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

[摘要] ENGLISH SUMMARY: In spite of numerous benefits of incorporating annual legumes ina rotation system, only 20% of a potential 1,7 million hectarein the western and southern Cape region of South Africa iscurrently under medics (annual Medicago spp.). Poor regenerationand persistence of annual medics is one of the major problemsdeterring farmers from incorporating medics in their croprotations.In this study, which consists of several experiments, variousaspects were investigated. These included the effect ofdigestion of medic seeds by sheep on the recovery thereof; theeffect of tillage methods used during seedbed preparation forwheat on the wheat yield and subsequent regeneration andproduction of medics; and the effect of soil type, planting depthand seeding rate on the establishment and dry mass production ofdifferent medic cultivars. A field study was conducted over twoyears to quantify the relationship between sward density ofregenerated medic pastures and seasonal dry mass and seedproduction.In the first experiment pod and seed characteristics of differentsub-species as well as the effect of digestion by sheep on seedrecovery were investigated. Although the M. truncatula cultivarsproduced larger pods, M. polymorpha and M. littoral is cuItivarshad higher seed to pod ratios's. Smaller seed size and higherhardseededness of cv. Santiago in addition to the high seedpercentage of this cultivar had resulted in significant higherseed recovery percentages after ingestion.In the second experiment the effect of different tillage systemson the placement of medic seed reserves in the soil profile andsubsequent regeneration was studied. Tine implements, regardlessthe depth of cultivation, maintained more medic seed reserves inthe 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 depthswere investigated, significant differences between soil typeswere found. Lighter textured soils resulted in higherestablishment percentages and dry mass production. Regardlessof soil type 10 mm was found to be the optimum planting depth.Deeper planting depths resulted in poor establishment andsubsequent dry mass production. When seed was left on the soilsurface, results for all parameters tested were inferior to allother planting depths.Due to the poor results obtained with deeper (>30 mm) plantingdepths, the cumulative strength of seedlings obtained from higherseeding rates on the negative effect of planting depth wasstudied. Increasing sward populations to more than 2000 to 3000plants m-2 however, resulted in self thinning due to interplantcompetition, especially in soil types that tend to form surfacecrusts. In general, plant size was more affected by swarddensities compared to planting depth.Seasonal production of the different Medicago species / cuI tivarswas significantly influenced by sward density. Early dry massproduction correlated with increased sward density butdifferences decreased towards the end of the growing season dueto higher rates of dry mass production obtained from the lowersward densities. In general, differences in seed productionobtained from the different sward densities at thegrowing season were small. Except for the 78end of theplants m-2treatment, all the sward densities produced enough seed to ensuresuccessful regeneration after a cereal crop.Although management of ley farming systems is complex, it becameclear that if good management is practised, crop rotations withmedics and cereals could be successfully implemented in thewestern and southern Cape areas of South Africa.

[发布机构] Stellenbosch University