Tato diplomová práce se zabývá analýzou současného elektronického obchodování a e-business. Na základě rozboru aktuálního stavu současného internetového obchodování je provedena analýza konkrétního e-shopu za pomoci dostupných metod.

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DC to DC step-down applications with high input voltage and high voltage conversion ratio operational requirements, such as photovoltaic battery chargers, are subject to high conduction losses, high switching losses and substantial reverse-recovery losses when minority carrier principle diodes are used. The recent introduction of silicon carbide diodes with high breakdown voltages has made possible the elimination of reverse-recovery losses at high voltage levels and as such has sparked interest in their use due to the potential efficiency improvements. This report presents the results of a comprehensive analysis on the use of silicon carbide diodes and their counterparts, ultrafast silicon diodes, in conventional buck converters and isolated current-fed buck converters in high input voltage and high voltage conversion ratio step-down applications. The analysis illustrates both theoretically, with the use of steady-state average models, and experimentally the substantial efficiency benefits of the use of reverse-recovery free silicon carbide diodes in the conventional buck converter and the small but significant improvement in the efficiency of the isolated current-fed buck converter. The improvements of the conventional buck converter paired with silicon carbide diodes are shown to be significant enough to grant the variant the most efficient position for power levels below 1 kW. In addition, the four variants are categorized based on their cost and performance; therefore, providing engineers with a convenient guide to aid their selection of the appropriate converter depending on the operational requirements.

DC to DC step-down applications with high input voltage and high voltage conversion ratio operational requirements, such as photovoltaic battery chargers, are subject to high conduction losses, high switching losses and substantial reverse-recovery losses when minority carrier principle diodes are used. The recent introduction of silicon carbide diodes with high breakdown voltages has made possible the elimination of reverse-recovery losses at high voltage levels and as such has sparked interest in their use due to the potential efficiency improvements. This report presents the results of a comprehensive analysis on the use of silicon carbide diodes and their counterparts, ultrafast silicon diodes, in conventional buck converters and isolated current-fed buck converters in high input voltage and high voltage conversion ratio step-down applications. The analysis illustrates both theoretically, with the use of steady-state average models, and experimentally the substantial efficiency benefits of the use of reverse-recovery free silicon carbide diodes in the conventional buck converter and the small but significant improvement in the efficiency of the isolated current-fed buck converter. The improvements of the conventional buck converter paired with silicon carbide diodes are shown to be significant enough to grant the variant the most efficient position for power levels below 1 kW. In addition, the four variants are categorized based on their cost and performance; therefore, providing engineers with a convenient guide to aid their selection of the appropriate converter depending on the operational requirements.

DC to DC step-down applications with high input voltage and high voltage conversion ratio operational requirements, such as photovoltaic battery chargers, are subject to high conduction losses, high switching losses and substantial reverse-recovery losses when minority carrier principle diodes are used. The recent introduction of silicon carbide diodes with high breakdown voltages has made possible the elimination of reverse-recovery losses at high voltage levels and as such has sparked interest in their use due to the potential efficiency improvements. This report presents the results of a comprehensive analysis on the use of silicon carbide diodes and their counterparts, ultrafast silicon diodes, in conventional buck converters and isolated current-fed buck converters in high input voltage and high voltage conversion ratio step-down applications. The analysis illustrates both theoretically, with the use of steady-state average models, and experimentally the substantial efficiency benefits of the use of reverse-recovery free silicon carbide diodes in the conventional buck converter and the small but significant improvement in the efficiency of the isolated current-fed buck converter. The improvements of the conventional buck converter paired with silicon carbide diodes are shown to be significant enough to grant the variant the most efficient position for power levels below 1 kW. In addition, the four variants are categorized based on their cost and performance; therefore, providing engineers with a convenient guide to aid their selection of the appropriate converter depending on the operational requirements. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

The objective of the present diploma thesis was to verify the influence of monosex populations in African catfish (Clarias gariepinus) fishfarming on growth rate, growth heterogeneity, feed conversion ratio, yield and product quality in the operating conditions of a small fish farm with a recirculating aquaculture system. In six tanks with a volume of 0,3 m-3, 60 individuals with an average unit weight of 614 ? 135 g, all-male, all-female and mixed- sex (in 2 repeats each) were deployed. The fish were fed with feed in the usual and registered feed doses 4 times a day. At the end of the experiment, 119 days average fish weight reached 1445 ? 911 g. The composition of the populations was found to have a statistically significant effect on the food conversion ratio, gonadosomal index, and fillet yield. Statistically significant differences were not found in specific growth ratio (SGR), growth heterogeneity, or muscle composition (fat and protein content). The yield of fillets without skin reached only 40.74 ? 1.81% for all-female group. An economically advantageous alternative to a mixed -sex groupe with a sex ratio of 1: 1 was a all-male fishfarning groupe where the yield of skinless fillets was 46.99 ? 1.12%. Food conversions were also more favorable for all-male groups (FCR = 1.08 ? 0.08 and 1.04 ? 0.12) compared to all-female (FCR = 1.18 ? 0.21 and 1.25 ? 0.19 ). The fat content of the muscle at the end of the experiment varied between 5.10-10.53% for each group without proven dependence on the type of the group, the protein content showed only minimal differences, varied between 15.18 and 16.46%, dry mass contained muscle from 24.23 to 27.95% without proven dependence on the type of occupancy. Significant economic impact has the composition of groups when evaluating the yield of fillets without skin as the final product.

Thesis (BSc. Agriculture (Animal Production)) -- University of Limpopo, 2019 / An experiment was conducted to determine the effect of bovine colostrum feeding period after hatching on performance and carcass characteristics of male Ross 308 broiler chickens. The experiment was based on 0, 12, 24, 36, 48 and 72 hours of liquid bovine colostrum feeding after hatching of broiler chickens. The experiment commenced with 180 male Ross 308 broiler chicks with an initial live weight of 42 ± 2g per bird and was carried out for six weeks. The chicks were randomly assigned to six treatments with three replications, resulting in 18-floor pens with 10 chicks per replicate. A complete randomized design was used in this experiment. Data was analysed using the General Linear Model (GLM) procedures of the Statistical Analysis of system, Version 9.3.1 software program. Where there were significant differences, mean separation was done using the Turkey test at the 5% level of significance. A quadratic regression model was used to determine the optimum productivity of the experiment while a linear model was used to determine the relationships between bovine colostrum feeding period and responses in the variables measured. Feed intake during Week 1, growth rate during Week 3 and feed conversion ratio (FCR) during Weeks 2 and 3 of the growing period of male Ross 308 broiler chickens were not affected (p>0.05) by bovine colostrum feeding periods after hatching. Similarly, bovine colostrum feeding had no effect (p>0.05) on diet dry matter (DM), neutral detergent fibre (NDF), acid detergent fibre (ADF) and ash digestibilities in male Ross 308 broiler chickens aged 14 to 21 days. Bovine colostrum feeding period after hatching had no effect (p>0.05) on nitrogen retention (N-retention) in male broiler chickens aged one to 21 days. Similarly, gut organ digesta pH, length and weight of male Ross 308 broiler chickens aged 21 days were not improved (p>0.05) by bovine colostrum feeding after hatching. However, bovine colostrum feeding improved (p<0.05) feed intake during Weeks 2 and 3 of the growing period of male Ross 308 broiler chickens. Similarly, bovine colostrum feeding after hatching improved (p<0.05) crude protein (CP) digestibility in male broiler chickens aged 14 to 21 days. Metabolisable energy (ME) intake of male Ross 308 broiler chickens aged one to 21 days was improved (p<0.05) by bovine colostrum feeding period after hatching. Similarly, growth rate of male broiler v chickens during Weeks 1 and 2 was improved (p<0.05) by colostrum feeding after hatching. Feed conversion ratio of male broiler chickens during Week 1 was improved (p<0.05) by bovine colostrum feeding period after hatching. Bovine colostrum feeding after hatching improved (p<0.05) live weight of male Ross 308 broiler chickens at the ages of 7, 14 and 21 days. Nitrogen retention and FCR of male Ross 308 broiler chickens aged 22 to 42 days were not affected (p>0.05) by bovine colostrum feeding after hatching. In addition, live weights of male Ross 308 broiler chickens aged 35 and 42 days were not affected (p>0.05) by bovine colostrum feeding period after hatching. Similarly, bovine colostrum feeding had no effect (p>0.05) on gut organ digesta pH, large intestine lengths, breast and drumstick weights and breast meat juiciness of male Ross 308 broiler chickens aged 42 days. However, bovine colostrum feeding improved (p<0.05) feed intake and growth rate of male Ross 308 broiler chickens aged 22 to 42 days. Live weights of male Ross 308 broiler chickens aged 28 days were improved (p<0.05) by bovine colostrum feeding period after hatching. Similarly, bovine colostrum feeding after hatching did not affect (p<0.05) diet DM, CP, NDF, ADF and ash digestibilities in male Ross 308 broiler chickens aged 35 to 42 days. Metabolisable energy intake of male broiler chickens aged 22 to 42 days was improved (p<0.05) by bovine colostrum feeding period after hatching. Similarly, GIT, small intestine and caecum lengths and crop, proventriculus, gizzard, small intestine, caecum, large intestine, carcass and thigh weights of male Ross 308 broiler chickens aged 42 days were improved (p<0.05) by bovine colostrum feeding after hatching. Bovine colostrum feeding after hatching improved (p<0.05) breast meat tenderness, flavour and shear force of male Ross 308 broiler chickens aged 42 days. It is concluded that reasons for differing responses to bovine colostrum feeding periods of up to 72 hours after hatching are not clear. Therefore, further studies in which longer bovine colostrum feeding periods are used after hatching are recommended. / National Research Foundation (NRF)

Thesis (M.Sc. (Animal Production)) --University of Limpopo, 2019 / Two experiments were conducted to determine the effect of dietary threonine level on production performance and carcass characteristics of Ross 308 broiler chickens. In each experiment the diets were isocaloric and isonitrogenous but with different dietary threonine levels.The first part of the study determined the effect of dietary threonine level on feed intake, growth rate, mortality and carcass characteristics of Ross 308 broiler chickens aged between Day 1-21. A total of 150 unsexed day-old chicks were used in a complete randomized design having 5 treatments (6.4, 7.5, 8, 8.5 and 9g of threonine/kg DM feed), replicated three times and having ten chickens per replicate. The second part of the study determined the effect of dietary threonine level on feed intake, digestibility, growth rate, mortality and carcass characteristics of male Ross 308 broiler chickens aged between Day 22-42. Seventy-five male chickens were used in a complete randomized design having 5 treatments (6.4, 7.5, 8, 8.5 and 9g of threonine/kg DM feed), replicated three times and having five chickens per replicate. A quadratic regression model was used to determine the optimal productivity of the chickens while a General Linear Model (GLM) procedures for the statistical analysis of variance was used to detect dietary treatment effects. Where there were significant differences (P<0.05), Turkey’s honestly significant difference test (HSD) was used for mean separation. The chickens were slaughtered at the ages of 21 and 42 days for Experiments 1 and 2, respectively, following ethical standards as recommended by the University of Limpopo Animal Research Ethics Committee (AREC/12/2017: PG). Two chickens per replicate for both studies were slaughtered for the determination of carcass characteristics (carcass and organ weights, gut organ digesta pH and gastro-intestinal length measurements). Dietary threonine levels used in this experiment affected (P<0.05) feed intake, growth rate, live weight, metabolisable energy (ME) intake, nitrogen retention, feed conversion ratio and gut organ weights and lengths of unsexed Ross 308 broiler chickens aged 21 days. Dietary threonine level did not affect (P>0.05) diet digestibility. Feed conversion ratio, pH of the proventriculus digesta, gut intestine length and caecum length of unsexed broiler chickens were optimized at different dietary threonine levels of 9.6, 8.5, 6.6 and 8.4 g/kg DM, respectively. Dietary threonine levels had an effect (P<0.05) on feed intake, diet digestibility, metabolizable energy, live weight, proventriculus pH values, GIT length, gut organ and carcass organ weights of male Ross 308 broiler chickens between 22 to 42 days of age. Proventriculus and large intestine weights were optimized at different dietary threonine levels of 7.5 and 9.1 g/kg DM feed, respectively. Dietary threonine level did not affect (P>0.05) growth rate, feed conversion ratio of male Ross 308 broiler chickens between 22 to 42 days of age. It is concluded that dietary threonine levels used in this study affected production performance of younger broilers (Day 1-21) more than that of older birds (Day 22-42). However, production variables were optimized at different dietary threonine levels. This has implication on diet formulation for the chickens and no linear response could be established / National Research Foundation (NRF), and the Department of Agriculture, Forestry and Fisheries (DAFF)

Energy-nutrient conversion ratios, as a measure of relative agricultural efficiency for Mexico, India, in the United States, and Japan between 1951 and 1974, were calculated for protein, B-vitamins, calcium, iron and food energy production. Estimates from the different nutrient ratios showed a range in percent change of production efficiency over the twenty-four year period. Indian estimates were the most consistent with only a 7 percent variability. Japan (23 percent), United States (42 percent), and Mexico (43 percent) each showed greater variability in their efficiency estimate ranges from aggregate energy-nutrient conversion ratios. Japan was the only country for which negative trends in efficiency were observed, as estimated from calcium and iron data. There did not appear to be any pattern in the relationship of one estimate to another in the countries studied. The variability of different nutrient efficiencies suggested that conventional methods of efficiency estimation based on a conversion ratio for a single nutrient may be inappropriate. There were significant differences in mean production efficiency estimates between countries but there was no uniformity among the determinations from different nutrients. The relative rank of efficiency for the countries changed for each of the efficiency estimates, except B-vitamins, over the time period studied. Diminishing returns in energy efficiency occurred as useful energy inputs increased. In addition, individual crops displayed tendencies toward diminishing returns. Plant foods were shown to be more efficient than animal sources of food. Efficiency increases appear to be related to increased use of effective energy inputs such as fertilizers.

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