摘要:

Seven defoliation treatments involving grazing at three-weekly intervals with sheep until mid-January or mid- March, or for the whole growing season, at each of two stocking rates, or resting for the whole growing season, were applied on veld during the 1985–86 season. Mass per unit basal area of the shoot components, green leaf, dry leaf and stem plus inflorescence of populations of tufts of the three most abundant grass species,Heteropogon contortus, Themeda triandraandTrachypogon spicatus, was determined at intervals during the periods of rest. Relative utilization was estimated by observations on samples of tufts after each grazing occasion. Within treatments, growth patterns during rest periods of the populations of the three species were broadly similar, though production per unit tuft basal area decreased in the ratio 100:65:40 forTrachypogon, Themeda and Heteropogonrespectively. Using the cumulative growth curves, absolute and relative growth rates for the whole shoot and green leaf in certain of the treatments were established. A pervasive feature of the growth patterns was a tendency towards senescence in the latter half of the growing season. Defoliation by grazing was only partially effective in delaying this process. Amounts of herbage of each species which were present at the start of the rest periods beginning in mid-January and mid-March were generally proportional to the stocking pressure prior to the rest. However, within species, there was a poor correlation between this result and estimated relative utilization.

ISSN:0257-1862    

DOI:10.1080/02571862.1989.10634471

出版商:Taylor&Francis Group    

年代:1989

数据来源: Taylor

摘要:

Seven defoliation treatments involving grazing at intervals with sheep until mid-January or mid-March, or for the whole growing season at each of two stocking intensities, or resting for the whole growing season (control treatment) were applied on veld during the 1985–86 growing season. Residual effects of the treatments on root mass and the mass of non-structural carbohydrates (NSC) in the roots of populations of the three most abundant grass species,Heteropogon contortus, Themeda triandraandTrachypogon spicatuswere determined in the winter of 1986. Shoot yields in the 1986–87 growing season were measured by harvesting species separately on both a tuft basal area and quadrat basis. In general, the defoliation treatments resulted in marked reductions in root and NSC mass in all species as compared with the control. Residual effects on shoot yields were relatively small, and, in sharp contrast to previous findings, correlations between effects on roots and on shoot yields were, except in the case ofTrachypogon, generally poor. Discrepancies were found between the residual effects on shoot yields of species measured on a per unit tuft basal area or a quadrat basis. The implications of such discrepancies are discussed.

ISSN:0257-1862    

DOI:10.1080/02571862.1989.10634472

出版商:Taylor&Francis Group    

年代:1989

数据来源: Taylor

摘要:

Morphological and selected chemical and granulometric properties of eleven soil profiles occurring on five hillslope sites (midslope, upper footslope, mid-footslope, lower footslope and valley bottom) were determined in order to evaluate the spatial pattern and variability of soils in a granitic area north-east of Nelspruit. A transect design was employed to compare the variance of selected soil properties within and between hillslope positions. Although a drainage-toposequence had developed in the colluvial deposits on the footslope, this soil pattern was interrupted in mid-footslope by sands of the Longlands form encountered between sands of the Avalon form. Meso-scale deflections of the bedrock floor in the form of hillslope terraces were presumably responsible for this variation in soil pattern. To study soil variability statistically, a two-stage nested design and an analysis of variance were employed. Although few significant results were obtained, certain tendencies were evident which pointed to a high degree of variability in selective soil properties in this landscape. Notwithstanding these findings, most of the spatial variability in soil properties could be explained in terms of pedogeomorphic processes.

ISSN:0257-1862    

DOI:10.1080/02571862.1989.10634473

出版商:Taylor&Francis Group    

年代:1989

数据来源: Taylor

摘要:

Controlled-environment studies confirmed that tomato plants subjected to a range of treatments involving the herbicide 2,4-D iso-octyl ester/ioxynil applied in the vapour and liquid phases develop symptoms of phytotoxicity in proportion to the severity of the treatment. Plants which received direct foliar applications of herbicide emulsion (40μg, 100μg and 1 000μg per plant) failed to survive longer than 35 days. In contrast, none of the plants subject to vapour treatments were killed as a consequence of herbicide activity in the vapour phase. Vapour treatments were, however, associated with a variety of sub-lethal plant responses which included changes in plant height, dry matter content and abnormal leaf development. Flowering was advanced by some herbicide treatments. The lower stems of many treated plants also developed root primordia and in the case of the more severe treatments, lesions of the stem occurred. Symptoms of phytotoxicity from which many plants recovered included apical stem bending and downward curving of the leaves and leaf petioles. An examination of the associations between the development of visual symptoms of phytotoxicity and measurable plant growth responses (height and dry matter yield) have been used to develop a diagnostic base for the identification of auxin-like herbicide damage in field-grown plants.

ISSN:0257-1862    

DOI:10.1080/02571862.1989.10634474

出版商:Taylor&Francis Group    

年代:1989

数据来源: Taylor

摘要:

Nineteen different cultivars and experimental lines of lucerne (Medicago sativaL.) were inoculated with sand-bran cultures ofColletotrichum trifoliiBain&Essary and evaluated for resistance to anthracnose and crown rot under local conditions. The various genotypes differed significantly in their reaction to these two diseases. Nine were resistant to crown rot, whereas five were resistant to both crown rot and anthracnose. The South African cultivar S A Standard was more resistant to crown rot and anthracnose than most other cultivars imported from Australia and North America. This cultivar, however, has a low yield potential in comparison with other cultivars. Frequent harvesting caused an increase of crown rot among all cultivars.

ISSN:0257-1862    

DOI:10.1080/02571862.1989.10634475

出版商:Taylor&Francis Group    

年代:1989

数据来源: Taylor

摘要:

The highest theaflavin (TF) contents associated with five tea clones grown in the Eastern Transvaal were encountered in the manufactured tea from‘MT 12’followed by‘SFS 204’, with‘Veld 35’and‘Veld 43’having the lowest values. TF contents of the latter two clones were not significantly different for hand-plucked as compared with machine-plucked tea, whereas with the other three clones hand-plucking resulted in higher TF values. A fermentation period of 90 as compared with 30 min increased TF content in‘MT 12’. Insufficient or no withering had a detrimental effect on the TF's of black tea when the leaves were processed on the same day of plucking, unless the leaves were left in the sun for 5 h, after which the highest TF value was obtained. Over-withering for 2 days gave the lowest TF concentration, probably as a result of a lowering of polyphenol oxidase activity. The possibility that this may affect tea flavour is discussed.

ISSN:0257-1862    

DOI:10.1080/02571862.1989.10634476

出版商:Taylor&Francis Group    

年代:1989

数据来源: Taylor

摘要:

The costs of the initial clearing of mixed stands of the alienAcacia cyclopsandA. salignainvading fynbos vegetation were compared in a replicated trial on the Pella Fynbos Research Site. Costs were estimated using a regression-based model of the individual cost components. Felling was the cheapest clearing method, with felling plus the stripping of bark from the cut stump, second. Applying arboricides to the cut stump generally doubled the cost relative to felling. Mattocking was the most expensive method with ringbarking second. The estimated cost of clearing a moderate infestation on the Pella site ranged from R26 to R286 ha−1for the different treatments. Similarly, estimations for an‘average’western CapeAcaciathicket ranged from R526 to R2 408 ha−1while those for the densest infestations encountered on Pella ranged from R299 to R5 170 ha−1. The precision of the model is thought to be low at higher densities. Variations in the density and height of the fynbos and the spatial distribution, size class distribution and growth form of theAcaciaplants are all thought to affect clearing costs.

ISSN:0257-1862    

DOI:10.1080/02571862.1989.10634477

出版商:Taylor&Francis Group    

年代:1989

数据来源: Taylor

摘要:

Solar radiation is the driving force which most directly influences the surface water balance and crop yields. In South Africa this variable is measured at a limited number of sites. In order to determine the geographic distribution of monthly mean daily totals of incoming solar radiation, an elementary model for solar radiation in terms of sunshine duration was used. It was found that summer solar radiant energy varied from 20 to 30 MJ m−2d−1and that the summer—winter differential is in the order of 15 MJ m−2d−1throughout the country. While data are available for each month of the year, the geographical distribution of monthly mean daily radiant energy received for only January, April, July and October are presented here. These are representative of summer, autumn, winter and spring. Irrespective of season, maximum incoming solar radiation occurs in the western and north-western interior, indicating that within these areas highest crop yields, coupled to highest water requirement, can be expected.

ISSN:0257-1862    

DOI:10.1080/02571862.1989.10634478

出版商:Taylor&Francis Group    

年代:1989

数据来源: Taylor

摘要:

Two foliar sprays of paclobutrazol applied at four different concentrations, viz. 50,100, 200 and 500 mg a.i. l−1onCitrus volkameriana(Ten.&Parq.) under greenhouse conditions, showed that concentrations of 100 mg l−1and above are suitable for reducing undesirable spindly growth of citrus rootstocks under crowded nursery conditions. Paclobutrazol at 200 and 500 mg l−1resulted in the greatest growth check and there was no significant difference between these two treatments. At these higher concentrations, however, there seemed to be a residual effect on the subsequent growth of the scions. The growth check on the rootstock at 100 mg l−1was significantly less than 200 and 500 mg l−1but still of practical benefit to nurserymen, besides having no residual effect on the growing scions. The treatments did not cause any significant differences in stem diameter and bud take.

ISSN:0257-1862    

DOI:10.1080/02571862.1989.10634479

出版商:Taylor&Francis Group    

年代:1989

数据来源: Taylor

摘要:

Colorimetrie methods for measurement of ammonium, nitrite and nitrate were modified (a) for application to small samples of nitrogen-poor soils, (b) by centrifugation of extracts to avoid contamination from filter papers and (c) to economize on apparatus and reagents. Estimates by modified methods on thirty soils chosen for their large differences in ammonium and nitrate concentrations were close to estimates on larger samples by a standard method. Ammonium nitrate added to five of the soils was recovered satisfactorily.

ISSN:0257-1862    

DOI:10.1080/02571862.1989.10634480

出版商:Taylor&Francis Group    

年代:1989

数据来源: Taylor