摘要:

A wheat crop growth model and stochastic dominance techniques were used to evaluate several wheat production strategies for the Orange Free State. Fifty seasonal grain yields and corresponding gross margins were calculated from daily weather data and soil data for each of five localities in the Orange Free State. Forty-five different strategies were tested at each locality. Strategies investigated were cultivar type, planting date and seeding rate. Highest long-term average yields were obtained at Bethlehem followed by Kroonstad, Wepener, Glen and Fauresmith. May, June and July plantings stochastically dominated other planting dates, and planting of winter type wheat generally dominated intermediate and spring wheat. Seeding rate had little influence on calculated production risk.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634816

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

Field trials were conducted on soybeans (Glycine max(L.) Merr.) in a hot, dry environment under irrigation and a warm, moist environment on soils on which the crop had not previously been grown. The aim was to assess the effectiveness of the method of inoculation withBradyrhizobium japonicum(no inoculation, seed dressing, spraying the planting furrow, and placement of inoculant 50 mm below seed depth) and the effect of two herbicides (metolachlor and trifluralin) on nodulation. Seed dressing and deep placement of inoculum achieved satisfactory nodulation and seed yield, with deep placement appearing to be marginally better under hot, dry conditions. Nodule proliferation on secondary roots was enhanced by deep placement of inoculum as opposed to seed dressing, which led to larger and greater numbers of nodules on the primary root. Herbicide application had no effect on nodule numbers or mass per plant. Satisfactory herbicide weed control significantly depressed soybean yield by 10% when compared with complete hand-weeding.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634817

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

The effect of temperature regimes on seedling diseases of grain sorghum (Sorghum bicolor(L.) Moench), was investigated in growth cabinets and for five planting dates in field trials. The period from planting to seedling emergence was increased by low pre-emergence temperatures (<13°C), with a concomitant increase in the incidence of pre-emergence damping-off and mesocotyl discolouration. Low post-emergence temperatures also promoted mesocotyl discolouration. Infection of the mesocotyl, as opposed to infection of the primary root, produced the primary post-emergence disease symptoms and was related to reduced seedling vigour and increased post-emergence damping-off. Seedlings grown in sterilized and unsterilized soil in growth cabinets, under conditions of varying day/night temperature regimes, were compared. Variation in pre-emergence damping-off, mesocotyl discolouration, post-emergence damping-off, seedling length and seedling mass could be attributed to low temperature and host predisposition to infection by soil-borne pathogens.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634818

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

The infestation patterns of the stem borer speciesBusseola fusca(Fuller) andChilo partellus(Swinhoe) and the effect on plant yield, tiller number, plant height, peduncle length and internal damage were determined under conditions of differential nitrogen fertilization and soil moisture. Planting date had a significant effect on the Infestation pressure and selective oviposition was observed between the different categories of irrigation and fertilization. Irrigated plants suffered higher yield losses than dry-land plants. Fertilizer application led to an increase in the number of tillers per main stem but no compensatory growth resulted from borer attack, irrespective of infestation pressure. Borer damage significantly influenced plant height and peduncle length. Under severe infestation pressure, dry-land plants suffered a higher degree of internal plant damage. These results stress the importance of differential economic thresholds for stem borer control in plantings of different yield potential.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634819

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

The competitive effect ofOxalis latifoliaH.B.K, (red garden sorrel) in maize has been investigated in the past four seasons at Cedara. The site chosen had a heavyO. latifoliainfestation. In each of the four seasons initial weed control was achieved on all plots using a mixture of atrazine, terbuthylazine and metolachlor. This treatment controlled a wide spectrum of weeds exceptO. latifoliawhich became the dominant species present. On ten plots this weed was allowed to compete with maize throughout the season. This was compared with another ten maize plots where most of theO. latifoliawas selectively controlled. Maize grain yields, plant lodging counts and the incidence ofDiplodiaandFusariumcob rots were not significantly altered by up to 329 well-developedO. latifoliaplants m−2compared with up to 55 insignificant plants m−2in the control plots.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634820

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

A simple procedure to determine micro-aggregate stability was developed. The micro-aggregates in aqueous suspension were subjected to controlled energy input imposed by sonic vibration to simulate the breaking down of aggregates by raindrop impact during a rain storm event. The change in aggregate size composition and the consequent increase in clay particle fraction were determined using the laser light scattering technique. Good correlations between soil surface susceptibility to runoff and erosion (as measured with final infiltration rates, soil losses and runoff volumes), spontaneous disintegration as well as disintegration resulting from sonic vibration, were obtained. A quadratic regression model has been proposed.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634821

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

Long-term field trials suggest that yields are declining where maize (Zea maysL.) is grown continuously in some parts of Natal. Available evidence indicates that the decline is not ascribable to soil fertility or climatic constraints and the possibility exists that a build-up of soil-borne pathogens is responsible. The work reported here was conducted to test this hypothesis. A field trial incorporating four cultivars and plots either untreated or treated with a soil fungicide (sodium N-methyldithiocarbamate) was conducted over three seasons at Cedara. Each season, yields, soil and plant analytical data and estimates of root and stalk disease incidence were recorded. For all seasons and cultivars the average yield response to fumigation (680 kg ha−1) was highly significant, and there was evidence to indicate differential cultivar response. Yield responses were associated with marked decreases In the incidence of stalk and root rots and were not ascribable to nutritional effects. The evidence presented indicates that root rots are more detrimental to yield than stalk rots and the suggestion is made that these diseases warrant far more attention than they currently receive.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634822

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

Mineralogical, chemical and electron microscopy studies were carried out on five weathering profiles developed on Archaean granites from the Nelspruit area to study changes in the distribution of primary minerals and their weathering products with profile development. The pedons differ significantly in their degree of pedogenesis. A gibbsite-kaolinite-quartz-hematite association has been established for the most highly-weathered and a feldspar-quartz-mica-kaolinite assemblage for the least-weathered surface horizon. The following observations apply to all profiles studied: Microcline is enriched in the sand—and plagioclase in the silt fraction. Plagioclase weathers much more readily than K-feldspar. Biotite is remarkably stable. Kaolinite and gibbsite contents increase towards the profile surface. Halloysite (7Å) is commonly associated with kaolinite, but decreases towards the top of each profile. Secondary iron phases comprise goethite and hematite. High gibbsite contents are associated predominantly with hematite as a secondary iron phase, while kaolinite is associated with goethite. Trends in major element composition with profile development mirror mineralogical findings: decreases in Na2O and CaO accompany a decrease in plagioclase contents and a decrease in K2O is consistent with decreasing proportions of microcline and/or biotite. Gradual increases in Al2O3and Fe2O3values reflect the formation of kaolinite/gibbsite and secondary iron phases, respectively. Mineral pangeneses are not related to elevation, topographic position or slope. Gibbsite is interpreted as being a relict of a more humid climate and bears evidence of palaeoclimatic changes in this part of South Africa.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634823

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

Opkomspatrone van 24 onkruidspesies, wat algemeen in bewerkte lande in somerreënvalgebiede voorkom, is ondersoek. Saad van onkruidspesies is gedurende die vorige herfs in die veld versamel. Een duisend sade van elk van die onkruide is ewekansig gekies uit die versamelde saad en in voorafgesteriliseerdegrand ingewerk. Meeste spesies toon gedurende die 1987/88, 1988/89 en 1989/90 seisoene 'n vroeësomer opkoms-piek.Acanthospermum glabraium, Datura stramonium, Galinsoga parvifloraenIpomoea purpureahet vroeg somer 'n hoëopkomspiek gevolg deur 'n lae opkomstempo gedurende die res van die seisoen. Siegs drie onkruide, naamlik,Cleome monophylla, Commelina benghalensisenXanthium strumarium, toon in die 1987/88 somer-groei- seisoen 'n tweede opkomspiek. Hierdie tweede piek is nie in die 1988/89 en 1989/90 somer-groeiseisoene herhaal nie. Onkruide soosChloris virgata, Crotalaria sphaerocarpa, Digitarla sanguinalisenEleusine indicahet gedurende al drie somer-groeiseisoene tot laat somer volgehou met kieming. Na drie groeiseisoene toon die totale opkomspersentasiesdat al die gesaaide sade nog nie opgekom het nie.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634824

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

Die verwantskap tussen bogrondse droëplantmassa, blaar- area-indeks en wortellengte vir mielies (Zea maysL. cv. PNR473) is op 'nClansthalgrondserie naby Potchefstroom ondersoek om sodoende 'n bydrae tot gewasmodellering te maak. Toekomstige wortellengtemetings kan moontlik by diéekotoop en cultivar uitgeskakel word deur gebruik te maak van regressievergelykings indien die blaararea-indeks of bogrondse droëplantmassa bekend is.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634825

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor