Department of Agronomy

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Browsing Department of Agronomy by Subject "Allelopathy"

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    The effect of soil residue cover on medicago pasture establishment and production under conservation agricultural practices
    (Stellenbosch : Stellenbosch University, 2015-04) Le Roux, Andries Abraham; Strauss, J. A.; Pieterse, P. J.; Stellenbosch University. Faculty of Agrisciences. Dept. of Agronomy
    ENGLISH ABSTRACT: Annual medic pastures play an important role in conservation agriculture (CA) practices in the Western Cape, because of the beneficial role it plays in rotation systems and the fact that it can re-establish on its own. In the Overberg medic pastures are the main pasture short rotation crop, but farmers in recent years shifted away from including medics. This was due to unsuccessful re-establishment and a visible decrease in dry matter production. This trend started after CA practices were implemented for a few years. A field study conducted during 2013 investigated medic re-establishment and production following a wheat, barley, oat and medic pasture production year ( WM, BM, OM and MM) of which residues were left on the soil surface at different cover percentage levels (100%, 75%, 50%, 25% and 0%). The objective of this study was to determine what the effect of different amounts of residues was on annual medic re-establishment and production. Data from this study suggest that management of annual medic pastures should aim to re-sow the medic pasture if plant count drops below 78 pants per square meter. Weed management is of cardinal importance as it competes for resources, light and space and decrease medic pasture re-establishment and production. The data also indicates that the wheat/medic sequence is the best option when applying a short cash crop/annual pasture cropping system. Producers should manage their animals to ensure that a 50% to 75% cover is left on top of the soil following the grazing of residues during the summer months. The study in 2013 should have been replicated, but due to the low levels of re-establishment and production a decision was made to re-plant the trial sites. The field study conducted during 2014 investigated the medic/clover establishment and production following a re-plant. Medics were replanted following a W, B, O and M season, respectively. Residues again were manipulated to different cover percentages (100%, 75%, 50%, 25% and 0%). The objective was again to look at the amount and type of residues on medic/clover establishment and production following re-plant. Data from this study indicated that it might be advisable for annual medic/clovers to be re-sown after a cereal production year rather than a medic pasture year. With the production of medic/clover pastures not being affected by the residue cover percentage, a 100% residue cover following re-plant is best in rotations, if the optimal effect of CA wants to be observed. If animals are included in the production cycle, grazing of residues during summer months can occur until 50% cover is left. Soils will take longer to reach its potential, but by including animals the gross margin is more stable year on year. Two supplementary studies were conducted to investigate the germination of annual medics under controlled conditions. The objectives of the first supplementary study was to investigate the physical barrier effect of residues at different percentage cover (100%, 75%, 50%, 25% and 0%) and a possible allelopathic action from different types of residues (wheat, barley, oat and medic) on the annual medic cultivar Cavalier (one of the cultivars used during 2014 field re-plant). The different amounts of residue had no significant effect on percentage emergence of Cavalier. The 0% residue cover having the slightly higher germination could be because there are no physical obstructions preventing seedlings to establish. The different types of residue cover had no significant effect on the germination of annual medics, as the germination under wheat, barley, oats and medic residues did not differ from the control. The control had a slightly higher germination percentage (85%), while germination under residues was just below the recommended germination rate of 80-85%. This could be an indication of allelopathy from residues. The objective of the second supplementary study was to investigate the allelopathic effects of different residue leachates (wheat, barley, oat and medic) at different levels of concentrations (100% leachate, 75%, 50%, 25% and distilled water being the control) on Cavalier germination. The interaction between leachate type and concentration were significant. Low levels of leachate concentration did not have a significant impact on medic germination when compared between each other and the control. When the concentration percentage was increased differences were detected. Cavalier germination decreased drastically when medic leachate concentration increased, indicating allelopathic effects. Cavalier germination followed the same trend, just not as drastic, when wheat leachate concentration increased. This indicates that wheat could also have a negative allelopathic effect. With oat leachate Cavalier germination did not decrease except when 100% concentrate was used, which could indicate a small allelopathic effect. Cavalier germination following barley leachate showed no effect as concentration increased, even showing the odd increase. Depending on repeatability or follow-up studies of these experiments, data suggest that re-plant of medic pastures is beneficial if plant count drops below sustainable levels. Management of weeds during the medic pasture year improves production. Annual medic pastures should be re-planted following a cereal production year rather than a previous pasture year. Thus single medic rotations are preferred, for example WMWM rotation. Greater amounts of residues are beneficial for CA effects, but allelopathic effects of wheat and oat residues should be taken in consideration during re-establishment and residue levels should be lowered.
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    The interference potential of nine selected South African spring wheat cultivars with selected weed species
    (Stellenbosch : Stellenbosch University, 2008-12) Nambili, Julia Nghituvali; Pieterse, P. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    The development of herbicide resistance in weeds is one of the major factors hampering profitable crop production worldwide. In South Africa resistance to herbicides in weeds is also a big problem, in particular in the Winter Rainfall Region of the country. The lack of sufficient different mode of action herbicide groups that can be rotated in these conditions necessitate the implementation of integrated weed management programmes to curb the development and spread of herbicide resistance. One of the alternative physical weed management strategies is to maximize crop competition to the weed population. One aspect of such a strategy is to plant crop cultivars that have greater interference potential than others.
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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.