Masters Degrees (Agronomy)

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Browsing Masters Degrees (Agronomy) by Subject "Agriculture -- Environmental aspects -- South Africa"

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    Physical and allelopathic effects of crop residue on wheat, barley and canola production
    (Stellenbosch : Stellenbosch University, 2021-03) Kotzé, Theunis Nicolaas; Swanepoel, Pieter Andreas; Pieterse, P. J.; Strauss, J. A.
    ENGLISH ABSTRACT: Retention of crop residue has many benefits such as moisture conservation, improvement of soil health and reduction in soil erosion. Residue retention together with no-tillage and crop diversification (crop rotation) are classified as Conservation Agriculture (CA). However, the adoption of CA comes with challenges of planting into large crop residue loads, especially when livestock is not part of the system. Certain crop residue types and loads may lead to yield penalties for the subsequent crop. Past studies have indicated that allelopathy, physical effects or chemical soil processes might be the cause. This study aimed to investigate the influence of crop residue on the subsequent wheat (Triticum aestivum), barley (Hordeum vulgare) and canola (Brassica napus) as well as identify the possible mechanisms responsible for driving productivity. Laboratory and glasshouse trials were conducted to evaluate effects of crop residue that had time to degrade prior to planting the next season’s crop, on the early growth of wheat, barley and canola. Extracts were made from various residues and the allelopathic effects of the extracts were evaluated on the germination, coleoptile and radicle lengths of seedlings. Germination was affected (p < 0.05) in barley and canola, but not in wheat (p > 0.05). The coleoptile and radicle lengths were affected more adversely (p < 0.05) than germination percentages. Some residue types led to decreases in the coleoptile and radicle lengths, while other residue types promoted them slightly. Crop residue still had an allelopathic potential even after degradation for one year in the field. However, in the presence of soil in the glasshouse, the allelopathic effects became negligible (p > 0.05). The canola with its small seed size was influenced (p < 0.05) by a large residue load of 8000 kg ha-1, which reduced early growth. A field trial evaluated performance of a single and a double disc planter and management of the residue loads, as well as the effect of various residue types on production of wheat, barley and canola. The double disc planter led to better wheat and barley establishment while the single disc planter led to better canola establishment (p < 0.05). The double disc planter cleaned the seed furrow more, while the single disc planter had better depth control. Allelopathy was negligible and physical effects was limited in this study due to relatively small residue loads, mostly under 5000 kg ha-1. The effect of crop residue on soil processes likely had the biggest influence on the subsequent crop. Crop residue types which resulted in the highest N mineralisation rate led to better yields in year two (p < 0.05), while in year one residue types which produced larger residue loads have led to slightly better yields due to moisture conservation. In a residue decomposition trial, effects of soil faunal communities and residue types on decomposition were tested. Soil fauna fragmented residue leading to faster decomposition. Residue types with lower C:N ratios decomposed faster. Retaining appropriate amounts of residue for a particular crop will minimise negative effects while retaining the benefits.