Doctoral Degrees (Horticulture)

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Browsing Doctoral Degrees (Horticulture) by Author "Kotze, Theunis Nicolaas"

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    Seedling dynamics and subsequent production of annual Medicago spp. as affected by pasture utilization, seedbed preparation and soil type
    (Stellenbosch : Stellenbosch University, 1999-03) Kotze, Theunis Nicolaas; Agenbag, G. A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticultural Science.
    ENGLISH SUMMARY: In spite of numerous benefits of incorporating annual legumes in a rotation system, only 20% of a potential 1,7 million hectare in the western and southern Cape region of South Africa is currently under medics (annual Medicago spp.). Poor regeneration and persistence of annual medics is one of the major problems deterring farmers from incorporating medics in their crop rotations. In this study, which consists of several experiments, various aspects were investigated. These included the effect of digestion of medic seeds by sheep on the recovery thereof; the effect of tillage methods used during seedbed preparation for wheat on the wheat yield and subsequent regeneration and production of medics; and the effect of soil type, planting depth and seeding rate on the establishment and dry mass production of different medic cultivars. A field study was conducted over two years to quantify the relationship between sward density of regenerated medic pastures and seasonal dry mass and seed production. In the first experiment pod and seed characteristics of different sub-species as well as the effect of digestion by sheep on seed recovery were investigated. Although the M. truncatula cultivars produced larger pods, M. polymorpha and M. littoral is cuItivars had higher seed to pod ratios's. Smaller seed size and higher hardseededness of cv. Santiago in addition to the high seed percentage of this cultivar had resulted in significant higher seed recovery percentages after ingestion. In the second experiment the effect of different tillage systems on the placement of medic seed reserves in the soil profile and subsequent regeneration was studied. Tine implements, regardless the depth of cultivation, maintained more medic seed reserves in the topsoil in comparison with disc and mouldboard ploughing. Deeper tine (150 mm) cultivations however, were required to maintain high wheat yields. When establishment of different medic species in coarse sand, loamy sand and sandy loam soils and at different planting depths were investigated, significant differences between soil types were found. Lighter textured soils resulted in higher establishment percentages and dry mass production. Regardless of soil type 10 mm was found to be the optimum planting depth. Deeper planting depths resulted in poor establishment and subsequent dry mass production. When seed was left on the soil surface, results for all parameters tested were inferior to all other planting depths. Due to the poor results obtained with deeper (>30 mm) planting depths, the cumulative strength of seedlings obtained from higher seeding rates on the negative effect of planting depth was studied. Increasing sward populations to more than 2000 to 3000 plants m-2 however, resulted in self thinning due to interplant competition, especially in soil types that tend to form surface crusts. In general, plant size was more affected by sward densities compared to planting depth. Seasonal production of the different Medicago species / cuI tivars was significantly influenced by sward density. Early dry mass production correlated with increased sward density but differences decreased towards the end of the growing season due to higher rates of dry mass production obtained from the lower sward densities. In general, differences in seed production obtained from the different sward densities at the growing season were small. Except for the 78 end of the plants m-2 treatment, all the sward densities produced enough seed to ensure successful regeneration after a cereal crop. Although management of ley farming systems is complex, it became clear that if good management is practised, crop rotations with medics and cereals could be successfully implemented in the western and southern Cape areas of South Africa.