摘要:

Effects of nitrogen (N) (0, 15, 30, 45 and 60 kg N ha−1) applied at five different times during autumn and early winter on the N fixation ofTrifolium repensL. (white clover) inLolium perenneL. (perennial ryegrass) / white clover pastures were examined (Experiment 1). Effects of applying different N fertilisers (e.g. urea and ammonium nitrate) at 45 kg N ha−1in autumn, with and without phosphorus (P), potassium (K) and sulphur (S) on N fixation were also tested (Experiment 2). In Experiment 1, percentage N derived from the atmosphere (percent Ndfa) by white clover varied between 87% and 91% in the period after (24 to 36 days) N application. Increasing rates of N and time of application did not influence percent Ndfa. The amount of N fixed by white clover ranged from 0.8 to 3.7 kg N ha−1in the period after N application. Increasing rates of applied N produced a significant (P<0.05) linear increase in the amount of N fixed. In Experiment 2, percent Ndfa and amount of N fixed (1.9 to 4.1 kg N ha−1) were unaffected by different N fertilisers, with and without P, K and S, in the period after (37 days) fertiliser application. For both experiments, fertiliser N increased dry matter yield while having no effect on white clover composition. It is concluded that single applications of N during the autumn and early winter benefit pasture growth without adversely affecting white clover N fixation or clover composition. This finding, however, will require further investigation.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647933

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Leucaena leucocephalais a fast‐growing tree that can provide both high quality forage and firewood. The objective of this trial was to determine the optimum height and frequency of cutting for both wood and forage production. Cutting heights at 0.3 m, 0.6 m and 1.0 m were superimposed on 3‐month and 6‐month cutting frequencies on mature rows ofL. leucocephalafor three years. Effects of year or its interaction with the other factors were not significant (P≥0.10). There was an increase (P=0.07) in wood production but not forage production (P≥0.10) with the longer interval. Cutting height had an effect on forage (P<0.001) production, with 5.47 t ha−1a−1for the 0.3‐m height, 7.62 t ha−1a−1for the 0.7‐m height, and 8.71 t ha−1a−1for the 1.0‐m height. Cutting height also had an effect on wood production (P<0.001), with 7.22 t hd−1a−1for the 0.3‐m height, 9.33 t ha−1a−1for the 0.7‐mheight, and 11.55 t hd−1a−1for the 1.0‐m height. In this experiment, the six‐month interval produced more wood than the three‐month interval, but there were no differences in quantity of forage dry matter inL. leucocephala; there were advantages in both wood and forage production with the taller trunk base.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647934

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Studies of the effects of woody vegetation on herbaceous vegetation in southern Africa have focused almost exclusively on savannas with isolated trees with no attention given to multi‐species bushclump savannas. The influence of multi‐species bushdumps on herbaceous vegetation was investigated in a mesic savanna in the Eastern Cape. Development of bushdumps appeared to reduce herbaceous production underneath the clumps by up to 90%. The herbaceous layer underneath the bushdumps was distinctly different to that of the open grassland and it also had slightly higher quality compared to that of grassland. Effective bush control measures are warranted if these savanna areas are to sustain beef and mutton production as is currently the case.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647935

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Tree crowns in semi‐arid areas have often been found to create micro‐habitats of improved soil physical and nutrient status and reduced evapotranspiration. These encourage the growth of mesic, palatable and high yielding perennial grasses in otherwise arid environments. A two‐year study, in areas with a long history of light grazing, at Matopos Research Station sought to establish whether the soil nutrient status and micro‐climate under tree crowns are different from those in open areas, and whether the differences are affected by soil type and tree species. The study trees wereTerminalia sericea and Acacia karroo. There were consistent patterns in soil nutrient status between understorey and open areas, although these patterns were largely not significant. There was also a consistent pattern in soil volumetric water content of open areas >T. sericea>A. karroounderstorey areas. Tree crowns reduced solar radiation reaching understorey areas by an average of 88% and rainfall by an average of 15%. The findings on soil nutrient status are consistent with results from disturbed systems, and challenge the dogma, at least for soil nutrient status, that conservative stocking rates are beneficial.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647936

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A two‐year study was conducted in lightly grazed areas of Matopos Research Station, Zimbabwe, to evaluate the impact of widely spaced trees on understorey grass composition, yield and quality. The study trees wereTerminalia sericeaandAcacia karroo. Ordination techniques using grass density and biomass as indices separated quadrats according to soil type but not grass species according to understorey or open areas or according to tree species. Grass yield under tree crowns was similar to open areas, in contrast with most reports where understorey areas had higher yields. The high understorey grass quality that has been reported from savanna areas exhibiting grass composition differences was not expressed. Selective grazing of palatable perennial grasses growing in association with tree crowns and their eventual replacement by low‐yielding and less‐palatable grasses that normally grow in open areas is proposed as an explanation of the unique tree‐grass interaction scenario of this study. In areas where this replacement has occurred, recovery may require management interventions.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647937

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The objective of this study was to investigate the influence of different nitrogen fertiliser application levels in combination with different defoliation intervals on the dry matter (DM) production and water‐use efficiency of ryegrass (Lolium multiflorum,) cv. Midmar. In a field trial, four nitrogen levels (0, 150, 300 and 450 kg N ha−1) were combined with five defoliation intervals (every 2, 4, 6 and 8 weeks and twice a season). Soil moisture levels were measured with a neutron hydroprobe and were used to schedule irrigation. The dry matter production, nitrogen‐use efficiency (NUE), plant DM content and water‐use efficiency were influenced (P<0.01) by nitrogen level, defoliation interval, and the interaction between these factors. Total DM production varied from 64 to 20 922 kg DM ha−1. Dry matter production per unit of applied N decreased with an increase in N level within a defoliation interval. The plant DM content varied from 14.0% to 35.6%. The plant DM content decreased (P<0.05) with increasing N applications, while a lengthening of the defoliation interval resulted in an increase (P<0.05) in the DM content. No difference (P>0.05) was found in total water used when plants received N. Water‐use efficiency varied from 1.2 to 25.8 kg DM ha−1mm−1water received.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647938

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Two experiments were conducted, one in the field and the other in the greenhouse, to investigate the effects of the intensity and frequency of grass defoliation on the survival and growth ofAcacia karrooseedlings. In the greenhouse, seedlings growing with heavily clipped grasses had higher biomass production than those competing with moderately clipped grasses. Root/shoot ratios were higher in treatments with undipped grasses. There was a negative relationship between grass root production andA. karroobiomass production. The field experiment was carried out in two paddocks, one previously heavily‐grazed and the other lightly‐grazed. Grazing in both paddocks was simulated by artificial defoliation. Generally moreA. karrooseedlings emerged under lightly defoliated treatments. Clipping frequency had a strong effect (P=0.066) on the survival of emerged seedlings during the wet season. There were no differences in survival rate at the end of the dry season. Though grass defoliation was shown to enhance seedling growth under controlled conditions, no evidence was found to suggest that seedling establishment during the first year is influenced by the intensity of grass defoliation.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647939

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In semi‐arid regions, the effects of grazing or sparing management on natural communities of long‐lived plants generally take decades to become evident. Event‐driven dynamic behaviour, unpredictable and low rainfall, and complicated interactions between species make it difficult to gather sufficient understanding of vegetation dynamics to be able to develop guidelines for sustainable management. Simulation models that consider the essential processes that determine vegetation dynamics offer scope for quantitatively exploring long‐term vegetation dynamics of arid and semi‐arid rangelands. In this paper we review three models that were aimed to provide an understanding of the vegetation dynamics and management of different typical vegetation types in South Africa, including the Karoo shrubland, the shrub‐grassland of the southern Kalahari, and pure semi‐arid grasslands.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647940

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The root distribution ofCommiphora pyracanthoidesEngl., a potential encroaching woody species in the sweet bushveld areas of the Northern Province, South Africa, was studied at the Mara Agricultural Development Centre. The root system of tenC. pyracanthoidesshrubs consisted of a taproot system supported by a well developed fleshy secondary root system. Minimum branching of secondary roots occurred, resulting in the absence of tertiary and feeder roots. Weak relationships existed between root distribution, plant height and canopy diameter, but subjectively estimated, secondary roots extended horizontally at an average depth of 15 ‐20 cm to a distance of more or less 1–1.5 times the canopy diameter and 1.5–2 times tree height. It is proposed that the correct placement of soil‐applied arboricides and/or absorption mechanisms may be critical for effective chemical control ofC. pyracanthoides.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647941

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Local farmer knowledge of species distribution patterns and plant palatability in Namaqualand, South Africa, was assessed using Participatory Rural Appraisal techniques. These findings were compared with the results obtained using phytosociological techniques (species distribution), and with palatability indices published in the literature. It was found that farmers were knowledgeable of species distribution close to the settlement, but less so further away from the settlement. Farmer plant palatability rankings did not correlate well with the palatability indices found in the published literature, which suggests that palatability in semi‐arid regions are locality specific.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647942

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor