Doctoral Degrees (Agronomy)

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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.
Effect of intercropping and phosphorous application on the growth and yield of sweetpotato, groundnut and soybean.
(Stellenbosch : Stellenbosch University, 2017-03) Munda, Eliah; Pieterse, P. J.; Andrade, M. I.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Sweetpotato (Ipomoea Batatas (L) Lam) is among the most important root crops in Mozambique. However, the yield is lower than its genetic potential due to poor soil fertility and poor agronomic practices. Inorganic fertilizers that could contribute to yield increase are too costly hence they are not accessible. One of the feasible option is the use of intercropping with legumes to recapitalize soil fertility and improve yield. In this study the effect of intercropping sweetpotato with groundnut and soybean at three phosphorus (P) levels on soil chemical properties, sweetpotato, groundnut and soybean vegetative growth, yield and sweetpotato nutritional quality was investigated. The study was carried out at Umbeluzi Research Station during the 2013/14, 2014/15 and 2015/16 growing seasons. A factorial design in a split plot arrangement was used. The main plot treatments were; sole sweetpotato, sole groundnut, sole soybean, sweetpotato-groundnut, sweetpotato-soybean, sweetpotato- groundnut- soybean and groundnut- soybean intercropping. The subplot treatments were 0, 20 and 40 kg P ha-1 applied at planting. Sweetpotato- groundnut, sweetpotato- soybean and soybean- groundnut intercropping at 40 kg P ha-1 in the 2015/16 growing season had more soil total nitrogen (N) compared to sole sweetpotato (P=0.038). Soybean-groundnut intercropping at 0 kg P ha-1 in 2013/14 growing season had more Olsen P than sole sweetpotato in all growing seasons (P=0.023). Sweetpotato- groundnut and sweetpotato- soybean had 21 % and 25.3 % more soil CEC respectively than sole sweetpotato at 40 kg P ha-1. Sweetpotato- groundnut and sweetpotato- soybean intercropping at 40 kg P ha-1 had 42.9 % and 32.9 % more CEC than at 0 kg ha-1 respectively (P=0001). All treatments involving legumes in the mix had lower soil pH in 2014/15 and 2015/16 compared to 2013/14 growing seasons. Soybean- groundnut intercropping, sole groundnut and sole soybean had higher soil available potassium (K) compared to sole sweetpotato in 2015/16 growing season (P=0.001). Sweetpotato- soybean intercropping at 20 kg ha-1 had higher sweetpotato main stem length compared to sole sweetpotato. There was no significant difference in sweetpotato main stem length between 20 and 40 kg P ha-1 in the intercropping treatments (P>0.05). Sweetpotato- groundnut intercropping at 40 kg P ha-1 had higher fresh root mass plant -1 compared to sole sweetpotato crop in 2013/14 and 2014/15 growing seasons. Sweetpotato-groundnut- soybean-, sweetpotato-soybean and sweetpotato- groundnut intercropping at 0 and 40 kg P ha-1 had higher number of leaves plant-1 compared to sole sweetpotato. Sole sweetpotato had higher sweetpotato stem diameter compared to sweetpotato- soybean intercropping in 2013/14 and 2014/15 growing seasons. Sweetpotato- groundnut intercropping at 0 and 20 kg P ha-1 had 32.7 % and 58.5 % more total storage root yield compared to sole sweetpotato. (P=0.0001). There was no significant increase in total storage root yield between 20 kg P ha 1 and 40 kg P ha-1 for sweetpotato- groundnut, sweetpotato-soybean intercropping and sole sweetpotato (P>0.05). Highest sweetpotato partial land equivalent ratio of 1.6 was attained on sweetpotato- groundnut intercropping at 20 kg P ha-1. Total storage root yield increased by 33.6 % at 20 kg P ha-1 compared to 0 kg P ha-1. Sweetpotato- groundnut intercropping had 48.3 % more commercial root yield compared to sole sweetpotato at 20 kg P ha-1 (P=0.036). Sweetpotato- groundnut intercropping at 20 kg P ha-1 had 27. 4 % more number of storage roots plant-1 and higher harvest index compared to sole sweetpotato (P=0.001). Sweetpotato- soybean intercropping decreased number of storage roots plant-1 compared to sole sweetpotato in 2014/15 growing seasons (P=0.008). There was no significant difference in the number of storage roots plant-1 between sweetpotato- groundnut intercropping and sole sweetpotato cropping system (P>0.05). Sole sweetpotato at 20 kg P ha-1 had higher storage root diameter compared to sweetpotato-soybean intercropping (P=0.049). Sweetpotato- soybean intercropping had higher storage root length at 20 kg P ha-1 compared to 0 kg P ha-1 in 2013/14 and 2015/16 growing seasons (P=0.027). Total biomass at 20 kg ha-1 was higher than at 0 kg ha-1 in all treatments (P=0.0001). Sweetpotato- groundnut, sweetpotato-groundnut- soybean intercropping and sole groundnut had a significantly higher pod yield at 20 kg P ha-1 than at 0 kg P ha-1 (P=0.005). Groundnut-soybean intercropping had a significantly lower shelled groundnut yield than sweetpotato-groundnut at 20 kg P ha-1 (P=0.017). Percent dry matter content was higher in sole sweetpotato at 40 kg P ha-1 compared to any other treatments involving soybean. Sweetpotato- groundnut and sole sweetpotato at 20 and 40 kg P ha-1 had more percent glucose content in 2014/15 and 2015/16 compared to the 2013/14 growing seasons (P<0.05). Percent starch content at 40 kg P ha-1 was higher than at 0 kg P ha-1 in all growing seasons (P=0.0001). There was a significantly higher β-carotene content in the storage roots in 2015/16 than 2013/14 growing seasons. Sweetpotato- groundnut intercropping at 0 kg P ha-1 had a higher iron (Fe) content in the sweetpotato storage roots compared to any other treatment (P=0.000). Sweetpotato –legume intercropping had more zinc (Zn) content in the storage roots and Zn yield in sweetpotato in 2015/16 compared to 2013/14 growing seasons (P=0.033). Farmers with the same environmentl conditions as where this study was carried out are recommended to intercrop sweetpotato and groundnut at 20 kg P ha-1. Key words: intercropping, legumes, micronutrient deficiency, nutritional quality, sweetpotato.
Exploring phosphorus, mucuna (Mucuna pruriens)and nitrogen management options for sustainable maize production in a depleted kaolinitic sandy loam soil of Zimbabwe
(Stellenbosch : University of Stellenbosch, 2009-12) Shoko, Munashe; Pieterse, P. J.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Continuous cropping without replenishing the nutrient component of soils will eventually lead to the depletion of soil nutrients. Small scale farmers in Zimbabwe often do not have the financial means to buy fertilizer and this problem is exacerbated by scarcity of commodities such as fertilizers. The use of herbaceous legumes such as mucuna (Mucuna pruriens) can assist to recapitalize soil fertility depletion and improve subsequent maize productivity in sandy loam soils in the small holder farming sector of Zimbabwe. In this study the effect of phosphorus (P) application to a mucuna crop, the effect of mucuna management options and the application of nitrogen (N) to the subsequent maize crop was investigated. The experiment was carried out during the 2007 to 2009 seasons at the Grasslands Research Station in Marondera in Zimbabwe. The soils are classified as humic ferrolsols and are predominantly of the kaolinitic order with sandy loams of low fertility and are slightly acid (pH CaCl = 5.2). A randomized complete block design was used for the effect of P on mucuna productivity and the effect of P and mucuna management options on the soil properties. The treatments were two P rates (P0 and P40 = 0 kg P ha-1 and 40 kg P ha-1 respectively) applied to a preceding mucuna crop, four mucuna management options [1) fallow (F) (no mucuna planted = control), 2) mucuna ploughed-in at flowering (MF), 3) all mucuna above ground biomass removed at maturity and only roots were ploughed-in (MAR) and 4) mucuna pods removed and the residues ploughed-in (MPR)]. A split-plot design was used to study the effect of P application to mucuna, mucuna management options and N rates on the growth and yield of the subsequent maize crop. The four N treatments [N0 = 0 kg N ha-1, N40 = 40 kg N ha-1, N80 = 80 kg N ha-1 and iv N120 = 120 kg N ha-1] were applied to a subsequent maize crop. Growth and development parameters such as biomass production, leaf area index, nutrient content of the foliage and grain yield were determined in the mucuna and maize crops. Soil parameters investigated included nutrient content, pH, bulk density and porosity. Phosphorus application in these particular soil conditions positively influenced mucuna biomass production and therefore enhanced the role of mucuna as a rotational crop by increased positive effects on the subsequent maize crop. The incorporation of above-ground biomass of mucuna had positive effects on all soil properties investigated. The MF and MPR management options increased the soil organic matter (OM) and reduced bulk density which leads to an improvement in porosity (f) of the soil. Mucuna incorporated at flowering (MF) and P40 treatment combination resulted in the highest mineral N, P, potassium (K), calcium (Ca) and magnesium (Mg) levels. A significant three-way interaction (P<0.05) between mucuna management options, nitrogen rates and time was observed in terms of biomass production and all nutrients in the leaves of the subsequent maize crop. The main findings were that the MF management option had the highest biomass and foliar nutrient accumulation through-out all the treatment combinations. In general the MF management option gave the highest maize yield across all the treatment combinations. Incorporation of mucuna biomass into the soil prior to planting a maize crop therefore improve soil physical and chemical qualities resulting in improved soil conditions for a subsequent maize crop which in turn lead to higher maize yields. Including a mucuna rotational crop have a similar effect on maize yield than application of 80 kg ha-1 of fertilizer N.
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.
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 .
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.
Rangeland potential, quality and restoration strategies in North-Eastern Ethiopia : a case study conducted in the Southern Afar region
(Stellenbosch : University of Stellenbosch, 2006-03) Gebremeskel, Kidane; Pieterse, P. J.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Agronomy.
Vegetation dynamics and restoration strategies of degraded rangeland were investigated near a watering point in the Allaidege communal grazing area in Administrative Zone 3 of the Afar Region in the northeastern lowlands of Ethiopia. The degradation gradient formed by grazing pressure in the study area was stratified into four different areas based on the vegetation cover; severely degraded (SD), moderately to severely degraded (MSD), moderately degraded (MD) and lightly degraded (LD) areas. The study was initiated at the start of the rainy season in June 2003 and lasted untill December 2004. The objectives were to study the effects of the grazing pressure on plant species composition; on plant biomass production and basal cover; on rangeland forage quality; on the rangeland soil status and to determine and quantify viable restoration strategies for forage species in severely degraded rangelands. The botanical composition of the different degradation areas was determined by making a 250 point wheel point method survey in each of four 30 m x 30 m quadrats in each degradation area using the nearest plant approach. The botanical composition of each degradation area was determined by measuring the frequency of occurrence of the different life forms (perennial grasses, annual grasses and forbs) of the species recorded in the field. Accordingly, a significant interaction was observed in both seasons between the different degradation areas and life forms considered. A high abundance of annual grasses was evident in SD and MSD areas in both seasons. In the MD and LD areas, a three-fold increase in frequency was recorded for perennial grasses compared to the MSD area in 2003. In 2004, the frequency of annual grasses, forbs and perennial grasses in the MD area was almost similar to that of the LD area. The abundance of perennial grasses in the MD and LD areas was two- and five-fold higher compared to perennial grasses in the MSD and SD areas respectively. Biomass production was recorded by cutting the vegetation in 1 m x 1 m quadrats in each grazing area at ground level. The dry matter content of subsamples was determined in order to calculate the dry matter production of the quadrat. The differences in dry matter yield recorded in the different degraded areas was not significant for the 2003 season, although an increasing trend in yield was observed from the SD to MD areas. Significant yield differences were however recorded when one outlier in the data was excluded from the analysis. The significant differences occurred between the MD and SD areas where the MD area produced 2.4 t ha-1 more dry matter than the SD area. Similarly, in 2004 no significant yield difference was observed between the degradation areas. However, the contribution of different species to dry matter yield varied in the different degradation areas. Setaria verticillata, Sporobolus ioclados and Paspalidium desertorum were found to be the major species contributing to the dry matter producion in the SD area, S. verticillata and P. desertorum in the MSD area, Chrysopogon plumulosus and P. desertorum in the MD area and C. plumulosus and Panicum coloratum in the LD area. The percentage basal cover was calculated from the number of basal strikes recorded at 1 000 points in each plot of each degradation area using the wheel point method. The total basal cover percentage did not significantly change along the degradation gradient in any of the seasons. However, data for both seasons showed an increasing trend of total basal cover percentage closer to the watering point compared to areas further away from the watering point, except for the SD area, which had the lowest basal cover percentage. The contribution to percentage basal cover by some species decreased while it increased for some other species in grazing areas near the watering point. Forage quality was investigated by analysing sub-samples of the forage samples taken to determine biomass production. The forage samples were analysed for neutral detergent fibre (NDF), acid detergent fibre (ADF), crude protein (CP), lignin, in vitro dry matter digestibility (IVDMD), phosphorus (P), and calcium (Ca) content. The forage showed a decrease in NDF and ADF content in areas close to the watering point in both seasons. This decrease in fibre content was accompanied by an increase in CP content close to the watering point. The increase in CP was significant for the SD area in both seasons. Although a similar trend was observed in both seasons, the CP content was found to be significantly higher in 2004 than in 2003. The results of the lignin analysis were inconclusive if the data of both seasons are considered. It does appear however as if the lignin content of the forage was generally higher in 2003. The 2 years pooled average of P content of the forages showed insignificant variation along the degradation gradient. However, an increase in P concentration of the forages was evident in areas far from the watering point. Contrary to this, Ca concentration was significantly higher in the SD area compared to areas further away from the watering point. Hand clipped forage samples and esophageal collected forage samples were analysed to compare the quality of the samples. Due to the fact that only two animals were available for esophageal collection, differences were in most cases not significant at the 5% level, but trends indicate that animals select higher quality forage than what is assumed based on hand clipping. Organic carbon (OC) content, total nitrogen (N) content, available phosphorus (P) content, available potassium (K) content, exchangeable calcium (Ca) and magnesium (Mg) contents, cation exchange capacity (CEC), total exchangeable bases (TEB), exchangeable sodium percentage (ESP), soil acidity (pH) and base saturation of soils in the different degradation areas were determined by means of acknowledged laboratory methods. No significant differences in OC, N, P, K, Ca, Mg and K content of soil in the different degradation areas could be observed. There was however an increasing trend for OC and N content with distance from the watering point. Sodium concentration and pH increased significantly in areas close to the watering point. Cation exchange capacity content of the soil was variable and no clear trend could be established. Significantly higher TEB and ESP contents were observed in the SD area. In general, the differences in plant biomass production and basal cover, botanical composition, forage quality and soil status over the degradation gradient clearly implicates the negative impact of unrestricted grazing pressure on the rangeland around the watering points. In the rangeland restoration trial, establishment of three local and three exotic grass species in the SD area was investigated. Treatments applied included application of inorganic fertilizer, dry dung organic manure and grass mulch. The mulch treatments caused a significant yield increase for all the sown species. Among all the species, Ischaemum afrum and Tragus berteronianus performed better and produced significantly higher dry matter yields than Enteropogon rupestris, Chloris gayana and Panicum coloratum. In general the study indicated the importance of mulching when planning to restore degraded rangeland under arid environmental conditions.
Resistance of ryegrass (Lolium spp.) to paraquat and glyphosate in the Western Cape
(Stellenbosch : Stellenbosch University, 2007-12) Eksteen, Frederik Heinrich; Cairns, A. L. P.; Pieterse, P. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: The origin of this study dates back to 2001 when ryegrass (Lolium spp.) from a vineyard near Tulbagh was found by the author to be resistant to glyphosate. Several other populations of glyphosate resistant ryegrass were found on nearby farms. In attempts to eliminate what was initially thought to be a very localised resistance problem, paraquat was applied at several of these sites. It soon became apparent that significant numbers of these ryegrass populations were also resistant to paraquat. Populations that were susceptible to glyphosate but resistant to paraquat were also found. This was the first recorded incidence of paraquat resistance in ryegrass. Further screening of Western Cape ryegrass resulted in the identification of populations that showed multiple resistance to glyphosate, paraquat and ACCase inhibitors. In laboratory trials this study established that the mechanism which confers resistance to paraquat is based on the failure of the herbicide to reach its site of action in the plant. This lack of trans location was ascribed to sequestration of the active ingredient in a non-physiologically active site in the plant, such as the vacuole. This form of resistance was overcome by applying glufosinate ammonium, either prior to, or together with the paraquat application. The same degree of synergy was observed using photosystem II inhibitors with paraquat. The resistance mechanism is overcome by these products by either preventing paraquat from being 'sequestrated and/or freeing up of herbicide which had already been bound. Mixtures containing paraquat, glufosinate ammonium and a photosystem II inhibitor such as terbuthylazine gave the best control of resi~tant ryegrass populations. Independent confirmation, of the findings in the laboratory perfonned trials, was done in field trials, conducted during the 2007 season by Syngenta SA on paraquat resistant ryegrass populations in the Western Cape.

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