Thesis (MS)--Montana State University--Bozeman, 2009. / Typescript. Chairperson, Graduate Committee: Alan T. Dyer. Includes bibliographical references.

Thesis--Auburn University, 2008. / Abstract. Includes bibliographic references.

Thesis (Ph. D.) -- Lincoln University, 2005. / Also available via the World Wide Web.

No description available.

The degradation of soil quality due to undesirable farming practices has reached alarming scales in the Eastern Cape and this has had negative repercussions on soil productivity and the environment in general. There is growing evidence that conservation agriculture (CA) practices involving minimal mechanical disturbance, maintaining permanent surface cover and embracing diverse crop rotations increase soil organic carbon (SOC) and therefore has potential to mitigate soil quality deterioration. A study was carried out at two sites located in two ecotopes to investigate the effects of crop residue retention and crop rotations in a no till system on overall soil quality using the Soil Management Assessment Framework (SMAF) as the soil quality assessment tool. The CA study trials were laid out in 2012 at two different locations, one at the Phandulwazi Agricultural High school within the Phandulwazi Jozini ecotope and the other one at University of Fort Hare Research Farm within the Alice Jozini ecotope. The experiment was laid out as a split-split plot arrangement in a randomized complete block design with three replicates. Tillage treatments were applied on the main plots while crop rotation treatments were applied as subplots. Crop residue retention treatments were applied as sub-sub plots. The rotational treatments were maize-fallow-maize (MFM), maize-fallow-soybean (MFS), maize-wheat-maize (MWM) and maize-wheat-soybean (MWS). The initial assessment of the overall soil quality of the two ecotopes using the SMAF soil quality index (SQI) revealed that the soils at the Alice site were functioning at 80% while the soils at the Phandulwazi site were functioning at 79 percent of their optimum capacity. The slight difference in the soil quality of the two ecotopes could be attributed to their different soil organic C contents where the Alice Jozini ecotope had significantly higher soil organic C contents than the Phandulwazi Jozini ecotope. After 3 years of continuous treatment application, crop residue retention significantly improved most of the measured soil quality parameters. Generally across the sites, more soil organic C, microbial biomass C (MBC), ß-glucosidase (BG) activity, mineral N, extractable P and K, Cu, Zn, Mn, Fe, and macro-aggregates were recorded in treatments where crop residues were retained. Crop rotations alone did not have a significant impact on most of the measured soil quality indicators. The crop rotations influenced significantly the availability of mineral N across the two sites, highlighting the importance of using a legume in rotations on available N for the subsequent crops. Most of the measured soil attributes were not significantly influenced after 3 years of continuously applying combined treatment of CA components. Mineral N (NO3 + NH4), K, Zn and Fe were significantly impacted on by the interactions of CA components at the Phandulwazi site, while N, Cu, Zn and Mn were significantly increased at the Alice site. Low response of SOC to combined CA treatments in the short-term prompted the need to examine treatment effects on individual soil carbon fractions. The interaction of crop rotation and residue management techniques were significant on the fine particulate organic matter – C fractions and microbially respired C. These soil C fractions were more sensitive to short-term treatments of combined CA components than SOC and MBC, therefore they can be used as short-term indicators of CA effects on SOM. Soil organic carbon, MBC, extractable P and K, soil pH, EC, b, AGS (aggregate stability) and BG activity were measured and the Soil Management Assessment Framework (SMAF) used to calculate soil quality index (SQI) values for each treatment. The combination of the crop rotations with crop residue retention showed the potential to significantly improve SQI values in the long term. The highest soil quality improvement at both sites was achieved by the maize-wheat-soybean (MWS) rotation with crop residue retention.

No information is available on the role of allelopathy in crop rotation systems of the Western Cape Province of South Africa, where more than 100 000 ha are under threat from herbicide-resistant rye grass. A study which investigated the use of allelopathic properties for the suppression of rye grass hybrid type (Lolium multiflorum x perenne) was undertaken. These objectives were accomplished by: a) exploring the use of allelopathic properties of crop residues for rye grass suppression; b) evaluation of the role of allelopathy from seeds, seedlings, roots and above-ground plant material of rotational crops; c) assessing the distribution of genetic and morphological variability of rye grass and d) determining the interactions among micro-organisms and allelopathic root leachates from rotational crops and rye grass. In the field trials, growth inhibitory or stimulatory effects were observed on crops exposed to the residues of others. Medic suppressed the weed type rye grass. The radicle length of rye grass was inhibited by seed leachates from wheat and lupine. Growth inhibition from lupine seed and seedling leachates was evident in rye grass radicle length and cumulative germination percentage. Morphologically, 50% of the total number of specimens was classified as rigid rye grass, 48% as the hybrid, namely L. multiflorum x perenne and 2% as perennial rye grass. The wide genetic and morphological variation detected in rye grass may be due to high genotypic plasticity and hybridisation for producing the weed type L. multiflorum x perenne. The faster growth rate of rye grass on Langgewens soil treated with barley root leachates was revealed by Principal Component Analysis (PCA) as a probable association with growth-promoting soil micro-organisms. Crop cultivars and weeds may modify the soil micro-organism populations to their advantage and to the disadvantage of other species by the release of root exudates that apparently differ in composition between plant species. The effect on microbial communities varied with source of exudates and between soils. / Thesis (PhD)--University of Pretoria, 2011. / Plant Production and Soil Science / unrestricted

Field and greenhouse studies were conducted during 2004 through 2006 at the Rodney R. Foil Plant Science Research Center, Starkville, MS. Six sorghum and soybean rotation treatments were tested to determine their effect on plant pathogen, insect, and nematode diversity and density levels. Treatments included 1) continuous sorghum, 2) continuous soybean, 3) sorghum-soybean-sorghum rotation, 4) soybean-sorghum-soybean rotation, 5) sorghum-soybean-soybean rotation, and 6) soybean-sorghum-sorghum rotation. Several nematode and insect species were identified during the study, but were always below economic thresholds. Six insect species were identified on soybean during each growing season and used as the indicator species for this study. The most prevalent were threecornered alfalfa hopper (Spissistilus festinus (Say) and bean leaf beetle (Cerotoma trifurcata (Forester). Sorghum webworm (Nola sorghiella Riley) and corn earworm (Helicoverpa zea (Boddie) were the most common insects on sorghum panicles. Rotations did not affect the diversity or density levels of the indicator soybean or sorghum insects during the three year study. Plant disease levels during the investigation showed variable results. Three foliar fungal pathogens including Diaporthe phaseolorum (Cooke & Ellis) Sacc. var. meridionalis, Septoria glycines Hemmi, and Cercospora sojina Hara on soybean, and Gloeocercospora sorghi D. Brain & Edgerton ex Deighton on sorghum were observed. The only virus disease on soybean was bean pod mottle, but levels were not affected by the rotations during the study. Zonate spot caused by G. sorghi was the most prevalent foliar sorghum disease, but was not affected by the rotations. Six frequently isolated fungal pathogens from either soybean or sorghum roots included Macrophomina phaseolina (Tassi) G. Goidanich, Rhizoctonia solani Kühn, D. phaseolorum, Aspergillus spp., Trichoderma spp and Fusarium spp. Aflatoxin contamination of sorghum seed was low (<20 ppb) the first two years of the study, but was high (790 ppb) in 2006. Significantly greater soybean and sorghum yields were obtained from rotated systems compared to monoculture systems in 2005. In a greenhouse test, M. phaseolina infection of soybean led to significantly greater root disease ratings, lower plant height and dry weight than the untreated control. Sorghum plant growth was not affected by M. phaseolina and R. solani.

Soil-borne oomycetes include plant pathogens that cause substantial losses in the agricultural sector. To better manage this important group of pathogens, it is critical to understand how they respond to common agricultural practices, such as tillage and crop rotation. Here, a long-term field experiment with a split-plot design with tillage as the main plot factor (conventional tillage [CT] vs. no till [NT], 2 levels) and rotation as the subplot factor (monocultures of soybean, corn, or wheat, and corn-soybean-wheat rotation, 4 levels) was sampled. Post-harvest oomycete communities were characterized over three consecutive years (2016-2018) by metabarcoding the Internal Transcribed Spacer 1 (ITS1) region of soil DNA extracts. The community contained 292 Amplicon Sequence Variants (ASVs) and was dominated by Globisporangium spp. (85.1% in abundance, 203 ASV) and Pythium spp. (10.4%, 51 ASV). NT decreased diversity and community compositional structure heterogeneity, while crop rotation only affected the community structure under CT. Soil and crop health represented by soybean seedling vitality was lowest in soils under CT cultivating soybean or corn, while grain yield of the three crops responded differently to tillage and crop rotation regimes. The interaction effects of tillage and rotation on most oomycetes species accentuated the complexity of managing these pathogens.

Thesis (MScAgric)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Sustainability issues and the structural over-supply of wheat in the Western Cape since the middle 1990‟s have caused the introduction of alternative crop rotation systems in the Middle Swartland, a dry-land winter cereal production area of the Western Cape. Crop rotation systems typically consist of cereals and oilseed crops and pastures. Alternative crop rotations systems are currently scientifically evaluated at the Langgewens Experimental farm. Currently more than half the cultivated area in the Swartland is still under wheat production, a third of which is wheat monoculture. An issue regarding the adoption of such a crop rotation system is the cash flow and affordability of implementing such an alternative system. The goal of this study is to determine the cash-flow implications of a shift from wheat monoculture to a crop rotation system. Typical strategies available to producers to support such a shift are investigated. The complexity of farm systems as well as the interrelationships between crops within such a crop rotation system necessitates the implementation of a systems approach. A multi-period, whole-farm budget model was constructed to capture the interrelationships of the farm system and to express the financial performance thereof in standard profitability criteria. The farm model is based on a typical farm for the Middle Swartland. The model was used to determine the expected profitability of various crop rotation systems and to evaluate alternative strategies to accommodate the shift to alternative systems. The Langgewens crop rotation trial results are used to determine expected profitability of various crop rotation systems. A wheat-monoculture system serves as basis for the shift to alternative systems with the focus on the practical implications of such as shift. The profitability calculations show that various crop rotation systems are expected to be more profitable than wheat monoculture. The most profitable system is one year canola followed by three years of wheat, followed by a wheat/medic system with Dohne Merino sheep on the medic pastures. The shift from wheat monoculture is simulated by four scenarios. The first evaluated the financial implications of a shift form monoculture to the three year wheat and one year canola system. The second simulates a shift from monoculture to a wheat/medic system within two years and using own funds. The third scenario simulate the same shift with own funding, but over a ten year period. The fourth is similar to the second, but borrowed money is used to fund the shift.Lower input costs and consistently higher yields results in higher expected gross margins for the crop rotation systems, especially with nitrogen fixing plants. The inclusion of medic and medic/clover pastures and alternative cash crops such as canola and lupins show a higher yield on investment than wheat monoculture. Insight into the factors that producers should consider was also generated by this study, concerning changes to crop rotation systems. These factors include; time period over which a shift is planned and the availability of financing options. It seems that a quicker shift, using borrowed funds, is more profitable over the longer term. / AFRIKAANSE OPSOMMING: Volhoubaarheidskwessies, en die strukturele ooraanbod van koring in die Wes-Kaap, het sedert die middel 1990‟s, gelei tot alternatiewe gewasproduksiestelsels in die Swartland, ʼn droëland wintergraanproduserende area van die Wes-Kaap. Gewasproduksiestelsels bestaan tipies uit graan- en oliesaad- en weidings gewasse. Alternatiewe gewas-wisselboustelsels word wetenskaplik gevalueer op die Langgewens proefplaas. Tans is meer as die helfte van die area in die Swartland steeds onder koring produksie, ʼn derde daarvan is koring monokultuur. ʼn Bekommernis rakende die aanneem van wisselboustelsels is die kontantvloei en bekostigbaarheid van die implementering van so ʼn alternatiewe stelsel. Die doel van hierdie studie is om te bepaal wat die kontantvloei implikasies van ʼn skuif van ʼn koringmonokultuurstelsel na ʼn wisselboustelsel is. Tipiese strategieë beskikbaar aan produsente om so skuif te finansier is ook ondersoek. Die kompleksiteit van boerderystelsels en die interverwantskap tussen gewasse in ʼn wisselboustelsel noodsaak die insluiting van ʼn stelselsbenadering. ʼn Multi-periode, geheelplaasbegrotingsmodel is ontwikkel om die interverwantskap van die boerdery te verenig en finansiële prestasie uit te druk in erkende winsgewendheid kriteria. Die boerderymodel is gebaseer op ʼn tipiese plaas vir die Middel-Swartland. Die model is gebruik om die winsgewendheid van verskillende wisselboustelsels te bepaal en om verskillende strategieë te assesseer wat die oorgang van wisselboustelsel kan akkommodeer. Die Langgewens wisselbouproefdata is gebruik om die winsgewendheid van verskillende wisselboustelsels te bepaal. „n Koringmonokultuurstelsel dien as basis vir die oorskakeling na alternatiewe wisselboustelsels, met die fokus op die praktiese implikasies van so ʼn skuif. Die winsgewendheid bepaling wys dat verskeie wisselboustelsels meer winsgewend is as koring monokultuur. Die mees belowende stelsels is een jaar canola gevolg deur drie jaar koring en ʼn koring/medic stelsel met Dohne Merino skape op die medic weidings. Die oorskakeling vanaf koring monokultuur is gesimuleer deur vier scenario‟s. Die eerste scenario evalueer die finansiële implikasie van ʼn skuif van koringmonokultuur na ʼn wisselboustelsel met een jaar canola. Die tweede scenario evalueer ʼn skuif na ʼn koring medic stelsel binne twee jaar met eie fondse. Die derde scenario simuleer dieselfde skuif maar oor ʼn tien jaar tydperk, met eie fondse. Die vierde scenario simuleer dieselfde skuif na koring/medics maar oor ʼn twee jaar periode met geleende fondse. Laer insetkoste en konstante hoër opbrengste lewer hoër brutomarges vir die wisselboustelsels, veral die met stikstofbindende weidingsgewasse. Die insluiting van medic en medic/klawer weidings en alternatiewe kontantgewasse soos canola en lupiene wys ʼn beter opbrengs op kapitaal investering in vergelyking met koringmonokultuur. Bykomende daartoe verskaf die resultate van die studie insig in die faktore wat graanprodusente behoort te oorweeg wanneer ʼn oorskakeling na alternatiewe wisselboustelsels oorweeg word. Die faktore sluit in, die tydperk waaroor die oorskakeling beoog word en die beskikbare finansieringsopsies. Dit blyk dat ʼn vinniger oorskakeling, selfs teen die koste van finansiering, oor die langtermyn meer winsgewend is.

Sanders, Donald W. The University of Manitoba, March, 2017. The effect of preceding crop on soybean (Glycine max) grain yield, mycorrhizal colonization, and biological nitrogen fixation. Major Professor:Yvonne Lawley. Manitoba has seen a twenty-fold increase in soybean acres seeded since 2000, with over 1.6 million acres seeded in 2016. This change presents unique opportunities and challenges to improve crop rotations in Manitoba. This experiment studied the effect of four crop sequences on soybean yield, mycorrhizal colonization, and biological nitrogen fixation. In the first year of this experiment, spring wheat, canola, corn and soybeans were grown at three sites in Manitoba (Carman, Portage la Prairie, and Kelburn). In the second year, soybeans were grown on these same plots as a test crop. This two-year sequence of crops was done twice at each site, in 2012-13 and 2013-14. To determine mycorrhizal colonization, root samples were collected at the V3 stage and then analyzed microscopically for mycorrhizal infection. Nitrogen fixation was estimated using the natural abundance method using soybeans collected at the R5 and R6 stage and canola as a reference crop. Soybean following soybean had significantly higher grain yield than all other crop sequences at one site year, and significantly lower grain yield than all other crop sequences at another site year. There were no other differences in soybean test crop yield between crop sequences. Crop sequence significantly affected mycorrhizal colonization. Soybean following canola had significantly lower mycorrhizal colonization than soybean following soybean or corn. Soybean following spring wheat also had significantly lower mycorrhizal colonization than soybean following soybean or corn. Soil test phosphorus levels also significantly affected mycorrhizal colonization, with increasing soil phosphorus resulting in decreased mycorrhizal colonization. Crop sequence significantly affected biological nitrogen fixation. Soybean following soybean or corn often had significantly greater biological nitrogen fixation than soybean following spring wheat or canola. Soil test nitrate levels affected biological nitrogen fixation, with increasing soil nitrate resulting in decreased biological nitrogen fixation. Soil test nitrate levels were affected by the carbon to nitrogen ratio of the preceding crop, with a higher carbon to nitrogen ratio associated with decreased soil nitrate. These results indicate that although there is often not a yield penalty associated with specific rotations, crop sequence has a strong impact on mycorrhizal colonization and biological nitrogen fixation. The soil organisms associated with those processes affect soil phosphorus uptake and nitrogen fixation. Producers should consider the importance of crop rotation when seeking to maximize productivity through symbiotic processes with mycorrhizae and nodule forming bacteria. / May 2017