O milho (Zea mays) é considerado como uma das culturas mais importantes cultivadas mundialmente devido a sua composição química, valor nutritivo e o seu potencial produtivo, apresentando assim, considerável relevância nos aspectos socioeconômicos. Diversos fatores interferem no desempenho da cultura e, dentre eles, o clima está entre aqueles que oferecem os maiores desafios para planejamento e manejo da cultura. Diante da importância da cultura no contexto nacional e mundial, é importante considerar as projeções futuras da produção do grão diante das mudanças climáticas e, consequentemente, na segurança alimentar nos próximos anos. Assim, avaliou-se neste estudo o impacto da mudança do clima sobre o crescimento e desenvolvimento da cultura do milho, utilizando um modelo de crescimento de plantas, baseado em processos biofísicos. Para a calibração do modelo foram utilizados dados obtidos experimentalmente e a simulação de cenários de mudanças climáticas foi realizada de acordo com a abordagem do programa AgMIP. Trabalhou-se com o modelo CERES-MAIZE/DSSAT, e três modelos de circulação climática global (GCM\'s) regionalizados e dois cenários econômicos (ou de emissão) para o período 2040-2069 (representando 2050). Com base nos resultados obtidos, pode-se concluir que se as práticas de gestão se mantêm como presentes, o rendimento de milho deverá diminuir em meio do século, no entanto, a diminuição dos rendimentos reais pode não ser tão dramática quanto previsto nos casos em que apenas o fator climático é considerado. Para isso fatores econômicos e tecnológicos devem ser considerados para aumentar os rendimentos. O aumento da [CO2] terá um efeito positivo sobre o crescimento do cultivo, mas não parece ser suficiente para compensar os efeitos negativos do clima futuro, notadamente o aumento da temperatura do ar. / Maize (Zea mays) is considered one of the most important crops cultivated worldwide due to its chemical composition, nutritional value and its productive potential, thus presenting considerable relevance in socioeconomic aspects. Several factors interfere in the performance of the crop and, among them, the climate is among those that offer the greatest challenges for crop planning and management. Given the importance of culture in the national and global context, it is important to consider the future projections of grain production in the face of climate change and, consequently, food security in the coming years. Thus, the impact of climate change on maize crop growth and development was evaluated in this study using a plant growth model based on biophysical processes. For the calibration of the model, data obtained experimentally were used and the simulation of climate change scenarios was performed according to the AgMIP program approach. We worked with the CERES-MAIZE / DSSAT model, and three regional climate circulation models (GCM\'s) and two economic (or emission) scenarios for the period 2040-2069 (representing 2050). Based on the results obtained, it can be concluded that if management practices remain as present, corn yields should decrease in the middle of the century, however, the decrease in real incomes may not be as dramatic as predicted in cases where that only the climatic factor is considered. For this economic and technological factors must be considered to increase incomes. The increase in [CO2] will have a positive effect on crop growth, but it does not appear to be sufficient to compensate for the negative effects of future climate, notably the increase in air temperature.

>Magister Scientiae - MSc / Maize streak disease (MSD), caused by variants of the Maize streak virus (MSV) A strain, is the world's third and Africa’s most important maize foliar disease. Outbreaks of the disease occur frequently and in an erratic fashion across Africa and Islands in the Indian Ocean causing devastating yield losses such that the emergence, resurgence and rapid diffusion of MSV-A variants in this region presents a serious threat to maize production, farmer livelihoods and food security. To compliment current MSD management systems, a total of 689 MSV-A full genomes sampled over a 32 year period (1979-2011) from 20 countries across Africa and the adjacent Indian Ocean Islands, 286 of which were novel, were used to estimate: (i) the levels of genetic diversity using MEGA and the Sequence Demarcation Tool v1.2 (SDT); (ii) the times of occurrence and distribution of recombination using the recombination detection program (RDP v.4) and the genetic algorithm for recombination detection (GARD); (iii) selection pressure on codon positions using PARRIS and FUBAR methods implemented on the DATAMONKEY web server; (iv) reconstruct the history of spatio-temporal diffusion for MSV-A using the discrete phylogeographic models implemented in BEAST v1.8.1; (v) characterize source-sink dynamics and identify predictor variables driving MSV-A dispersal using the generalized linear models, again implemented in BEAST v1.8.1. Isolates used displayed low levels of genetic diversity (0.017 mean pairwise distance and ≥ 98% nucleotide sequence identities), and a well-structured geographical distribution where all of the 233 novel isolates clustered together with the -A1 strains. A total of 34 MSV inter-strain recombination events and 33 MSV-A intra-strain recombination events, 15 of which have not been reported in previous analyses (Owor et al., 2007, Varsani et al., 2008 and Monjane et al., 2011), were detected. The majority of intra-strain MSV-A recombination events detected were inferred to have occurred within the last six decades, the oldest and most conserved of these being events 19, 26 and 28 whereas the most recent events were 8, 16, 17, 21, 23, and 29. Intra-strain recombination events 20, 25 and 33, were widely distributed amongst East African MSV-A samples, whereas events 16, 21 and 23, occurred more frequently within West African MSV-A samples. Events 1, 4, 8, 10, 14, 17, 19, 22, 24, 25, 26, 28, and 29 were more widely distributed across East, West and Southern Africa and the adjacent Indian Ocean Islands. Whereas codon positions 12 and 19 within motif I in the coat protein transcript, and four out of the seven codon positions (147, 166, 195, 203, 242, 262, 267) in the Rep transcript (codons 195 and 203 in the Rb motif and codons 262 and 267 in site B of motif IV), evolved under strong positive selection pressure, those in the movement protein (MP) and RepA protein encoding genes evolved neutrally and under negative selection pressure respectively. Phylogeographic analyses revealed that MSV-A first emerged in Zimbabwe around 1938 (95% HPD 1904 - 1956), and its dispersal across Africa and the adjacent Indian Ocean Islands was achieved through approximately 34 migration events, 19 of which were statistically supported using Bayes factor (BF) tests. The higher than previously reported mean nucleotide substitution rate [9.922 × 10-4 (95% HPD 8.54 × 10-4 to 1.1317 × 10-3) substitutions per site per year)] for the full genome recombination-free MSV-A dataset H estimated was possibly a result of high nucleotide substitution rates being conserved among geminiviruses such as MSV as previously suggested. Persistence of MSV-A was highest in source locations that include Zimbabwe, followed by South Africa, Uganda, and Kenya. These locations were characterized by high average annual precipitation; moderately high average annual temperatures; high seasonal changes; high maize yield; high prevalence of undernourishment; low trade imports and exports; high GDP per capita; low vector control pesticide usage; high percentage forest land area; low percentage arable land; high population densities, and were in close proximity to sink locations. Dispersal of MSV-A was frequent between locations that received high average annual rainfall, had high percentage forest land area, occupied high latitudes and experienced similar climatic seasons, had high GDP per capita and had balanced maize import to export ratios, and were in close geographical proximity. / National Research Foundation (NRF), the Poliomyelitis Research Foundation (PRF), and the Thuthuka Board

The widespread increase in use of maize-based feedstuffs in California dairy cow rations has raised concerns of reduced efficiency of CP utilization due to the low lysine content of maize and maize by-products. The objectives of this research were to determine the impact of increased dietary maize CP levels on animal performance using three metabolic models of dairy cows in order to predict limiting AA’s in California dairy rations to identify a ruminally protected AA package to supplement similar rations. Since lysine was the most consistently predicted limiting AA among dairies, and metabolic models, the dairy with the ration calculated to be the most limiting in lysine was chosen to determine effects of feeding an RPL product on milk production and composition, as well as on N balance. Nutrient profiles of 16 high multiparity cow rations were evaluated and limiting AA predicted by the metabolic models Amino Cow, CPM Dairy and Shield. Higher inclusion levels of maize products in rations increased the contribution of maize CP to the total CP content of the TMR, to between 20 – 40%, but had no impact on cow performance. Even though the lysine to methionine ratio decreased as more maize CP was included in the TMR, it did not have a major impact on the final predicted AA profile of MP, or milk component levels, but, according to Shield, it had an effect on milk yield. Methionine, isoleucine and lysine were predicted to be most limiting according to Amino Cow, CPM Dairy and Shield respectively. The models suggested three dramatically different AA packages but the high degree of consistency within model in predicting the limiting AA sequence among dairies, suggest that there may be sufficient consistency in the nutrient profiles among rations to support production of a ruminally protected AA complex. The second experiment used a double (i.e., early and mid-lactation multiparity dairy cows) 2 x 2 factorial with 28 day experimental periods. Feeding the RPL, with estimated rumen escape of lysine between 18 and 23% suggesting an increased intestinal absorption of lysine between 8 and 22 g/d/cow, did not influence DMI or milk, true protein and lactose yields. Milk fat yield and concentration decreased, while MUN percentages increased when RPL was fed. Blood plasma levels of almost all AA’s, except lysine, decreased when RPL was fed, suggesting that lysine was the limiting AA and that its supplementation led to increased absorption and utilization of other AA. The lack of response in milk protein synthesis and the decrease in plasma 3-MH concentrations when RPL was fed suggests that muscle protein synthesis was stimulated, and degradation reduced, with RPL feeding. It is possible that lysine had an effect, either directly or indirectly, on muscle protein turnover and energy metabolism that, impacted intakes, metabolism and absorption of AA and milk production in mid lactation cows, but it had no major impact on early lactation cows. Copyright / Dissertation (MSc(Agric))--University of Pretoria, 2009. / Animal and Wildlife Sciences / unrestricted

Four trials were conducted to evaluate the effects of ensiling whole plant maize with wet distillers grains with solubles (WDGS) on its preservation and nutritive value. In the first study, WDGS was blended with whole maize plants at 0% (control), 10%, 20%, 30% and 40%, and ensiled for 120 days in bottle silos in a complete randomized design. Fermentation was monitored by taking samples at day 0, 7, 21, 42 and 120. Results showed a steady decrease (P<0.05) in dry matter (DM) concentration with increasing level of WDGS inclusion. Final silage pH was lowest (P<0.05) for the 40% WDGS treatment (pH 3.62) and highest for the 10% WDGS treatment (pH 3.79). There was no effect (P>0.05) of level of WDGS inclusion on initial buffering capacity (day 0), as well as for day 42 and 120 samples. Lactic acid was higher (P<0.05) at day 120 for the control treatment than those blended with WDGS, which did not differ significantly. The concentration of acetic acid was higher value (P<0.05) for silage treatments blended with WDGS than the control, with that of 40% WDGS level of inclusion recording the highest (P<0.05). The control and 40% WDGS treatments had lower (P<0.05) ammonia nitrogen concentration (g/kg N) than the other treatments at day 120. The second trial involved monitoring ruminal fermentation characteristics of cannulated sheep fed three experimental diets. Formulated diets contained maize silage mixed with 24% dried distillers grains with solubles (MS DDGS treatment ), maize silage mixed with 19.5% sunflower oilcake meal (MS SOM treatment), and silage blend of 91% of whole maize plant/9% WDGS (WDGSMS treatment), all on a DM basis, to obtain iso-nutrient diets. The mean value for rumen pH, NH3N and total VFA concentrations among diets did not differ (P>0.05) among treatments. The individual VFA were similar with only butyric acid being lower (P<0.05) in sheep fed WDGSMS. There were no differences in the acetic:propionic acid ratio among treatments. The third trial involved the determination of DM degradability of the three formulated experimental diets using the nylon bag technique. Effective degradability was measured at two outflow rates, 2% and 5%. The diets did not differ (P>0.05) for washing losses (avalue), slowly degradable DM fraction (b-value) and rate of degradation of DM (c-value). Treatment MS SOM recorded the highest effective degradability with MS DDGS having the lowest at 2% outflow rate. The effective degradability value at 5% outflow rate for WDGSMS was not significantly different from that of MS SOM and MS DDGS, which differed significantly. In the final study, experimental diets were fed to three groups of eight lambs in a growth performance trial. The groups offered MS DDGS and MS SOM had superior (P<0.05) final mass (g/head), average daily intake (gDM/kg0.75) and average daily gain (g/head/day), than those offered WDGSMS. The feed conversion ratio did not differ (P>0.05) among all treatments. / Dissertation (MSc(Agric))--University of Pretoria, 2010. / Animal and Wildlife Sciences / unrestricted

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Background:In angiosperms, the endosperm plays a crucial placenta-like role in that not only is it necessary for nurturing the embryo, but also regulating embryogenesis through complicated genetic and epigenetic interactions with other seed compartments and is the primary tissue in which genomic imprinting occurs.Results:We observed a gradual increase of paternal siRNA expression in the early stages of kernels and an expected 2:1 maternal to paternal ratio in 7-DAP endosperm via sequencing of small interfering RNA (siRNA) transcriptomes in developing kernels (0, 3 and 5 days after pollination (DAP)) and endosperms (7, 10 and 15 DAP) from the maize B73 and Mo17 reciprocal crosses. Additionally, 460 imprinted siRNA loci were identified in the endosperm, with the majority (456/460, 99.1%) being maternally expressed at 10 DAP. Moreover, 13 out of 29 imprinted genes harbored imprinted siRNA loci within their 2-kb flanking regions, a significant higher frequency than expected based on simulation analysis. Additionally, gene ontology terms of "response to auxin stimulus", "response to brassinosteroid stimulus" and "regulation of gene expression" were enriched with genes harboring 10-DAP specific siRNAs, whereas those of "nutrient reservoir activity", "protein localization to vacuole" and "secondary metabolite biosynthetic process" were enriched with genes harboring 15-DAP specific siRNAs.Conclusions:A subset of siRNAs subjected to imprinted expression pattern in maize developing endosperm, and they are likely correlated with certain imprinted gene expression. Additionally, siRNAs might influence nutrient uptake and allocation processes during maize endosperm development.

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