摘要:

Experimental maize hybrids with various levels of resistance to the maize stalk borer,Busseola fusca(Fuller), were evaluated at different infestation levels during seasons of favourable and unfavourable rainfall. Limited plant resistance sufficed to reduce plant damage and yield losses significantly. In spite of plant damage being more severe during the unfavourable season, yield losses were less severe, which was attributed to a greater seasonal effect on undamaged than on damaged plants. Yield reduction by seasonal effects was shown to manifest as a constant proportion of attainable yield in the favourable season, independent of infestation level. The significance of the results is discussed in relation to the development of differentiated economic thresholds, in order to improve the predictive estimation of yield losses for various categories of yield potential.

ISSN:0257-1862    

DOI:10.1080/02571862.1996.10634377

出版商:Taylor&Francis Group    

年代:1996

数据来源: Taylor

摘要:

There is an increasing awareness of the ecological and agricultural importance of hydric (wetland) soils. Because of the scarcity of information in South Africa, this study aimed to critically examine systems currently being applied to the hydric soils of South Africa, recommend the best criteria to be used from a management point of view, and suggest future research.Soil taxonomy(Soil Survey Staff, 1975) was found to be superior to the South African soil classification system for describing hydric soils, primarily because it accounts for depth of waterlogging. Nevertheless,Soil taxonomydoes not account adequately for degree of wetness, with some series including a wide range of water regimes. A review of soil morphology/water-regime studies revealed that along the continuum from temporarily wet to permanently wet areas: matrix chroma decreases; the most intensively mottled zone becomes progressively shallower; mottle abundance increases then decreases; and soil organic matter increases. Based on these trends and observations of wetlands in KwaZulu-Natal, a three-class water-regime classification system with distinguishing field characteristics was developed. Provisionally, this system could be used in combination withSoil taxonomyor the South African Soil Classification system for enhancing the description of hydric soils in South Africa. However, this three-class system has not been adequately tested and it is strongly recommended that local studies be undertaken to improve the capacity for clearly describing hydric soils.

ISSN:0257-1862    

DOI:10.1080/02571862.1996.10634378

出版商:Taylor&Francis Group    

年代:1996

数据来源: Taylor

摘要:

Interplot competition in plant breeding experiments can substantially reduce the efficiency of plant selection. In diallel experiments on outcrossing species such as maize (ZeamaysL.), competing stresses might result from differences in vigour between inbred and hybrid entries. A partially balanced 8×8 triple lattice is presented. The design accommodates the inbred parents and F1single crosses of a 10×10 half diallel cross of maize in such a way that competing stresses are minimized. Practical field-plot considerations in the design are also discussed.

ISSN:0257-1862    

DOI:10.1080/02571862.1996.10634379

出版商:Taylor&Francis Group    

年代:1996

数据来源: Taylor

摘要:

The root organic matter (OM) per unit volume of soil ofLolium perenneL. (perennial ryegrass) was evaluated over two years under subtropical conditions. Root OM was measured on five separate occasions in four soil-depth categories (0–50, 50–100, 100–200 and 0–200 mm) under six combinations of grazing frequency and intensity. The root systems were unaffected by treatments during the establishment year. During the second year, treatments infrequently grazed had higher root OM contents per unit volume of soil in both the 0-50- and 0-200-mm soil-depth categories than treatments frequently grazed. Grazing intensity did not affect root OM per unit volume of soil in any depth class. Overall, perennial ryegrass was found to be shallow-rooted, with approximately 75% of the root mass occurring in the top 50, mm of the soil.

ISSN:0257-1862    

DOI:10.1080/02571862.1996.10634380

出版商:Taylor&Francis Group    

年代:1996

数据来源: Taylor

摘要:

Die studie is onderneem om die invloed van twee faktore, N-bemesting en atmosferiese N-binding, op opbrengs en proteïengehalte van Suid-Afrikaanse sojabone vas te stel. Drie N-misstowwe (ureum, NH4+, NO3−) met15N verryk, is in 'n potproef teen 20 kg N ha−1na die opkoms vanGlycine max(kultivar Prima) saailinge toegedien. Die saad is met tweeBradyrhizobium japonicum-rasse, WB1 en WB74, geïnokuleer. Die plante is in die R8 (oesryp) groeistadium geoes en die saad vir totale N en15N ontleed. In simbiose met WB1 en WB74 het die kultivar respektiewelik 68.4% en 65.2% van die N in die saad uit die atmosfeer bekom. N uit die drie bronne is beter in WB1 se teenwoordigheid benut. Die NO3-bron is die beste benut. Tussen 45.2% en 54.4% van toegediende N is in die saad herwin. Die invloed van toegediende misstof op atmosferiese N- en grond-N-opname het tussen die twee inokulante verskil. Die NH4-bron het veroorsaak dat meer atmosferiese en grond-N in die saad opgeneem is. Die NO3-bron het meer misstof-N in die saad laat akkumuleer. WB1 het tot meer atmosferiese en misstof-N in die saad gelei, terwyl met WB74 die saad meer grond-N bevat het. Alhoewel WB74 as inokulant teenoor WB1 'n kleiner persentasie atmosferiese N aan die saad voorsien het, het dit die massa totale N in die saad, teenoor die kontrole (nie geïnokuleer nie), laat toeneem. Die twee rasse het dieselfde proteïenproduksie per hektaar vir al die toegediende N-misstofvorm gelewer. Die toegediende 20 kg N ha−1het 'n merkbare (statisties nie betekenisvol nie) toename teenoor die kontrole in proteïenproduksie veroorsaak. Die verhoging in opbrengs en proteïeninhoud van sojaboonsaad deur N-bemestingbehoort met hoër N-toedieningspeilebevestig te word.

ISSN:0257-1862    

DOI:10.1080/02571862.1996.10634381

出版商:Taylor&Francis Group    

年代:1996

数据来源: Taylor

摘要:

Koringkultivars se aanpassing word grootliks bepaal deur 'n gepaste fenologie en dit word sterk deur saaityd beïnvloed. Om verskille in opbrengs beter te kan verklaar, is kennis van die groeiduurtes van die verskillende ontwikkelingsfases van koring noodsaaklik. Min inligting oor die fenologie van koringkultivars is in Suid-Afrika vir verbouingsgebiede met 'n Mediterreense klimaat beskikbaar. Die lentekoringkultivars SST 25, Palmiet, Adam Tas en Chokka is in veldproewe op Elsenburg-, Langgewens- en Tygerhoekproefplase gesaai om die invloed van saaityd op drie voorantese groeifases, naamlik blaarvorming, aarvorming en aargroei van koring (Triticum aestivumL.) te ondersoek. Die proewe is in 1991 gesaai op vier verskillende saaidatums wat op Elsenburg en Tygerhoek drie weke en op Langgewens twee weke na mekaar gevolg het. Die gemiddelde duurtes van die blaarvorming-, aarvorming- en aarverlengingfases was onderskeidelik 36, 22 en 43 dae of 452, 262 en 541 daggrade. Die duurtes in aantal dae by die blaar- en aarvormingsfases het aanvanklik eers toegeneem met later saaitye en daarna weer afgeneem. Vir die aarvorming-, aarverlengingsperiode en die periode van saai tot antese, het die duurtes in daggrade verminder met later saaitye. Vir kultivars SST 25, Palmiet, Adam Tas en Chokka is die aantal daggrade wat tot antese nodig was met onderskeidelik 3.4, 4.0, 4.0 en 3.9 verlaag vir elke dag wat later gesaai is.

ISSN:0257-1862    

DOI:10.1080/02571862.1996.10634382

出版商:Taylor&Francis Group    

年代:1996

数据来源: Taylor

摘要:

Verskille in opbrengs en proteïen by koring (Triticum aestivumL.) kan beter verklaar word indien meer kennis beskikbaar is oor die effek wat saaityd daarop het. Vier lentekoringkultivars is in 1991 op drie lokaliteite met 'n tipiese Mediterreense klimaat gesaai by vier opeenvolgende saaitye vanaf Mei tot Julie. Saaitye het twee tot drie weke na mekaar gevolg. Die vestiging van koring is beïnvloed deur heersende klimaat en grondtoestande net voor en net na die saai van die koring. Later saaitye het gemiddelde droëmassaproduksie en opbrengs met onderskeidelik 45.6% en 61.2% verlaag vanaf die eerste saaityd vroeg in Mei tot die laatste saaityd in begin Julie. Graanopbrengs is lineêr verlaag met 1% vir elke dag wat later gesaai is as 18 Mei. Die vier kultivars se reaksie het in diéopsig nie van mekaar verskil nie. Opbrengs is grootliks verminder deur die afname van gemiddeld 44.1% in aantal korrels m−2wat geproduseer is, maar die massa korrel−1het ook gedaal. 'n Goeie verband (r= 0.87 - 0.94) tussen bogrondse droëmassa by oesstadium en die aantal korrels geproduseer is verkry. Adam Tas het gemiddeld 11.9 korrels g−1droëmassa geproduseer teenoor 9.9 korrels van Palmiet. Vroeësaaityd het die proteïeninhoud van die saad verhoog in vergelyking met saaitye twee tot ses weke later. Die laaste saaityd se proteïeninhoud was egter weer hoër as diévan 'n saaityd twee of drie weke vroeër. Laat saaityd het ook die oesindeks verlaag.

ISSN:0257-1862    

DOI:10.1080/02571862.1996.10634383

出版商:Taylor&Francis Group    

年代:1996

数据来源: Taylor