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Integrated weed management using intercropping and weeding frequency for sorghum, dry bean and cowpea.

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Weed management is a complex topic in agriculture. This is despite the fact that it may be one of the oldest crop management techniques to improve yield. Currently, agriculture predominantly uses chemical weed control because it produces results quickly, but it has many disadvantages regarding sustainable agriculture. Mechanical weed control also has a high risk regarding the need to mitigate sustainable agriculture challenges from the soil management perspective. Biological weed control is not reliable and could have counterproductive effects. This study aimed to expand the current knowledge about cultural weed control in the context of sustainable integrated weed management practices. Planting highly competitive crop species and manipulation of planting density are among the suggested approaches to cultural weed control. Sorghum (Sorghum bicolor (L.) Moench), dry beans (Phaseolus vulgaris L.), and cowpea (Vigna unguiculata (L.) Walp.) are traditional crops of southern Africa. Small-scale and subsistence farmers continue to rely on these crops as sources of food and fodder. The Agricultural Research Council of South Africa solicited an investigation into intercropping and frequency of weeding culturally in order to produce some indicators to advise small-scale farmers who have no capacity to use the predominant weed control strategies for commercial agriculture. The aim is also to promote sustainable agriculture practices that minimise or avoid use of herbicides. The study was designed to collect relevant data on the response of each one of the three crops to intercropping and cultural weeding frequency. The structure of the thesis is that it has a general review of literature on the topic and a focus on each crop response to the same field trial treatments over two growing seasons is presented in three different chapters. Finally, an integrated analysis of intercropping and weeding frequency interactions is presented in a separate chapter, before the general discussion of all the results are presented as a concluding chapter. Each chapter was written using a format for a separate complete manuscript. Crop growth traits associated with plant height, leaf number, chlorophyll content index, land equivalent ratio (LER), biomass, yield, legume pod production and sweet sorghum Brix value were the major crop performance variables of focus over two growing seasons under sole-cropping, intercropping of all three crops and weeding frequencies. Overall, the study revealed that sweet sorghum benefitted from intercropping with legumes while weeds caused a significant pressure on crop agronomic performance, including land equivalent ratio (LER) and final yield compared with sole cropping. The effect of sweet sorghum on legumes during intercropping is minimised by the earlier legume growth and development to escape competition for light and possibly water and nutrients, respectively. It is generally recommended that reduction of weeding frequency may have less effect on sorghum yield in an intercropping system with dry beans or cowpeas. The principal component analysis (PCA) evaluation allowed the study to identify the predominant weed species based on the relative importance value (RIV%). Although this finding cannot be used to associate the weed species with the cultural weed management approach used in this study, it is a significant guideline for predicting weed occurrence, especially in the context of providing extension advice to small-scale farmers. Keywords: beans; cowpeas; cultural weed management; intercropping; sorghum; yield; weeding frequency


Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.