Seedling dynamics and subsequent production of annual Medicago spp. as affected by pasture utilization, seedbed preparation and soil type

Kotze, Theunis Nicolaas (1999-03)

Dissertation (Ph.D. Agric.) -- University of Stellenbosch, 1999.

Thesis

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.

AFRIKAANSE OPSOMMING: Ten spyte van die verskeie voordele wat die ins lui ting van eenjarige peulgewasse in wisselboustelsels inhou, is slegs 20% van 'n potensieele 1,7 miljoen hektaar in die Wes- en Suidkaap tans onder medics (eenjarige Medicago spp.). Swak hervestiging en produksie van eenjarige medics is een van die hoof oorsake wat boere weerhou om medics in hul wisselboustelsels in te sluit. Hierdie studie het uit verskeie eksperimente bestaan. Faktore soos die verteerbaarheid van medicsade en die deurvloei daarvan deur die spysverteringskanaal van skape; die effek van bewerkingsmetodes wat gebruik word vir saadbedvoorbereiding op die opbrengs van koring en daaropvolgende hervestiging en produksie van medics; asook die effek van grondtipe, saaidiepte en saaidigtheid op die vestiging en droemateriaal produksie van verskillende medic kultivars is ondersoek. Om die verwantskap tussen plantestand van hervestigde medicweidings en seisoenale droemateriaal- en saadproduksie te kwantifiseer, is 'n veldstudie oor twee jaar uitgevoer. In die eerste eksperiment is die peul- en saadeienskappe van verskillende subspesies asook die effek van verteerbaarheid en die deurvloei daarvan deur skape se spysverteringskanaal ondersoek. Alhoewel M. truncatula kultivars groter peule gehad het, het M. polymorpha en M. littoralis kultivars hoer saad tot peul verhoudings gehad. Die kleiner sade en hoer hardskaligheid van kultivar Santiago, tesame met die hoe saadpersentasie het tot hoer saad herwinnings persentasies na inname en deurvloei deur die spysverteringskanaal gelei. In die tweede eksperiment is die effek van verskillende bewerkingsmetodes op die plasing van medic saad reserwes in die grondprofiel en daaropvolgende herves tiging bestudeer. Tand implemente, ongeag die diepte van bewerking, het meer medic saad reserwes in die bogrond gelaat as skottel- en skaarploeg bewerkings. Dieper tand bewerkings (150 mm) word egter benodig om hoe koring opbrengste te verseker. In die eksperiment waar die vestiging van verskillende medic spesies in growwe sand, leem sand en sanderige leem gronde by verskillende saaidieptes ondersoek is, is betekenisvolle verskille tussen grondtipes gevind. Ligter gronde het hoer vestigingspersentasies en droemateriaal produksies gelewer.Die optimale saaidiepte in aIle grondtipes was 10 mm. Dieper saaidieptes het swak vestiging en laer droemateriaal produksie veroorsaak. Waar saad op die grondoppervlakte gelaat is, is die swakste resultate verkry. As gevolg van die swak resulta te wa t met dieper (>30 mm) saaidieptes verkry is, is die gesamentlike krag van saailinge verkrygbaar deur hoer saaidigthede op die negatiewe effek van plantdiepte bestudeer. Verhoogde plantestande bo 2000 tot 3000 plante m-2 het egter tot self ui tdunning as gevolg van interplant kompetisie gelei. Hierdie tendens is versterk deur grondtipes wat tot toeslaan geneig is. In die algemeen is plantgrootte meer deur plantestand as saaidiepte beinvloed. Seisoenale produksie van die verskillende Nedicago spesies / kultivars is beduidend deur plantestand belnvloed. Aanvanklike droemateriaal produksie was in ooreenstemming met verhoogde plantestand dighthede, maar verskille het gaandeweg gedurende die groeiseisoen verminder as gevolg van die hoer tempo van droemateriaal produksie wat by die laer plantestande verkry is. Saadproduksie aan die einde van die groeiseisoen het min verskil. Behalwe vir die laagste plantestand (78 plante m-2), het al die plantestande voldoende saad geproduseer om suksesvol te hervestig na 'n kleingraan siklus. Alhoewel die bestuur vanwisselboustelsels wat peulgewasrusoeste ins lui t baie kompleks is, het hierdie navorsing getoon dat sodanige stelsels weI suksesvol in die Wes- en Suidkaap gebiede van Suid Afrika toegepas kan word.

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