Masters Degrees (Agronomy)

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Agroecosystem diversification for sustainability : the effects of crop rotation on soil microbial diversity, fertility and yield
(Stellenbosch : Stellenbosch University, 2016-03) Venter, Zander Samuel; Hawkins, Heidi-Jayne; Jacobs, Karin; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Agricultural intensification can involve the simplification of agroecosystems to crop monocultures requiring agrochemicals to maintain soil fertility and control pests. This can have negative impacts on the ecosystem services flowing to and from agroecosystems and thus, more sustainable management practices are necessary. Broader ecological theories propose that ecosystem biodiversity is important for ecosystem function. Within agriculture, increasing aboveground diversity through crop rotation, a component of conservation agriculture, can enhance the ecosystem services supporting an increase in cash-crop yield. The extent to which the belowground microbial diversity can be manipulated through crop rotations and may facilitate the yield increase is unclear. This thesis aimed at investigating (1) the relationship between above and belowground diversity in crop rotation systems, and (2) the relationship between diversity in (1) and agroecosystem function in terms of soil fertility, including nitrogen fertility, and crop yield. A quantitative review of the literature using a meta-analysis of 27 studies from around the world found that soils under a higher diversity of crops in rotation produced higher microbial richness (+15.11%) and diversity (+3.36%) scores. This effect was significantly influenced by the type of microbial analysis method used, the length of the study trial, and the percentage annual ground cover. There was a high between-study heterogeneity and no correlation to soil nitrogen fertility. A field study at Langgewens Experimental Farm (Western Cape Department of Agriculture’s 19-year wheat-legume crop rotation trial in the Swartland Local Municipality of South Africa) added to the meta-analysis. Community level physiological profiling (CLPP) and automated rRNA intergenic spacer analysis (ARISA) were used as measures of functional and genetic microbial diversity, respectively. Increasing crop diversity through rotations of wheat with medic (Wm) or a combination of medic and clover (Wmc) resulted in greater wheat plant stem length and N concentrations when compared to wheat monoculture (WW). This effect seemed to be less linked with microbial diversity per se than with the Rhizobium species present because both microbial analyses found no differences in soil microbial activity, richness or diversity with increasing crop diversity. The lack of relationship between above and belowground diversity is likely due to other abiotic drivers of microbial community structure such as P availability, Na and K excess, and pH, all of which correlated to microbial activity and functional richness in our study. The role that microbial diversity plays in the agroecosystem diversity-function relationship remains complex as revealed by the lack of correlation between functional and genetic diversity scores. However, the relation between crop diversity and functional components including wheat yield and soil N followed a hump-shaped curve. The results of this thesis suggest that soil biodiversity and function are decoupled in agroecosystems. This provides support for the hypothesis that ecosystem function may be a product of either specific productive species (selection effect), or the facilitative interaction of multiple species (complementarity effect). Further investigation into the role of specific functional microbial groups in the yield increase of crop rotation systems using next-generation sequencing is required.
Application of Hazard Analysis Critical Control Points (HACCP) in improving the global competitiveness of the fish industry with special reference to the Nile Perch (Tanzania)
(Stellenbosch : Stellenbosch University, 2000-03) Mlolwa, Mwanaidi Ramadhani; Hoffman, Louwrens C.; Stellenbosch University. Faculty of AgriScience. Dept. of Agronomy.
ENGLISH ABSTRACT: Not only have non-trade barriers been applied in Europe and the US to agricultural products from developing countries, but protective policy is also likely to continue. Owing to the omission of fish from the discussions in the final round of agreement at Uruguay, where tariffs were significantly reduced for other agricultural products, the fish industry has not had any non tarriff barier relief in recent years. The failure of the November 1999 World Trade Organisation Meeting at Seattle to reach any agreement, has again shattered hopes of international relief for fishery products. The main assumption underlying the study was that the production of fish in Tanzania could improve significantly if the HACCP (Hazard Analysis Critical Control Points) principles were applied rigorously. It was believed that neither the prerequisite programmes, nor HACCP principles were probably applied as stringently as international standards require. The aim of the study was to review the literature on HACCP, programmes that are prerequisite for HACCP and factors that_ determine competitiveness in business and to examine the Tanzanian Nile perch industry against this background. The overall goal was to determine how the industry could become more internationally competitive. This was achieved by comparing the socio-economic reality of Tanzania with factors that are considered in the literature to affect competitiveness in general and in the fish industry specifically. To achieve the first of these aims, Tanzania's socioeconomic position as a developing country was examined. To achieve the latter, the commitment of fish processing factories to the application of HACCP principles and their compliance with prerequisite programmes were evaluated in a field study. The literature study confirms that HACCP has received international recognition in the fish industry as a factor to prevent hazards and gain competitiveness in the global fish market. However, conditions in Tanzania were shown to be insufficient to support the fisheries sector in attempting to gain such a competitive advantage. Although, Tanzania has the comparative advantage of a fishery resource base, a favourable trade policy and foreign technology and management, the macro-factors that include the infrastructure, human resource capital, foreign policy environment and the supporting industries are not supportive. For the time being, therefore, improvement can only come from the government and from the industry itself. There are several factors to which the government needs to pay attention in order to support this industry - and others - in achieving international success. These include the provision of infrastructure and facilities, the improvement of the transportation network, technological capabilities, human resource capital and the provision of safe water and electric power. However, the fish industry cannot rely on the government alone. The improvement of their product is also dependent on their own efforts. According to the findings of this study, the industry needs to comply with the basic sanitation factors and provide the necessary training to the employees and fish suppliers. Having examined the situation more closely, the assumption is proved to be correct. The fish industry in Tanzania does indeed appear to have the potential to become internationally competitive. Such success, however, will be heavily dependent on cooperation between the government and the industry
Aspects of honeybush tea (Cyclopia species) propagation
(Stellenbosch : Stellenbosch University, 2008-12) Mbangcolo, Mongezi Morrison; Reinten, E.; Agenbag, G. A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
Honeybush (Cyclopia spp. Fabaceae) is indigenous to the fynbos botanical biome of the Eastern and Western Cape of South Africa. The increase in the international demand for honeybush tea for health benefits, concern over exploitation of wild populations and the lack of published agronomic information necessitated this study to evaluate different aspects of honeybush propagation. The main objectives of this study were to evaluate the effect of species and cutting position on rooting of cuttings of Cyclopia species using different rooting hormones, to evaluate the effect of an organic plant fertilizer and cutting position on growth and establishment of rooted cuttings and to study the influence of different seed pre-treatments on germination of Cyclopia species. Terminal and sub-terminal cuttings of C. intermedia and C. genistoides treated with different rooting hormones were rooted under day/night temperature controlled glasshouse conditions. Intermittent mist was used as means of moisture supply to the cuttings for 45-60 seconds daily every 30 minutes. C. genistoides rooted significantly better compared to C. intermedia as measured by rooting percentage, number of roots per cutting, length of longest root and mean root length during the summer season. The cutting position had a significant effect on rooting of the cuttings in summer compared to winter and spring season. The interactive effect of species, treatment and cutting position resulted into 86% of rooting in summer from the terminal cuttings of C. genistoides, while only 4% was recorded as the highest rooting percentage in both winter and spring seasons. The highest number of roots and the greatest root length per cutting were obtained with 2 and 4 g L-1 IBA from terminal cuttings of C. genistoides and these hormone concentrations were not significantly different to each other. To evaluate the effect of an organic plant fertilizer and cutting position on plant growth and establishment, rooted cuttings of two Cyclopia species (C. intermedia and C. genistoides) from two cutting positions (terminal and sub-terminal) were transferred to pots (576 cm3) and treated with Nitrosol fertilizer at application rates of 3.33 ml.L-1, 1.67 ml.L-1 and 0 ml.L-1 (control). Cyclopia plantlets were uniformly inoculated once with a symbiotic Rhizobium bacteria to improve the formation of nodules. Nitrosol® at 3.33 ml.L-1 significantly affected fresh and dry plant weight, fresh and dry root weight, number of shoots and nodules per plant compared to either 1.67 ml.L-1 or the control. Relative to species, C. genistoides performed better in terms of fresh and dry plant weight, fresh and dry root weight, and number of shoots and nodules per plant compared to C. intermedia. The origin of the cutting position did not significantly affect the above mentioned parameters. Plant mineral analysis revealed that most of the essential elements increased with increasing Nitrosol® application rates, with C. genistoides having higher levels of mineral elements than C. intermedia. This could be an indication of the differences between the two species in terms of nutrient uptake, utilization and distribution within the plant tissues. In the germination studies, seeds obtained from different seed sources of Cyclopia species were subjected to different pre-sowing treatments. Seed treatments were sulphuric acid (95%), hot water (100°C), water with smoke paper disk, and demineralised water (control). The study revealed that all the treatments had a significant effect on germination with the exception of eight year old seeds obtained from C. subternata (seed source two). Although hot water treatment improved germination compared to smoked paper disk and the control, seeds treated with hot water degenerated rapidly. The highest overall germination (77.33%) was found with one year old seeds compared to other seed sources older than one year. Although smoked paper disks generally did not improve germination compared to the control, in one year old seeds from seed source one, this treatment greatly influenced germination, suggesting that seed age might have influenced germination of these seeds. In terms of germination rate, germination generally started after four days in most treatments.
Controlled release fertiliser as a management tool for productivity of tunnel-grown tomatoes
(Stellenbosch : Stellenbosch University, 2022-04) Potgieter, Johan; Le Roux, Marcellous; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: To be able to grow crops such as tomatoes on a commercial scale under stringent controlled conditions in what is termed controlled environment agriculture (CEA), requires a great amount of expertise and technology. Therefore, the aim of this study was to see if the use of controlled release fertilizer as an alternative low expertise and low technology-based fertilization method could produce the same yields and marketability than the conventional fertigation fertilization methods used in greenhouse tomato production. The study assessed this by (1) determining if the ratio of perlite to coco coir in the growth medium had any effect on the yield response to an industry recommended controlled release fertilization recommendation, (2) trying to establish an appropriate mixing ratio of controlled release fertilizer (CRF) and liquid fertilizer (LF) to determine if a follow up fertilization application of the pre-plant applied CRF can obtain improved yields. From the results it was evident that by applying a mixture of 80% CRF (based on the fraction of the total % nitrogen applied) and 20% LF with additional monthly manual application of calcium sulphate or calcium nitrate to each planting bag, CRF could potentially replace a 100% LF fertilization programme in greenhouse tomato production. Some results here indicate that a spike in temperature at the beginning of the growing season may have contributed to the premature release of nutrients from the CRF prill, causing a spike in EC. This stunted the growth of the plants for the rest of the season, which could have been attributed to an initial toxic level of salts in the rootzone and a prolonged deficit of other essential nutrients. The growth media trial for the determination of the optimal perlite: coco coir ratio revealed that a mix consisting of 20% perlite to 80% coco coir, or 40% perlite and 60% coco coir were the best ratios that yielded the highest. Thus, for tomato production the effect of CRF would be greatly improved if applied in an environment where the temperature and growth media properties are favourable for the slow release of the nutrients. The evidence here did not support the utilisation of 100% CRF as a replacement of the currently employed LF for commercial greenhouse tomato production. In addition, it is imperative that additional calcium (Ca2+), Sulphate (SO42-) and Nitrate (NO3-) be supplemented to reap the full benefits of CRF due to the ongoing technological research into the ability to coat calcium-based fertilizer products as a CRF.
Crop response in the Western Cape of South Africa to liming soil under no-tillage and following once-off tillage in a no-tillage regime
(Stellenbosch : Stellenbosch University, 2021-03) Liebenberg, Adriaan; Swanepoel, Pieter Andreas; Hardie-Pieters, Ailsa G.; Labuschagne, Johan; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Soil acidity, and the stratification thereof, was found throughout the Western Cape Province. Soil acidity is especially prevalent in the Swartland, where 19.3% of soils in this region have been found to contain at least one soil layer, in all cases deeper than 5 cm, with pH(KCl) ≤ 5.0. The mean acid saturation percentage of the Swartland region was above the 8% threshold for wheat production. The wide adoption of no-tillage has presented challenges to address subsoil acidity. Since soil acidity is a limiting factor for wheat (Triticum aestivum), barley (Hordeum vulgare) and canola (Brassica napus) grown in these regions, acidity should not remain unaddressed. Therefore, it is crucial that liming is done with the correct combination of liming material, method of application and physical incorporation, or lack thereof. These variables were evaluated on sandy loam soil with pH(KCl) 5.5. Results from this field trial indicate that micro-fine lime pellets and Class A calcitic lime yield similar results on soil chemical properties and crop response under the soil and climatic conditions that prevailed during this study. The in-row application of a small amount (40 kg ha-1) of micro-fine lime pellets had a negligible effect on soil chemical properties and the treatment where only 40 kg ha-1 of micro-fine lime pellets were applied was the only treatment, along with the control, where soil pH(KCl) decreased over the course of this trial. Comparison between samples taken in-row and between crop rows in the treatments where liming material was applied in-row and broadcast, showed a greater (p ≤ 0.05) increase in Ca content in the samples taken between crop rows than in-row. Of the crop response variables measured, canola showed treatment responses (p ≤ 0.05) in leaf area index (LAI), aboveground biomass and oil content. Canola LAI’s only differed at 90 days after emergence (DAE), with the treatments where soil was disturbed and where micro-fine lime pellets were applied at 19% below the recommended rate having the highest LAI’s. Where a disc plough was used and where micro-fine lime pellets were applied in-row only, oil contents were the lowest (p ≤ 0.05). The crop responses in only some variables can be ascribed to the resilience of canola and the fact that lime application was done in the same year, thus the liming materials did not have sufficient time to react with soil acidity. In the following year, wheat was planted on the same site. This was done to monitor treatment effects over two years. Wheat showed treatment responses (p ≤ 0.05) in plant population and aboveground biomass at 150 DAE. Where a disc plough was used, both the plant population and aboveground biomass was the highest. Increases in soil pH in the 5 – 15 cm soil depth layer positively correlated with increased aboveground biomass and wheat grain protein content. Increasing effective cation exchange capacity also correlated with increased aboveground biomass in wheat. The amount of rainfall, as well as rainfall distribution, may have contributed to the few treatment differences in 2020.
Cultivation aspects of hydroponic cut tulip (Tulipa gesneriana) production in South Africa
(Stellenbosch : Stellenbosch University, 2013-12) Derbyshire, Geline; Kempen, Estelle; Hoffman, L. (Lynn) (Horticulturalist); Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Tulip cut flowers are considered speciality flowers, but are cultivated on a limited scale in South Africa. Published research done on the cultivation aspects of Tulipa spp. in warm climates is sparse and insufficient. The production potential of this valued floriculture crop under South African conditions however prompts a need for research as profitable returns can only be realised when cut tulip producers deliver high quality tulips, both for the local and potentially the export market. Understanding how cut tulips react to different cultivation aspects under local conditions is key in developing successful and profitable forcing programs for cut tulip production in South Africa. In order to produce quality tulips it is vital that producers minimize the occurrence of physiological disorders by optimizing cultivation aspects such as nutrition, bulb quality and age, cultivar selection as well as postharvest treatments. To study the nutritional requirements of cut tulips produced in a hydroponic system under South African conditions two similar experiments were conducted, using early-forcing and late-forcing tulip bulbs respectively. In each experiment the effect of four different nutrient solution formulations (“Current SA”, “Standard Steiner”, “Europe”, “Europe+NH4”) and four different cut tulips cultivars (‘Leen van der Mark’, ‘Jan van Nes’, ‘Ill de France’, ‘Royal Virgin’) on growth, quality and vase life was evaluated. Results showed that although nutrient solutions did not significantly affect the scape growth of cut tulips, vase life was significantly affected by nutrient solution formulation for both early- and late-forcing bulbs. For early-forcing bulbs nutrient solution “Europe” produced tulips with a significantly longer vase life than other nutrient solutions and for late-forcing bulbs nutrient solution “Standard Steiner” produced tulips with a significantly longer vase life than other nutrient solutions. Cultivars differed significantly in terms of growth, quality and vase life for both early- and late-forcing bulbs. It was found that the cultivar ‘Leen van der Mark’ presented the longest stem length, greatest fresh weight and longest vase life of all cultivars evaluated for both early- and late-forcing bulbs and it seems to be a cultivar with a low risk in terms of quality and vase life for forcing hydroponically in warm climates. As stem topple, a physiological disorder which can be prevalent in hydroponically produced tulips, has been shown to be reduced by the application of calcium fertilizers, the next experiment was conducted to investigate the role of foliar sprays to reduce or eliminate this common disorder. Various foliar spray treatments, including calcium nitrate (CaNO3), CalTrain, NonTox Silica® and an untreated control, were evaluated for their effect on the occurrence of postharvest stem topple and the vase life of the two tulip cultivars ‘Jumbo Pink’ and ‘Strong Gold’ respectively. Trends indicated that foliar sprays containing calcium, “CaNO3” and “CalTrain”, reduced the incidence of postharvest stem topple. The vase life of hydroponically forced cut tulips was significantly increased by the use of foliar fertilizer sprays as compared to the control. The vase life of cut tulips is an important parameter directly affecting quality. In a final experiment, the efficacy of three postharvest treatments to extend the vase life and minimize postharvest stem elongation of the four cut tulip cultivars ‘Deshima’, ‘Synaeda Orange’, ‘White Marvel’ and ‘Margarita’ was evaluated. Treatments consisted of an untreated control, an overnight stem pulse of Chrysal BVBPlus, a four hour fumigation of Chrysal Ethylene Buster® (1-MCP) and a combination of the latter treatments. Vase life, postharvest stem elongation and vase-solution used were parameters measured. No statistically significant effects on any parameters measured could be identified between the various treatments. Trends observed suggest that treatments containing BVBPlus may prolong the vase life, minimize postharvest stem elongation and improve the usage of vase life solution of hydroponically produced cut tulips. BVBPlus thus shows potential for use as an essential postharvest treatment for cut tulips produced hydroponically under warm climate conditions, but further research is needed for conformation. This research is a first report on key agronomical aspects of the hydroponic cultivation of cut tulips in South Africa and aims to serve as a basis for future research, ultimately to support successful commercial cultivation of tulips on a larger scale in South Africa as well as internationally.
Developing management strategies to support sustainable production of lucerne in long-rotation cropping systems
(Stellenbosch -- Stellenbosch University, 2020-03) Van der Westhuizen, Christoff George; Swanepoel, Pieter Andreas; Labuschagne, Johan; Brand, T. S.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Lucerne (Medicago sativa L.) forms part of long-rotation cropping systems integrated with livestock in the southern Cape of South Africa. The lucerne phase is five to seven years long, followed by five to seven years of cash crops. Lucerne swards allow for the integration of livestock into cropping systems, improved resource utilisation, is a high-quality animal forage, biologically fix N, break disease cycles and may improve subsequent cash crops yields. However, some farmers consider excluding lucerne from crop production cycles. The main reasons for the exclusion of lucerne swards are low summer and winter herbage production due to moisture stress and lucerne’s natural winter dormancy. Low herbage production during these periods create considerable fodder flow deficits and make the management of fodder flow programmes challenging for farmers. The oversowing of dryland lucerne swards with annual winter growing forage crops, to create lucerne-based multiple species pastures, was investigated to determine if fluctuations in fodder flow programmes could be reduced through increased winter herbage production. Field experiments were conducted at Tygerhoek Research Farm (Riversonderend) during the 2018 and 2019 growing seasons. Both single species treatments and mixes were oversown into an existing lucerne base. Single species treatments included black oats, forage barley, stooling rye, Westerwolds ryegrass, forage radish and canola. Mixes consisted of various combinations of hybrid ryegrass, Italian ryegrass, forage barley, black oats, various annual Medicago and clover species, vetch and forage radish. The effect of oversown species and mixes on herbage production, pasture and soil quality was monitored for the duration of this study. Drought conditions after oversowing restricted the performance of the oversown species and mixes and had a knock-on effect that persisted for the duration of this trial. No treatment had a higher herbage yield to that of the control at any stage in the growing season (p>0.05). Small grains and mixes that contain small grains did however show the most potential to improve herbage production, especially in late winter. Due to poor performance of oversown treatments, herbage samples mainly consisted of the lucerne base and ryegrass, both as an oversown species and weed. The relatively similar species composition from different treatments yielded no clear and or obvious treatment that improved pasture quality, however, grazing management ensured that all treatments were of a high quality at the time of sampling. Similar returns of organic matter, both quantatively and qualitatively resulted in soil quality that was similar between all treatments. Different results may be obtained if oversown species establish well and this study should be replicated in years of normal rainfall distribution to fully comprehend how changes in pasture composition will affect herbage production and pasture quality. Soil physical, chemical and biological parameters should also be monitored over an extended period of time as changes in soil quality may take several years in Mediterranean climates.
Developing nitrogen fertiliser management strategies for canola (Brassica napus L.) under conservation agriculture practices in the Western Cape
(Stellenbosch : Stellenbosch University, 2018-12) Du Toit, Etienne; Labuschagne, Johan; Swanepoel, Pieter Andreas; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Nitrogen (N) is one of the most important nutrients in canola production systems. Improper N supply due to lack of knowledge regarding canola N management and inappropriate fertiliser guidelines, frequently results in low canola yield and profitability in the Western Cape. Current N guidelines are based on international literature or adapted from wheat guidelines. Conservation agriculture practices have also changed soil N dynamics. More N is mineralised from soil organic matter than conventional systems, which were historically practised. Canola N guidelines should therefore be refined to account for the abovementioned changes. The aim of this study was to determine the effect of different topdress N rates, foliar N application at stem elongation and N source on plant parameters, canola seed yield, oil content and N use efficiency, whilst monitoring the effect of different topdress N rates on the soil mineral N concentration at plant, pre-topdress, post topdress and at harvest. This study was conducted at five canola producing areas in the Western Cape during 2016 and 2017. The trial was laid out as a randomised block design consisting of seven different topdress N rates (0, 25, 50, 75, 105, 135 and 165 kg N ha-1) applied at the rosette stage. For all the above mentioned treatments, 25 kg N ha-1 was applied at planting. A control treatment was included that received no N. A foliar N application that consisted of 20 kg N ha-1 (urea ammonium nitrate) was applied at stem elongation. Five N sources were evaluated, applied as topdressing at rosette stage. Increasing topdress N rate increased (p<0.05) soil mineral N concentration. Plant population at harvest and biomass production did not respond (p<0.05) to topdress N rates, a result not expected but could be ascribed to the relative dry seasons experienced in 2016 and 2017. Canola yield responded (p<0.05) to topdress N rate. Maximum yield response was recorded at lower topdress N rates than expected. The N use efficiency (NUE) decreased (p<0.05) as topdressed N rate was increased, with a drastic reduction in NUE when total N application was increased above 25 kg N ha-1. Foliar N application at stem elongation did not (p>0.05) influence yield or oil content at most sites. Nitrogen source did not influence (p>0.05) plant population, biomass production or yield, except at one site (Langgewens) in the Swartland in 2017 where the urea + inhibitor outperformed LAN. No differences (p>0.05) were recorded in oil content between different N sources in 2016. Generally, in 2017, oil content was lower compared to 2016 and inconsistent results were recorded between N sources. This was possibly due to the dry conditions during 2017, which may have influenced oil production. It is apparent that N fertiliser recommendations have to be adjusted for certain areas. Current N recommendations may result in over-fertilisation and reduced profitability at sites in the southern Cape. Current N recommendation at the Swartland sites has a low NUE and further increase in topdress N rates would likely result in pollution of the environment. Nitrogen source did not affect canola productivity. Selection of N source should be based on cost. In general, CA practices tended to decrease fertiliser N requirement for canola production. Refined N fertiliser guidelines may result in more consistent canola yield and ensure profitability. Guidelines will only be finalised on completion of the research project after at least four years of data capturing.
Developing nitrogen fertiliser management strategies for wheat (Triticum aestivum) under conservation agriculture practices within the Western Cape
(Stellenbosch : Stellenbosch University, 2018-03) Neethling, Paul Johannes; Labuschagne, Johan; Swanepoel, Pieter Andreas; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Nitrogen (N) is one of the most limiting plant nutrients. Supplying enough N to growing crops is one of the most critical factors influencing wheat production. There has been a strong drive towards conservation agriculture in South Africa, especially within the Western Cape Province. Conservation agriculture brings forth changes in soil physical, chemical and biological properties that influence the plant-available soil mineral N content, specifically an increased potentially mineralisable N content. The aim of this study was to do a complete analysis of the effect that different preceding crops, N rates, forms of N, and timing of N application would have on the yield, yield components and protein content of wheat, whilst monitoring the effect of different N rates on the soil mineral N concentration throughout the growing season. The first objective of the study was to determine the effect of different fertiliser rates on the grain yield, selected yield components, quality parameters and N use efficiency. The second objective was to determine the effect of a late-season foliar N application on the yield and grain protein content of the wheat crop. The third objective was to test the effect of different N sources on crop growth, yield and quality of wheat. This study was conducted during 2016 and 2017 on nine sites within the dryland grain producing areas of the Western Cape. The trial was subdivided into two separate studies: i) topdressed N rates with or without foliar applications of N and ii) N sources at topdress. Increasing topdress N rates had a less profound effect on crop yields than expected, where most of the sites in both years showed no increase (P > 0.05) in yield with increasing topdress N rate. Five of the research sites in Year 1 and all the research sites in Year 2 showed no response (P > 0.05) in the number of ear-bearing tillers, as influenced by increasing topdress N rates. An increase (P < 0.05) in grain protein content with the increasing topdress N rates was recorded at all the research sites in Year 1. The N use efficiency of wheat decreased (P < 0.05) with increasing topdress N rates in both years. Foliar N application at post-anthesis had limited success in increasing yield and grain protein content of wheat. No profound effect of fertiliser N source on the yield, yield components or quality of wheat was recorded. After doing a complete analysis of the N requirement of wheat produced under conservation agriculture practices and dryland conditions in the Western Cape Province, it was apparent that fertiliser N recommendations will possibly have to be adjusted. The N guidelines to produce wheat lead to over-fertilisation in some areas, which may, in turn, lead to environmental pollution and economic losses. Determining the optimal N source might entail choosing the most cost-effective and accessible source.
Direct and indirect methods of estimating lucerne (Medicago sativa) yield
(Stellenbosch : Stellenbosch University, 2019-12) Makuni, Josiah; Swanepoel, Pieter Andreas; Labuschagne, Johan; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Lucerne (Medicago sativa) is an important drought tolerant fodder crop which plays an important role in providing feed for livestock in South Africa. Currently, the standard method for determining fodder on offer is the cut-and-dry method, which is time consuming, costly and labour intensive. There is therefore a need to find alternative non-destructive methods that can be used to accurately estimate lucerne yield in a time-efficient manner. The aim of this study was to calibrate a rising plate meter (RPM), ceptometer, meter ruler and canopy cover using an unmanned aerial vehicle (UAV) to estimate lucerne herbage yield. Data was collected from January 2015 till February 2018 with a break from June 2016 to June 2017. The study trial was conducted from July 2017 to February 2018 on existing lucerne trial plots under full irrigation at the Elsenburg Research Farm outside Stellenbosch. The first objective was to determine yield potential of lucerne cultivars available commercially in South Africa. Herbage yield data for 2015 was used to determine yield potential of different cultivars. Dormancy class did not affect herbage production in this study. The second objective was to calibrate indirect methods namely RPM, ceptometer and meter ruler, for estimating lucerne yield. Linear and quadratic regressions were calculated to estimate the accuracy of the RPM, ceptometer and meter ruler. Coefficients of determination derived from three yield estimations were significant (p<0.05). The RPM had the best coefficient of determination of r2 = 0.69 (p<0.05) compared to the other instruments. Operation was fairly easy and it achieved its objective of cutting down on time. It worked best on the months where lucerne production was low. The ceptometer (r2 = 0.55) was highly weather dependent as it worked best on clear sunny days and was affected on days with clouds and morning dew. The meter ruler was quick and easy to use to collect data. However, it could not produce a high coefficient of determination (r2 = 0.50). The third objective was to develop ways to use digital data collected with a UAV for estimating lucerne yield. Linear and quadratic regressions were also calculated to estimate accuracy of the UAV canopy cover. The UAV canopy cover estimations produced the lowest coefficient of determination of r2 = 0.45 compared to the other instruments. The drought experienced in the Western Cape Province during 2017 and part of 2018 cut the data collection period down to seven months from the expected twelve months. For the current study, it was concluded the RPM could be the best yield estimation instrument for estimating yield albeit there is room for it to be calibrated to get higher yield estimation accuracy. It is recommended the study is repeated over a longer period to properly calibrate all yield estimation instruments over all seasons of the year.
Does zinc influence germination, vegetative growth or yield of wheat (triticum aestivum)?
(Stellenbosch : Stellenbosch University, 2018-03) Van der Linde, Jacques Henry; Pieterse, P. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Nutrients are essential to all life on earth. Nutrients are divided into macro and micro-nutrients. Macro nutrients such as proteins and carbohydrates for example are needed by organisms in large quantities to remain healthy while micro nutrients such as vitamins and minerals for example are needed in very small quantities. A lack in sufficient uptake of either macro or micro nutrients by organisms can lead to serious health problems, due to the occurrence of nutrient deficiencies. Emphasis has been laid on the importance of zinc (Zn; a micro nutrient) as more than 50% of soils on which staple foods (e.g., wheat) are produced globally are considered as being Zn deficient. Various secondary advantages such as decreased emergence rate, increased disease resistance, better stand density and yield have been linked to an improvement in wheat vigour due to an increase in the Zn concentration [Zn], either in wheat seeds, in the soil or due to foliar applications of Zn. The main aim of the study was to determine what the causal effects of increased [Zn] are on these various parameters. The influence of seed [Zn], priming and foliar applications of Zn fertilisers were either studied in uncontrolled or controlled environmental conditions. Various stress aspects, including water stress, increased planting depth and weed competition were also incorporated into some of the controlled environmental experiments. It is noteworthy that no Zn deficient (< 22 mg kg-1) seeds were used during these experiments as none could be found. An increase in wheat seed [Zn] did have a significant positive influence (p < 0.05) on the germination percentage of wheat seeds during the germination experiment. Soil moisture and planting depth had a significant influence on seedling growth (p < 0.05). Seemingly, insufficient amounts of soil moisture led to decreased seedling growth while an increase in planting depth led to a decrease in seedling emergence. Wheat seed emergence was also significantly (p < 0.05) improved due to an increase in wheat seed [Zn]. Wheat seed germination and seedling growth was not influenced by the presence of ryegrass, but the presence of only one wheat plant had a significant influence on the dry mass (DM) production of ryegrass (p < 0.05). Wheat stem length and DM actually increased in one of the experiments as the number of wheat plants decreased and the number of ryegrass plants increased. This finding was corroborated by other similar studies and also by two of the controlled environmental condition experiments in this study. Seed [Zn] and different fertilizer treatments had an indifferent influence on plant growth and nutrient composition at all three localities where field trails were conducted. Results were inconclusive as to whether seed [Zn] did have a positive influence on growth and vigour of wheat, which may have been attributed to the absence of truly Zn deficient wheat seeds in this instance.
The ecological sustainability of potato production in the sandveld region of the Western Cape: nutrient and water use efficiencies
(Stellenbosch : Stellenbosch University, 2019-12) Kayes, Malcolm Jeremy; Swanepoel, Pieter Andreas; Franke, Angelinus; Steyn, Martin; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Uncertainty regarding the rate at which water and nutrients move and are distributed throughout the soil profile is key in managing potato production systems in the Sandveld region of the Western Cape. The sandy soils with low nutrient and water holding capacities complicate irrigation water management and fertiliser practices. Information on efficient water management practices is scarce due to the difficulties of measuring water losses to the environment. Thus, the aim of this study was to quantify inputs and losses in potato production systems in the Sandveld region to close the gap in knowledge with regards to water and nutrient leaching under current management practices. The study was conducted on nine potato fields (processing cultivar FL2108 and table cultivar Sifra) between March 2018 and March 2019 under centre-pivot irrigation systems. Water inputs were monitored with flow meters and pressure transducers. Nutrient and water losses (drainage and leaching) was assessed using drainage lysimeters and soil water movement throughout the profile was monitored with the use of capacitance probes. Tuber yield was determined when the crop was mature, and soil-water balance components as well as water and nutrient-use efficiencies were calculated. The regular evaluation of irrigation systems is recommended to prevent over or under application of water to combat inefficiencies and meet the evapotranspiration demands of the crop. The simulation of evapotranspiration through adjusted basal crop coefficient curves to meet the demands of the specific areas was indicated to be a good measure of crop water use. Evapotranspiration values obtained ranged from 188 to 647 mm. Irrigation is generally not adjusted to crop physiological needs, resulting in over application of water, particularly during winter due to the effect that rainfall has on the increased potential of drainage. The rainfall recorded ranged from 54 to 271 mm. Substantial drainage occurred in summer planted crops as a result of irrigation water exceeding crop requirements. However, as a result of the rapid depletion of water in the soil profiles due to low water holding capacities, farmers cannot leave substantial room in the profile for rainfall. Weather station data and soil capacitance probes provided good information regarding the potential occurrence of drainage events and are recommended as management tools. Large nutrient losses were associated with substantial drainage, occurring on average at 70 kg N ha-1, 52 kg P ha-1 and 138 kg K ha-1. Drainage collected ranged from 4 to 302 mm per season. Water use efficiency observed was average (65.4 to 122.2 kg mm-1), which is accredited to low yields and high drainage losses in winter. Yields ranged from 34.7 to 118.2 t ha-1. Relatively low yields in winter and autumn resulted from cool temperatures and less available solar radiation in these periods. Yields during winter where below 60 t ha-1, compared to summer crops, which yielded 59.0 and 118.2 t ha-1. Keywords: water-use efficiency, nutrient use efficiency, nutrient leaching, drainage lysimeter, soil water balance, evapotranspiration.
The effect of crop rotation and tillage practice on soil moisture, nitrogen mineralisation, growth, development, yield and quality of wheat produced in the Swartland area of South Africa
(Stellenbosch : Stellenbosch University, 2013-03) Wiese, Jacobus Daniel; Labuschagne, Johan; Agenbag, Andre; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: This study was done during 2010 and 2011 as a component study within a long-term crop rotation/soil tillage trial that was started in 2007 at the Langgewens Research Farm near Moorreesburg in the Western Cape Province of South Africa. The aim of this study was to determine the effect of crop rotation and soil tillage on the soil moisture content, mineral-N levels of the soil, leaf area index, chlorophyll content of the flag leaf, biomass production, grain yield and grain quality of spring wheat (Triticum aestivum L). The experimental layout was a randomised complete block design with a split-plot treatment design replicated four times. Wheat monoculture (WWWW), lupin-wheatcanola- wheat (LWCW) and wheat-medic (McWMcW) crop rotation systems were included in this study and allocated to main plots. This study was confined to wheat after medic/clover, wheat after canola and wheat monoculture. Each main plot was subdivided into four sub-plots allocated to four tillage treatments namely: Zero-till (ZT) – soil left undisturbed until planting with a star-wheel planter No-till (NT) – soil left undisturbed until planting and then planted with a no-till planter Minimum-till (MT) – soil scarified March/April and then planted with a no-till planter Conventional-till (CT) – soil scarified March/April, then ploughed and planted with a no-till planter. Soil samples were collected every two weeks from just before planting until before harvest, from which gravimetric soil moisture and total mineral-N (NO3--N and NH4+-N) were determined. Plant samples were collected every four weeks until anthesis, starting four weeks after emergence. From these leaf area index and dry-matter production were determined. Chlorophyll content and light interception were measured at anthesis. At the end of the growing season the total biomass, grain yield and grain quality was determined. Crop rotations which included medics (McWMcW) or canola/lupins (LWCW) led to higher mineral-N content of the soil at the start of the 2011 growing season when compared to wheat monoculture, but did not have an effect on soil moisture. Conservation tillage (minimum- and no-till) practices resulted in higher soil moisture whilst conventional-till resulted in the highest mineral-N content for 2010. There was however no differences in mineral-N content between tillage methods for 2011, whilst soil moisture content was affected in the same way as the previous year. Both crop rotation and tillage influenced crop development and biomass production. In general, increased soil disturbance together with wheat after medics and wheat after canola resulted in better development of the wheat crop with regards to dry matter production and leaf area index. The positive effect of medic and canola rotations was also evident on chlorophyll content and light interception. Grain yield was positively influenced by wheat after medics and wheat after canola, with both systems out-yielding wheat monoculture in 2010 and 2011. Minimum- and no-till resulted in the highest grain yield in both years. Crop rotation and tillage practice showed no clear trends with regards to grain quality. This illustrated the important effect of environmental conditions during grain-filling. Environmental factors such as rainfall and temperature had significant effects in both years of the study, but the importance and advantages of crop rotation, especially with a legume crop such as medics included, was evident even though this component study was done early in terms of the long-term study. The positive effect of implementing conservation tillage practices such as minimum- and no-till were also clearly shown in results obtained throughout this experiment.
The effect of foliar micronutrient applications on nutrient use efficiency in tomatoes
(Stellenbosch : Stellenbosch University, 2024-03) Wolf, Anchen; Kempen, Estelle; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Plant nutrient management through the application of a balanced nutrient solution is critical for the success of crops in intensive production systems. Over-application or unbalanced applications however can result in wasted resources and environmental pollution. This study explores the impact of micronutrient foliar applications of silicon (Si) and boron (B) on the post-harvest yield and quality of two tomato varieties in a fertigation system where the calcium (Ca²⁺) and potassium (K⁺) application ratio is adjusted. Using these fertigation systems, the objective is to determine whether the nutrient use efficiency (NUE) can be improved for tomato production systems with the respective treatments. The study was conducted in a greenhouse on the Welgevallen Experimental Farm in Stellenbosch, Western Cape. Two varieties of tomatoes were used, Floradade and Solarino RZ F1 (72-150). Seedlings were planted into 20 L bags with coconut peat as the growing medium. The trial comprised of four treatments with an open-hydroponic system where a foliar feed of Si, B, or a combination of the two was applied in conjunction with a fertigation solution where the Ca²⁺ % was adjusted or followed standard recommendations. This adjusted solution maintained an EC of 2mS.cm⁻¹ where the Ca:K and Ca:Mg ratios were changed with reduced Ca²⁺ and increased K⁺ and Mg²⁺, whereas the standard solution had ratios according to the Steiner solution concentrations. Analysis of the data collected during the trial, both developmental and post-harvest, was done using R (R Core Team, 2023) and STATISTICA Version 13.3 (TIBCO Soft Inc. 2016) for statistical software. A full nutrient analysis was also conducted on the fruits. Standard fertigation treatments, specifically treatment 2 with the added Si foliar feed had the most significant effect on the vegetative growth parameters. The reduced fertigation treatments had a less significant impact on these parameters, although the rate of Ca²⁺ supplied to tomatoes can be reduced if managed correctly, and if amendments of B and Si are given. The adjusted Ca²⁺ % reported an 18% increase in NUE for the marketable weight of the harvested product, and an 8% increase for the marketable weight of all the treatments combined, allowing growers to receive a larger percentage of marketable product at the same input cost and volume of nutrients compared to the standard fertigation treatments. It is confirmed that the NUE can be improved in hydroponic tomato production where Si and B are applied as foliar feed substitutes for Ca²⁺ without compromising the quality and post-harvest shelf- life of tomatoes. The reduction of Ca²⁺ % in the fertigation and the addition of Si and B did not compete with standard fertigation treatments. This reduction of the input, however, allowed for an increase in the NUE of these treatments, allowing growers to receive a larger percentage of marketable product for the input cost and volume of nutrients compared to standard fertigation treatments.
Effect of magnesium and boron on potato tuber quality
(Stellenbosch : Stellenbosch University, 2022-04) Hindley, Renoir; Le Roux, Marcellous; Combrinck, Nicolaas; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Magnesium (Mg) is an important macronutrient with the main function of facilitating various biochemical reactions and physiological responses throughout the development of plants. Magnesium serves as the central atom of chlorophyll molecules in green tissues, contributing extensively to plant assimilation. However, Mg-related studies in potato nutrition, particularly in terms of keeping quality, are poorly documented. Boron (B) is another nutrient that is often overlooked. This nutrient acts as a stabilizer of the cell wall pectin network and as a result enhances cell wall structure. Boron also facilitates the movement of sugar in the plant by forming sugar borate complexes or by linking with the cell membrane in order to enhance sugar permeability. The conversion of starch-to-sugar in tubers is known to affect the nutritional value of potatoes. In this study, greenhouse experiments were conducted to evaluate the influence of different levels of Mg and B against various parameters of potato tuber quality. Magnesium was shown to have a positive response on tuber weight and a shortage of it was detrimental to the storability of tubers. The most significant finding for B was its moderate improvement of starch content in tubers. In the first experiment, two cultivars were evaluated. Tuber number for each treatment combination was determined on day of harvesting. Tuber weight and specific gravity (SG) were performed using a weighing scale and the water displacement method, respectively. The harvests were stored in a dark room, at room temperature, for a six-week period in order to monitor the rate of weight loss. This was obtained by routinely weighing tubers on days 0, 7, 14, 21, 28, and 42 after harvesting. Thereafter, tubers were sent for starch determination that was done by means of the AMG / α-amylase / HK method. In the second experiment, the study to determine the effect of Mg and B in tuber quality was extended. In addition to number, weight, storage losses, and starch content, tubers were divided into weight classes of 50-100 g and 100-150 g, in order to analyse elemental distribution and periderm thickness, respectively. The distribution of elements was accomplished using a Scanning Electron Microscope (SEM). Potato samples that were used for the analysis of periderm thickness, was done by the help of MicroCT scans. The visualization of these samples was performed in Volume Graphics VGStudioMax 3.1. Taking into account the work of Rian Gericke (2018) in predicting the keeping quality of potato by way of tuber characteristics, where Mg correlated negatively with weight loss, the hypothesis in this study that Mg content would have a positive effect on the storage quality of tubers was true. Boron was shown to moderately improve starch content. The distribution analysis showing that Mg tended to locate near the stem-end, at a decreasing gradient towards the bud-end of tubers, was consistent with several previous studies. Unfortunately, the low levels of B were not detected in any of the samples by this method. In comparison to foliar applications, the supply of Mg through the soil was shown to have a more significant influence on periderm thickness, whereas the effect of B was unclear.
Effect of mycorrhizal inoculation and phosphorus levels on growth and yield of wheat and maize crops grown on a phosphorus deficient sandy soil
(Stellenbosch : University of Stellenbosch, 2010-12) Pharudi, Joseph Albert; Agenbag, G. A.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Agronomy.
ENGLISH ABSTRACT: See full text for the abstract
The effect of Phosphorus on the growth, plant mineral content and essential oil composition of Buchu (Agathosma betulina)
(Stellenbosch : University of Stellenbosch, 2007-12) De Villiers, Chris Johan; Agenbag, G. A.; Langenhoven, P.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Agronomy.
An increase in the demand of buchu (Agathosma betulina) oil has lead to an increase in the commercial cultivation of buchu in fields and also in hydroponic systems. A nutrient solution for hydroponically grown buchu is still required to ensure optimal growth and yield. ASNAPP (Agribusiness in Sustainable Natural African Plant Products) South Africa has done some trials to achieve optimal EC and pH in the nutrient solution. Phosphate concentrations in the nutrient solution might play a significant role due to reports by a variety of researchers on the sensitivity of Protea plants to phosphate. Buchu and Proteas are both part of the Fynbos biome and are found in regions with similar soil (sandy soils with a low pH and mineral contents) and climatic conditions. Two separate experiments were conducted to determine the effect of increasing phosphate concentrations (ranging from 0.00 to 1.40 me L-1) in the nutrient solution on buchu growth. The first experiment was done in a plastic covered structure with a pad and fan and the objective of this trial was to determine the effect of increasing phosphate concentrations in the nutrient solution on the general growth, biomass production, oil composition, mortality rate and chemical composition of the buchu plants. The second experiment was done in a glasshouse with mechanical temperature control and the aim of this trial was to determine the response of buchu to increasing concentrations of P at two different root temperatures. A chemical analysis of the plants was done and the general growth, yield and root mass were recorded to determine the response of buchu plants to the phosphate and temperature treatments. In the greenhouse experiment an optimum growth and yield response of buchu plants was found at a phosphate concentration of 0.7 me L-1 in the nutrient solution. Phosphate concentrations lower or higher than 0.7 me L-1 lead to a decrease in growth and yield. An increase in the phosphate concentration in the nutrient solution lead to a general increase in N, P, K, Ca, Mg and B content in the buchu plants and a decrease in Fe content. The mortality rate of the buchu plants increased with an increase in the phosphate concentration from 0.0 to 1.4 me L-1 in the nutrient solution. The phosphate concentration in the nutrient solution only made a significant difference on one major component of the buchu oil which was Ψ-Diosphenol, but no general trend with Ψ-Diosphenol content and P concentration could be found and the significant difference in Ψ-Diosphenol observed in this trial may only have been due to genetic variation between the plants. The effect of the different root temperatures in the glasshouse experiment was very clear. The buchu plants grown at the high root temperature (20°C) produced a higher yield and better overall growth than the plants grown at lower (10°C) temperatures. The buchu plants grown at 20°C had a significantly higher N, K, Na and B content than plants grown at 10°C. Buchu plants grown at 10°C showed no significant response in terms of growth and yield to the phosphate concentration in the nutrient solution, but plants grown at 20°C exhibited growth and yield peaks at phosphate concentrations of 0.35 and 1.4 me L-1. The peak observed in the plants growth at high phosphate concentrations is unexplainable and can possibly be ascribed to the limitation of the plants per experimental unit and/or amount of replications. The increase in P concentration in the nutrient solution caused a general increase in N, P and K content in the buchu plants. A significant interaction between the phosphate concentration and root temperature was observed for the P, Mn en Zn contents of the plants which meant that the buchu plants respond differently towards phosphate concentrations at different root temperatures.
Effect of salinity on germination and seedling growth of Canola (Brassica napus L.)
(Stellenbosch : University of Stellenbosch, 2007-03) Bahizire, Francois B.; Agenbag, G. A.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Agronomy.
Soil salinity is a serious constrain to crop production in many countries. In the Western Cape canola production area, large variability in soil properties within the same field is a very common feature. Some of this variability may be ascribed to differences in salinity. These saline patches affected the germination and survival of cereal crops like wheat and may therefore also be a reason for the high variability and generally low canola yields obtained in this area. It will therefore be important to quantify the effect of such saline conditions on the germination and seedling establishment of canola in an effort to improve yields. In this study several experiments were conducted under controlled conditions in either Petri dishes in an incubator or pots in a temperature controlled glasshouse to determine the salinity response of canola. Results of the experiments done in Petri dishes, showed that the germination response of canola (Brassica napus L., cv. Tornado) to increasing levels of salinity is very similar to that of wheat (Triticum aestivum L., cv SST 88). Although canola germinated more rapidly than wheat at a temperature of 20 ºC, the final percentage germination for both crops decreased severely when NaCl induced salinity exceeded levels of an EC of 8 dS m-1, but less severe responses were obtained with NaCl/CaCl2 induced salinity. Under soil conditions an interaction between soil type and salinity level showed that the responses in sandy soil to an increase in salinity from 0 to 8 iv dS m-1 were less severe than in loamy soil and especially when salinity stress was induced by NaCl. Although cultivar Spektrum showed more tolerance to saline conditions during the germination and seedling stages in pot experiments when compared to other cultivars tested, these results will have to be confirmed in field trails where grain yield and quality can be measured.
Effect of seasonal distribution and rate of nitrogen fertilisation on canola production and soil bacterial communities in the Western Cape
(Stellenbosch : Stellenbosch University, 2017-12) Becker, Frederick; Le Roux, M. R.; Swanepoel, Pieter Andreas; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Canola (Brassica napus) was introduced into crop rotation systems of South Africa in 1994. Ever since, canola production is expanding as the benefits of canola is recognised. Canola has a higher nutrient demand than most other crops such as wheat and barley. Of these nutrients, nitrogen (N) comprise most of the production costs as it is applied at high rates. Currently, in South Africa N fertiliser guidelines for canola production is adopted from guidelines for wheat or from international literature. Losses of N is not only economically inefficient, but can also be detrimental to the environment and human health. Sustainable production necessitates a reduction of these losses and lower dependency on inorganic fertilisers without compromising high yields. The soil biological component renders ecosystem services such as nutrient cycling. Soil bacterial communities are principally involved in the cycling of N and could therefore determine the fate of fertilisers. Results of studies done on the effect of N fertilisation on soil bacterial communities lack consistency and is often contradictory and thus not well understood. The aim of this study was to evaluate different N fertilisation rates and seasonal distribution for canola production in South Africa, and to determine the effect of the fertiliser N on soil bacterial communities. The study was conducted during the 2016 production season under dryland conditions in the Western Cape. It was replicated at three different localities, representative of the important canola production regions, namely Langgewens Research Farm, Altona and Roodebloem Experimental Farm. Langgewens and Altona are situated in the Swartland region and Roodebloem in the southern Cape. Each of the trials were laid out as a randomised complete block design with six N fertilisation treatment-combinations including a control without added N. The treatment-combinations was replicated in four blocks. Two factors were evaluated, i.e. N fertiliser rate and distribution of N. Two N fertiliser rates (60 and 150 kg ha-1) were applied. Twenty kg ha-1 was applied at planting and the remainder was distributed at either only 30 days after emergence (DAE), 30 and 60 DAE or 30, 60 and 90 DAE. Results indicated that the increase in N fertilisation from 60 kg ha-1 to 150 kg ha-1 did not increase yields (P>0.05). Soil bacterial community changed through time (P<0.05), but fertilisation treatments had no effect (P>0.05). Soil bacterial biodiversity and species richness decreased over time (P<0.05) at Langgewens. It is therefore recommended to apply 60 kg ha-1, split into three increments, i.e., 20 kg ha-1 at planting and the remainder in two equal applications at 30 and 60 DAE. These applications can vary in amount and timing due to weather conditions of the specific growing season.
The effect of shoot heterogeneity on the physiology and grape composition of Shiraz/Richter 99 grapevines
(Stellenbosch : Stellenbosch University, 2004-12) Cloete, Hanle; Archer, E.; Hunter, J. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: The effect of shoot heterogeneity on vegetative and reproductive growth parameters, vine physiology and grape composition was investigated in a ShirazlRichter 99 vineyard. Comparisons between underdeveloped (typically shorter and less ripened at véraison) and normally developed shoots in both shaded (non-manipulated) and well-exposed (manipulated) canopies were made. Compared to underdeveloped shoots, normal shoots had a larger total leaf area, due to the higher occurrence of secondary shoots as well as larger leaves on primary and secondary shoots. Since the photosynthetic activity of the leaves from normal shoots was higher than those from underdeveloped shoots, higher levels of carbohydrates were produced and stored in the former. Starch was more evenly distributed over the length of the whole shoot in normally developed shoots compared to underdeveloped shoots. Normally developed shoots were longer and thicker in diameter than underdeveloped shoots. The larger clusters of the normally developed shoots are evidence of their more favourable total leaf area per gram berry mass. Berries from the normally developed shoots were smaller at five weeks after véraison than those from underdeveloped shoots, displaying a higher skin to pulp ratio and therefore higher anthocyanin and total phenolic extraction potential for winemaking. The smaller clusters and fewer berries per cluster found for the underdeveloped shoots indicate an imbalance between vegetative and reproductive growth initiated during the vegetative growth phase and continued during the ripening period. The peculiar absence of statistically significant differences in grape composition between normally and underdeveloped shoots indicates that assimilates needed for berry ripening of the latter originated in organs other than the leaves [e.g. from adjacent normal shoots and the rest of the permanent structure of the vine (cordon, trunk, roots)]. The larger differences in berry size that occurred between shoot types in the shaded compared to the well-exposed canopies may be evidence for this. The photosynthetic activity of shoots was lower in shaded than in exposed canopies. The total carbohydrate production of the normal shoots in shaded canopies seemed insufficient to supply in the ripening needs of the shoot itself, their own clusters as well as the ripening of stem tissue and clusters of the underdeveloped shoots in the canopy. This is illustrated by the lower levels of starch that accumulated in the normal shoots from shaded compared to that of exposed canopies. Vine shoot heterogeneity clearly leads to visible and physiological imbalances that would impact negatively on grape and wine quality as well as production costs and should therefore be avoided on any terroir.

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