Doctoral Degrees (Agronomy)

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    New perspectives on plant population and row spacing of rainfed maize
    (Stellenbosch : Stellenbosch University, 2020-03) Haarhoff, Stephanus Johannes; Swanepoel, Pieter Andreas; Kotze, Theunis Nicolaas; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: Recent maize grain yield increases are attributed to genetic advances and changes in soil and crop management practices, including no-tillage (NT) and additional conservation agriculture (CA) practices. Management practices such as plant population and row spacing should be adapted for NT and other CA practices to optimise maize grain yield and promote sustainable production. However, there is a lack of information reporting on the influence of environmental and management factors and its relationship with plant density and maize grain yield. This study was initiated to generate novel perspectives on the complex concept of interplant competition of rainfed maize under various soil and crop management practices and climate conditions. The study entailed five research themes. The first research theme consisted of a critical review of the current soil and crop management practices followed in rainfed maize production regions of South Africa. Sustainable and alternative agronomic management approaches were highlighted. Alternative agronomic management practices, such as NT, crop intensification and diversification, crop residue retention, and livestock integration may provide pathways to increase the sustainability of these rainfed maize systems. Improved soil water content may support higher plant populations. The second research theme entailed consolidation of global published data from rainfed maize plant population field trials to investigate the effects on yield and to determine the influence of rainfall, soil tillage and nitrogen on the relationship between plant population and yield. Data was extracted from 64 peer-reviewed articles. Maize grain yield responded positively to increased plant population in high rainfall environments, while yields in rainfall limited environments were highly variable. The optimal plant population under NT was lower than under conventional tillage. However, at a given plant population, maize grain yield under NT outperformed the yield obtained under conventional tillage. As a third research theme, the effects of plant population and row spacing on soil water, soil temperature and maize grain yield under CA in a sub-tropical environment, were evaluated over three seasons. Although maize grain yield was not affected by plant population in the season with the highest early-season rainfall, maize grain yield increased with increasing plant population in the average rainfall and drier seasons. The fourth and fifth research themes involved a two-year trial in a semi-arid environment. In this trial, the effects of plant population and row spacing on the aboveground growth, water use efficiency and root morphology were evaluated under NT. A row spacing of 0.76 m was advantageous in the drier season. Plant populations of 20 000 to 50 000 plants ha-1 out-yielded plant populations more than 25 000 plants ha-1 at 0.52 m row spacing. Rainfall affected maize root growth while plant population had a small effect on maize root morphology. Optimising maize grain yield using plant population and row spacing requires a flexible systems-based (i.e., CA) approach. Conservation agriculture should incorporate management practices (such as plant population and row spacing) tailored for specific context.
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    Factors influencing inhibition of glutamine synthetase enzyme in grass weeds by glufosinate ammonium under different temperatures
    (Stellenbosch : Stellenbosch University, 2019-12) Mucheri, Tendai; Pieterse, P. J.; Reinhardt, Carl Frederick; Kleinert, Aleysia; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: Evolution of weed resistance emphasized the need to implement integrated weed management strategies, however, farmers still immensely rely on chemical weed management. Glufosinate ammonium is an alternative herbicide that can replace or be used in rotations with herbicides such as glyphosate and paraquat, but it poses a problem due to its inconsistencies in controlling weeds. Studies in this dissertation aimed to investigate the influence of temperature on glufosinate ammonium efficacy. Chapter 3 of this dissertation investigated the influence of temperature on ryegrass cuticle thickness, phenolic acid concentration and calcium accumulation, and subsequently, the effect of the afore-mentioned factors on glufosinate ammonium efficacy. Ryegrass was grown at 10/15, 15/20, 20/25, and 20/30 °C (night/day) temperatures and treated with 0, 1.5, 3, 4.5, 6 and 7.5 L ha-1 glufosinate ammonium dosage rates. The grass was treated six weeks after planting and assessment was done four weeks after glufosinate application. Control of ryegrass decreased with increasing temperature. Results indicated that cuticle thickness and calcium content increased as temperatures increased, probably due to production of phenolic compounds responsible for plant defence mechanisms against herbicide stress, hence resulting in poor control of ryegrass under warmer temperatures. Chapter 4 investigated ammonia accumulation, glutamine synthetase, glutamate dehydrogenase, nitrate reductase activity and ryegrass photosynthesis in roots and leaves of control (0 L ha-1) and treated ryegrass (4.5 L ha-1) harvested 24 hours after glufosinate ammonium application. There was a significant increase in glutamine synthetase enzyme activity with increasing temperature after glufosinate ammonium application. Better control of ryegrass under cooler temperatures with glufosinate ammonium was mainly attributed to the plants’ inability to produce adequate glutamate and α-ketoglutarate, which form the carbon skeleton for transamination processes. Response of glutamine synthetase to glufosinate ammonium was significantly higher at warmer temperatures, such that the use of an alternative glutamate dehydrogenase pathway was not vital. The significant increase in glutamine synthetase activity in ryegrass under warm temperatures was able to circumvent photosynthetic inhibition. A comparative study to investigate the response of different grass weed species to glufosinate ammonium was conducted on ryegrass (Lolium spp.), ripgut brome (Bromus diandrus L.) and wild oats (Avena fatua L) in Chapter 5. The grasses were grown at 10/15, 15/20, 20/25, and 20/30 °C (night/day) temperatures and treated with 0, 1.5, 3, 4.5, 6 and 7.5 L ha-1 glufosinate ammonium dosage rates. The study observed that glufosinate ammonium control differed among weed species. Control of ryegrass increased with decreasing temperature. Temperature had no effect on wild oats. Control of ripgut brome was initially poor at 10/15 °C then increased at 15/20 and 20/25 °C and finally decreased again at 25/30 °C. Such differences in the grass response to glufosinate ammonium, even after being grown under the same conditions, was attributed to their differences in morphological characteristics such as cuticle thickness, calcium accumulation and photosynthesis after herbicide application. Increase in cuticle thickness decreased mortality of all grasses. The study perceived that negative effects of calcium on mortality can only be noticed if the cytosolic and mitochondrial calcium is mobile and active, thus, allowing it to react with glufosinate ammonium. A possible solution to mitigate problems arising from calcium level, cuticle thickness and phenolic compounds was investigated in Chapter 6. The study investigated the role of adjuvants in increasing glufosinate ammonium efficacy. Ryegrass was grown at 20/25 °C and treated with 0, 1, 2, 3 and 4 L ha-1 glufosinate ammonium. Glufosinate ammonium was applied solo and in tank mixtures with Velocity Super™ (ammonium sulfate, L 9603), Summit Super (nitrogen solution/non-ionic surfactant, L 8539) and Class act NG™ (ammonium sulfate plus a non-ionic surfactant, L 10477). Better control of ryegrass was observed when treated with glufosinate ammonium in a tank mixture with Class act NG™ and Velocity Super™ than its solo application as well as in a tank mixture with Summit Super. Ammonium sulfate exhibits surfactant and humectant properties and it facilitates movement of glufosinate ammonium into the plant while non-ionic surfactants aim to reduce water surface tension only. This explains better control observed with glufosinate ammonium in tank mixture with adjuvants containing ammonium sulfate than with Summit Super. The study suggests that adjuvant Class act NG™ and Velocity Super™ can be used to mitigate the defensive response of phenolic compounds after glufosinate ammonium application, hence, increasing its efficacy. The practical relevance of glasshouse observations in Chapter 3, 4, 5 and 6 was confirmed in Chapter 7. The study was conducted under rainfed conditions at Langgewens and Roodebloem farms in 2018 and 2019. Glufosinate ammonium was applied at different times of the day (8:00 am, 12:00 pm and 5:00 pm). The dosage rates applied were 0, 2.5, 5 and 7.5 L ha-1. The study observed that morning (8:00 am) and evening (5:00 pm) applications showed better control of ryegrass than mid-day application provided relative humidity during application time was greater than 75%. Application at mid-day (when temperatures were higher than morning temperatures) showed good control only if relative humidity was recorded above 80%, however, higher dosage rates of 5 or 7.5 L ha-1 were required to achieve greater than 90% control.
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    The influence of granular and liquid top-dressed nitrogen on nitrogen use efficiency (NUE), grain yield and quality parameters of spring wheat (Triticum aestivum L.)
    (Stellenbosch : Stellenbosch University, 2018-12) Mbangcolo, Mongezi Morrison; Pieterse, P. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: Nitrogen use efficiency (NUE) of major cereal grains including wheat (Triticum aestivum L.) is estimated to be approximately 50% due to losses from leaching, soil denitrification, gaseous plant emissions, volatilization and surface runoff. Use of liquid nitrogen fertiliser to improve grain yields and quality parameters and N use efficiencies has demonstrated positive results; however, responses are inconsistent. Low N use efficiencies indicate the need to improve crop N recoveries and possible lower environmental pollution and the already high production costs. Studies on application of granular and liquid N topdressings to wheat are limited in South Africa. Studies were conducted from 2013-2015 to evaluate the response of NUE, yield and quality parameters of spring wheat to granular (broadcast) and liquid (sprayed) N topdressings under field conditions at two locations (Roodebloem - 34o 13’31.55”S; 19o 26’13.76”E and Langgewens - 33° 16' 33.96" S; 18° 42' 14.4" E) of the Western Cape Province, and controlled glasshouse conditions (2013, 2014 and 2016, and 2014, 2015 and 2016) at the Department of Agronomy, University of Stellenbosch, South Africa. Following applications of N as limestone ammonium nitrate (LAN 28%) at 30 kg N ha-1 at sowing, granular [(LAN (28%), granular urea (46%)] and liquid [urea ammonium nitrate (UAN 32%), liquid urea (46%) solution] N topdressings (30 and 60 kg N ha-1) were applied by means of single (tillering), and split (tillering and flowering) application on spring wheat. The field study results showed that the interaction between locality and growing season significantly affected NUE and grain yields and Roodebloem showed significantly better responses in grain yield in two (2014 and 2015) of the three study years compared to Langgewens. The effect of N rate showed that higher mean grain yield was produced through the application of N at 60 kg ha-1 (3 920 kg ha-1) compared to 30 kg ha-1 (3 577 kg ha-1) at Langgewens in 2013. The N rate x method of N application interaction showed that grain yield was significantly improved by liquid N topdressing at 30 kg ha-1 compared to granular N at 30 kg N ha-1 and liquid N at 60 kg N ha-1. Roodebloem (3 090 kg.ha-1) produced significantly higher mean grain yield compared to Langgewens (2 084 kg ha-1). The protein content and falling number were not significantly affected by N treatment. In the first glasshouse experiment, UAN applied at 60 kg N ha-1 significantly improved grain yield compared to other treatments. The responses showed that 60 kg N ha-1 promoted significantly higher yields and yield parameters compared to 30 kg N ha-1 and that liquid N topdressings were superior compared to granular applied N throughout the study. Similarly, in the second glasshouse experiment, plant responses increased with increasing N rates. The method x timing interaction showed significant differences due to timing of N application for liquid N topdressings. Plants treated with liquid N once at tillering showed superior responses compared to split applications of liquid N. NUE studies showed that different N use efficiency parameters were significantly improved by liquid N topdressings where the effects were significant both under field and glasshouse conditions. Seasonal rainfall was overall the main contributing source of variation in the studies conducted under field conditions .
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    Allelopathic effects of green manure cover crops on the germination and growth of blackjack (Bidens pilosa L.) and rapoko grass [Eleusine indica (L.) Gaertn]
    (Stellenbosch : Stellenbosch University, 2018-03) Rugare, Joyful Tatenda; Pieterse, P. J.; Mabasa, Stanford; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: The use of green manure cover crops to improve the nitrogen levels of soils and other physical as well as biological soil properties has been extensively researched and documented in several parts of the world. However, there is paucity of information on their use in weed management particularly through the exploitation of their allelopathic properties on some of the difficult to control weeds in arable crop production. Identification and integration of cover crops with weed suppressive ability is likely to reduce overreliance on herbicides and tillage-associated soil degradation. Moreover, the use of allelopathic mulches is likely to solve the problem of early season weed pressure commonly experienced in conservation agriculture where the use of pre-emergence herbicides is practically ineffective due to the presence of mulch at the time of planting. This study sought to establish the allelopathic potential of cover crops and evaluate their efficacy in suppressing weeds when their aqueous extracts and residues were used for weed control in maize. The study comprised different sets of experiments and was conducted between 2014 and 2017 at the University of Zimbabwe. The first study involved laboratory evaluation of 0, 1.25, 2.5, 3.75 and 5% wv-1 aqueous extracts of dry leaf, stem and root residues of ten cover crops on the germination, plumule and radicle growth of two test weed species namely, goosegrass (Eleusine indica (L.) Gaertn) and blackjack (Bidens pilosa L.) as well as of the crop species maize (Zea mays L.). The cover crops used included jack bean (Canavalia ensiformis (L.) DC), velvet bean (Mucuna pruriens (L.) DC var utilis), hyacinth bean (Lablab purpureus L), red sunnhemp (Crotalaria ochroleuca G. Don), showy rattlebox (Crotalaria grahamiana Wight & Arn.), common bean (Phaseolus vulgaris L.), common rattlepod (Crotalaria spectabilis Roth.), radish (Raphanus sativus L.), tephrosia (Tephrosia vogelii L.) and black sunnhemp (Crotalaria juncea L.). There was a significant interaction between extract concentration and extracted tissue with all cover crops and with all studied germination parameters of goosegrass and blackjack. Results showed that extracts of all cover crops exhibited an allelopathic potential on both test weeds that decreased in the order leaf > stem > root extract except with radish whose root extracts were more phytotoxic than extracts of the other tissues on goosegrass. In contrast, maize germination was not affected by aqueous extracts of the cover crops except for extracts of jack bean and common rattlepod. Most of the cover crops reduced seedling growth of maize except extracts of hyacinth bean and tephrosia. The second study involved an assessment of the allelopathic potential of soil incorporated biomass of the different cover crop tissues on the emergence and dry weight of goosegrass, blackjack and maize. The experiment which was laid out as a Complete Randomised Design (CRD) was carried out in the greenhouse. The soil incorporated leaf, stem and root residues of all cover crops significantly affected seedling emergence, dry weight and vigor indices of both weeds. Overall jack bean and hyacinth bean residues exhibited the highest phytotoxic activity on weed seedling emergence and growth. On the other hand, none of the cover crop residues exhibited deleterious effects on maize emergence, dry weight and vigor indices. Samples of the cover crop tissues were subjected to Liquid Chromatography Mass Spectrometry (LC-MS) at the Central Analytical Facilities (CAF) at Stellenbosch University in order to determine the phenolic composition of the extracts. The analyses showed the presence of kaempferol, naringenin, rutin, and genistein in jack bean. On the other hand genistein, atropine and kaempferol were detected in velvet bean tissues. A new compound was detected in the tissues of both cover crops and was tentatively identified as quercetin 3-O-glucoside 7-O-rhamnoside. Thereafter, standards of compounds detected in the samples were used to carry out germination bioassays using goosegrass and blackjack as test species. Highest inhibition of seedling growth of the two test weed species was obtained with standards of kaempferol and rutin. The effect of post emergence sprays of aqueous extracts of jack bean and velvet bean alone or in combination with reduced atrazine dosages on the test weed species and maize was evaluated in a greenhouse experiment that was laid out as a CRD. Application of the different treatments at the 3-4 leaf stage of test species significantly reduced chlorophyll content, plant height, and dry weight and resulted in irreversible weed damage at 6 and 10 days after spraying of blackjack and goosegrass, respectively. Maize was not affected by the repeated post emergence applications of the different herbicidal treatments. Field experiments were carried out during the summer cropping seasons 2014-15 to 2016-17 on two fields at the University of Zimbabwe and on one field at CIMMYT, Harare station in Zimbabwe. A Randomised Complete Block Design (RCBD) was used to evaluate the effect of different maize-cover crop rotations on the weed community composition. Soil samples were taken at 0-5, 5-10 and 10-15 cm depths in a bid to evaluate the influence of maize-cover crop rotations on weed seed bank size and species composition in an experiment that was laid out as a split plot design under greenhouse conditions. The results showed that there were no significant differences in weed density and species composition among maize-cover crop rotations in all seed banks. However, there was a significant decline in weed densities in the second maize phase of the rotations. Weed species richness, evenness and diversity was also significantly affected in the second year of the rotation although there were no significant variations among maize-cover crop rotations. Generally, cover crop rotations significantly reduced blackjack density in the second maize phase of the rotation but the density of goosegrass remained unchanged. The effect of maize-cover crop rotation type on weed emergence was significant. Maize-velvet bean and maize-red sunnhemp rotations consistently reduced total weed density and biomass across seasons. However, the different cover crop rotations did not exhibit deleterious effects on maize emergence and height. Based on the findings of this study it can be concluded that all the cover crops are allelopathic to goosegrass and blackjack and could be used to manage weeds either as surface mulches in maize-cover crop rotations or as post emergence sprays when weeds are still very small.
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    Optimizing harvesting procedures of Amaranthus hybridus L. and A. tricolor L. under different watering regimes during hot and cool seasons in southern Mozambique
    (Stellenbosch : Stellenbosch University, 2017-03) Ribeiro, Jeronimo E. M. M.; Pieterse, P. J.; Famba, Sebastiao; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: Drought has been the major constraint for vegetable and food crop production in arid and semi-arid regions as is the case in southern Mozambique with a tropical dry savanna climate that is prone to droughts. In this vulnerable region, malnutrition associated with scarcity of vegetables imposed a serious constraint in the diet of rural communities. Rural communities are forced to use wild plants, such as amaranth, as a way to supplement their nutrition. Here Amaranth species grow naturally and the leaves are regularly collected manually to be consumed as tender greens. There is no evidence of grain consumption. Although few growers cultivate amaranth in small areas or in their gardens, the intensity and frequency with which the leaves are collected has not been tested yet. The production of multi-purpose amaranth, a C4 plant widely distributed in the tropics and relatively drought-tolerant crop, offer a great potential to play a beneficial role in nutrition and food security. Three field experiments with Amaranthus hybridus and A. tricolor repeated six times each (three during the rainy season and three in the dry season) were carried out during the period from December 2013 to October 2015 in Maputo. A randomized complete block design in a factorial arrangement was used in each experiment. Experiment one aimed to assess the vegetative growth, flowering, leaf and grain yields, as well as nutrient contents in leaves and grain of those species when subjected to watering regimes of 80%, 50% and 20% of total available water. The relationship between temperature and day length on the leaf yield and the time to flowering were also assessed (Chapters 3 and 4). The treatments were laid out in a 3 x 2 factorial arrangement with six replications. In experiments two (Chapther 5) and three (Chapter 6), under the same watering regimes and with the same species, the vegetative growth, leaf yield and nutrient content were assessed as affected by harvesting intensity (plants topped by 25% and 50% of their heights) and harvesting frequency (every two weeks and every three weeks) respectively. In these two experiments, the treatments were laid out in a 3 x 2 x 2 factorial arrangement with three replications. Results from experiment one revealed that vegetative and reproductive growths were sensitive to soil water contents of 50% and 20% of total available water. However, the vegetative growth was less susceptible to water deficits that occurred in short intervals throughout the rainy season. Higher calcium and crude protein contents in the leaves were found at low water levels with the highest values obtained in A. tricolor. Temperature significantly affected the high leaf yield during the rainy season while day length had a noticeable influence on the low leaf yield during the dry season. The onset of flowering was determined by day length and minimum temperature with day length the most determining factor. The vegetative growth of both species showed similar behavior in response to different soil water and climate conditions over the year. The highest grain yield and harvest index was obtained in A. tricolor with an increase in minimum temperature which delayed flowering mainly when the day length was above 12 hours day-1 during the rainy season. The results from experiment two and three indicated insufficient evidence to support the hypothesis that the combined effects of watering regimes and harvesting intensity, and watering regimes and harvesting frequency affect vegetative growth in both species. However, the vegetative growth of both species was tolerant to water deficit at 50% of total available water with successive cuttings. The best harvesting intensity and frequency found was 25% of their heights and two-week intervals respectively, since this frequency yielded more small and tender green leaves which are preferred by the consumer. Amaranthus hybridus showed better performance and higher leaf yield compared to A. tricolor over the year. Results also revealed that the multiple harvests extend the vegetative growth phase which is an advantage for amaranth leaf production, especially under short days during the dry season. In plants harvested several times, the calcium and crude protein were not affected by watering regimes. However, the highest calcium and crude protein content in the leaves were obtained at final and first harvests respectively. As a leafy vegetable, A. hybridus showed to have potential to become a suitable crop throughout the year and to supplement calcium and protein requirements in the diet of rural communities. It is recommended to be cultivated in the rainy season under rain-fed condition with supplemental irrigation and in the dry season as an irrigated crop at 50% of total available water. In both seasons, the leaf harvesting should be topping by 25% at 2 weeks intervals.