000 years ago (Beadle, 1939). Long-term breeding for more desirable traits ultimately resulted in the maize we see today. This long-term breeding likely impacted the processes within the rhizosphere of maize, however, to what extent is not well understood. This study examined the microbial communities between an inbred maize line (B73), a hybrid of two isogenic lines (B73xMo17), and two genetically modified maize hybrids (DKC63-55RIB and DKC64-69RIB) to determine if the plant’s ability to attract beneficial microbes changed with breeding. The hypothesis was that the isogenic cultivar forms better relationships with bacteria and fungi compared to the newer cultivars, especially in low P soil. It was also expected that the greater the difference between the cultivars the more distinct their soil microbiome. To test these hypotheses, experiments were conducted under greenhouse and field conditions. Analyses consisted of root staining to test symbiotic relationships, phospholipid fatty acid analysis (PLFA) for microbial communities, total plant and root biomass, and nutrient content to understand plant responses. Based on the field results, there was no impact on root and shoot biomass and nutrient content by differences in cultivar. Differences in cultivar did impact arbuscular mycorrhizal fungi (AMF) colonization, which decreased over time and depth for all. Soil AMF also saw a significant effect by cultivar. Other microbial groups were not impacted by cultivar, were greatest in the control, and decreased over time. Greenhouse results showed a cultivar by time interaction for root and shoot biomass. Soil P also impacted shoot biomass, but not root biomass. Shoot nutrient content was greater in high P soil, while roots only saw an impact for root P. No cultivar effect was found for soil microbial groups except for fungi, while all microbial groups were reduced in the control soil. Most soil microbial groups were also impacted by soil P as indicated by reduced concentrations in low P soil. AMF was the only microbial group that was not negatively impacted by limited soil P. In addition, all soil microbial groups increased over time, although fungi saw a decrease at R1. No significances were observed for percent AMF colonization.

To combat insect infestation of maize and maize products during storage without using chemical fumigants, a possible physical treatment method, microwave technology was investigated. Through its selective heating between cereals and insects, microwave technology is a possible physical treatment for eradication of insects and their eggs. Eradication of five insect species, namely Sitophilus zeamais, Rhizopertha dominica, Ephestia cautella, Cryptolestes ferrugineus and Tribolium Confusum was studied. Different microwave parameters such as power dosage, microwave mode, length of microwave cavity, maize exposure method and exposure time were investigated. The effective microwave treatment conditions were then selected, and used to treat maize kernels at laboratory scale. The effect of microwave treatment on the physicochemical properties of maize kernels was investigated. Microwave single exposures did not result in total insect mortality when maize was dropped through the microwave cavity (free falling) as the exposure times were too short. But utilising a pulley system, total insect mortality was achieved in a single exposure of 9 sec. A long microwave cavity (728 mm) resulted in maize kernel damage in terms of swelling, popping and discolouration. Redesigning the cavity by shortening it appeared to reduce these effects. The pulsed microwave mode was found to be better than continuous mode. The selected treatment that eradicated all five insect species with no visible kernel damage was pulsed mode at 2450 MHz frequency, using a 483 mm long microwave cavity, at a power level of 1.5 kW, with an exposure time of 9 sec. The selected conditions (normal treatment) and a more harsh treatment (2 kW power dosage, 18 sec exposure time) were applied to 4 kg samples of white and yellow maize kernels. The normal microwave treatment significantly decreased the moisture content and kernel weight of maize kernels but had no significant effect on test weight, stress cracks, germination and translucency. The harsh microwave treatment also had significant adverse effects on test weight, translucency, germinability, hardness and stress cracks. Additionally, reduced extractability of certain proteins was observed by 2D PAGE with the harsh microwave treatment. Both normal (power dosage of 1.5 kW, for 9 sec exposure time) and harsh (2 kW, 18 sec exposure time) treatment conditions eradicated adult insects and their eggs, but only the former maintains maize quality. The use of microwave technology has potential to be used as an insect control measure of maize products prior to packaging of the products. It is recommended that the effect of pulsed microwave disinfection on nutritional quality (starch and protein digestibility of maize products) be studied. Heat transfer phenomena should be studied and improved if possible to reduce the power usage and possibly shorten the exposure time from 9 sec. / Dissertation (MSc)--University of Pretoria, 2009. / Food Science / unrestricted