摘要:

The ecophysiological responses of two karoo shrubs to sheep browsing at different stocking rates were studied. The responses of a relatively unpalatable shrub (Pteronia tricephala), and of a shrub of greater palatability (Eriocephalus ericoides) were examined at high, intermediate and at low stocking rates. These responses were compared with the ecophysiological attributes in the absence of browsing (control). Despite the two species being of different palatabilities, the estimates of the extent of plant utilization confirmed that both species were browsed at all stocking rates. No significant changes in ecophysiological processes were apparent in E.ericoidesat high stocking rates. However, non‐structural carbohydrate status decreased at the highest stocking rates, which may signify a loss in plant vigour. In contrast,P. tricephalaresponded to high stocking rates with an improvement in plant water relations, and an elevation in net photosynthetic rates. The carbohydrate status of heavily browsedP. tricephalaplants remained unchanged, and leaf polyphenol concentrations decreased. These findings suggest that relatively more palatable shrubs such asE. ericoidesmay not be able to survive continuous heavy browsing, and that less palatable shrubs, such asP. tricephala, have compensatory mechanisms that operate only at high stocking rates. These compensatory responses may increase the acceptability (low polyphenol levels) of relatively unpalatable shrubs, and may confer a competitive advantage (high carbon reserves and elevated photosynthesis) on these shrubs when subjected to browsing at high stocking rates.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647947

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Vegetation change on different substrate types was measured over 15 years in semi‐arid Mopani Veld. Baseline data were collected in the terminal year (1982) of a decade of above‐average rainfall, whilst the period of monitoring was characterised by below‐average rainfall, with severe drought years occurring in 1988/1989 and 1991/1992. Grazing pressure was, on average, alleviated over the period of recording so that changes reflect primarily the effect of rainfall. Long‐lived perennial grasses had declined markedly, annual and micro‐perennial grasses had increased markedly, and the proportion of forbs had increased. Despite 7–17 years since the replacement of livestock grazing with wildlife at lower stocking rates, there was no evidence of an improvement in sward or soil condition, owing ostensibly to the effect of drought. The extent of change in botanical composition and in sward condition was influenced by substrate type. The rate of growth of the woody component had been slow: the average per annum increase in the radius of trees was 0.67 mm. For the dominant species,Colophospermum mopane, only individuals of less than 1 m in height consistently showed an increase in size. The basal area of dead standing trees had increased threefold. Complete mortality or substantial topkill of shrub species, in particular ofGrewiaspp.,Ximenia americana, andCombretum apiculatum, and of adults of the latter, had occurred owing to drought. Shrub mortality was most pronounced on colluvial soils and sandveld. All species exhibited partial dieback of the crown. Overall, shrub recruitment has been limited except for that ofH. mopaneon colluvial soils but not on other substrate types, which is therefore likely to increase further its dominance on this substrate at the expense of species richness. Grazing pressure on desirable perennial grasses and browsing pressure on palatable woody plants has probably increased because of their decline, even if animal abundance has not increased, which will have a negative effect on their re‐establishment. Acting in opposition, dieback of trees and shrubs has probably created a window of opportunity for the recovery of perennial grasses, but it would likely require an extended run of above‐average years to reverse the changes in herbaceous vegetation, for which the availability of seed of perennial grasses is likely to be a major constraint.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647948

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The response of wall barley (Hordeum murinum L) to clipping height at various stages of growth was investigated in the semi‐arid grassland of northeastern Jordan in the 1994/1995 and 1995/1996 growing seasons. Treatments included combinations of clipping heights (5 or 10 cm above soil surface) and plant growth stages (tillering, jointing or booting), in addition to an unclipped control. Results showed that clipping height had less of an effect on shoot weight and tiller number than time of clipping. Clipping at the tillering stage produced 8.1 to 9.4 g per plant dry matter and 9.3 to 11.2 tillers per plant, compared to 7.7 and 8.1 g per plant and 7.1 and 8.5 tillers per plant for the undipped control. Clipping at the jointing and booting stages reduced shoot mass and tiller number. Defoliation during tillering stage did not have an impact on plant shoot mass or tiller number. It is therefore recommended that wall barley plants are defoliated early at the tillering stage before the plant reaches the jointing or reproductive stages, or after the plants reach physiological maturity.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647949

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Understory areas in semi‐arid rangelands can be a habitat for high‐yielding palatable grasses. Selective grazing of these grasses can result in their replacement by less palatable grass species, normally found in open areas. An experiment, at Matopos Research Station, sought to establish whether the understorey microenvironment improves the quality and structure of open‐area grasses that invade understorey areas. Herbaceous quality (ash, nitrogen, neutral detergent fibre, acid detergent fibre) and structure (height, leaf:stem ratio, tuft circumference, tiller number) of the dominant species of open‐area grasses were measured. Tuft circumference and tiller numbers were the only measurements that differed between the same species growing under tree crowns and in open areas in one of the three sites. The appropriate management responses to changes in understorey species composition are discussed.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647950

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Experiments were conducted at three sites to study effects of different nitrogen (N) fertilisers (e.g. urea, ammonium nitrate, di‐ammonium phosphate and ammonium sulphate), applied at 45 kg N ha−1, with and without phosphorus (P), potassium (K) and sulphur (S), on pasture dry matter (DM) yield and botanical composition. Autumn and spring applied N increased DM yield at all sites. Different N fertilisers generally produced similar DM yield increases. Where different N fertiliser types were balanced with respect to P, K and S there was a similar marginal, but usually nonsignificant (P≥0.05) increase in DM yield over unbalanced N fertiliser types. Average autumn N response efficiencies (excluding P, K and S) at Sites 1, 2 and 3 (after 47, 34 and 37 days, respectively) were 10:1 (10 kg DM per kg N), 16:1 and 13:1, respectively. Including P, K and S, N response efficiencies were 12:1, 17:1 and 15:1, respectively. Residual N response efficiencies without P, K and S (after 50, 41 and 29 days at Sites 1, 2 and 3, respectively), were 5:1, 5:1 and 3:1, respectively. When P, K and S were included, N residual response efficiencies were 6:1, 6:1 and 4:1, respectively. Average spring N response efficiencies at Sites 1, 2 and 3 (without P, K and S) after 29, 30 and 26 days, respectively were 9:1, 6:1 and 16:1, respectively. Including P, K and S, N response efficiencies were 12:1, 7:1 and 17:1, respectively. Residual N response efficiencies (excluding P, K and S) were 6:1, 6:1 and 7:1 at Sites 1, 2 and 3 after 21, 20 and 21 days, respectively. Including P, K and S, average N residual response efficiencies were 5:1, 7:1 and 8:1, respectively. The botanical composition of the pasture was unaffected by the application of N.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647951

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Experiments were conducted at three sites to examine effects of applying different nitrogen (N) fertilisers at 45 kg N ha−1, with and without phosphorus (P), potassium (K) and sulphur (S) on dairy pasture nutritive value and mineral content in autumn and spring. Pasture nutritive value was assessed by measures of crude protein (CP), metabolisable energy (ME), water‐soluble carbohydrates (WSC) and neutral detergent fibre (NDF). Pasture mineral content was assessed by estimates of the major minerals phosphorus (P), potassium (K), sulphur (S), sodium (Na), calcium (Ca), and magnesium (Mg), and trace minerals manganese (Mn), iron (Fe), copper (Cu) and zinc (Zn). Apart from some exceptions, applications of N, with and without additional P, K and S generally increased pasture CP and ME content in autumn and CP content in spring. Again with some exceptions, pasture WSC content was unaffected by fertiliser application in autumn, but was decreased by some treatments (containing N and N with P, K and S) in spring. Fertiliser treatments generally had no effect on pasture NDF content at Sites 1 and 3 during autumn and spring. At Site 2, some treatments (containing N and N with P, K and S) reduced pasture NDF content in autumn while during spring some treatments (containing N and N with P, K and S) increased pasture NDF content. Different N fertilisers applied at the same rate, with and without additional P, K and S, generally produced the same effect on pasture nutritive value. The application of P, K and S in the absence of N generally had no effect on pasture nutritive value. Apart from some exceptions, pasture P content was increased by fertiliser treatments including P, K and S, but N on its own did not influence pasture P content. Pasture K content was generally not influenced by treatment. At Site 1 during autumn and spring, and at Site 3 during spring, treatments containing P, K and S increased pasture S content while applications of N had no effect. At Site 2 during autumn, P, K and S increased pasture S content relative N only, while during spring pasture S content was generally increased by all treatments. At Site 3 during autumn, applications of N decreased pasture S content while P, K and S increased pasture S content relative to no fertiliser. Pasture Na and Ca content were generally not influenced by treatment. Pasture Mg content was not influenced by treatment at Sites 2 and 3. At Site 1 during autumn and spring, however, some treatments (containing N and N with P, K and S) increased pasture Mg content. Fertiliser treatments produced no consistent patterns or differences with respect to trace minerals (Mn, Fe, Cu and Zn).

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647952

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Methods of determining stocking rate based on metabolic mass, animal type and biomass are reviewed in the context of equilibrial/disequilibrial paradigms. In South Africa, the calculation of ‘carrying capacity’ is based on conversion of animal species to metabolic mass equivalents. This assumes homogenous systems that tend to some point of equilibrium. It is applied widely in commercial livestock systems involving one or two species. Examination of a case study in the Lowveld of the Northern Province, South Africa, showed that the determination of ‘stocking rate’ in this multi‐herbivore and heterogenous system, overestimated the ‘carrying capacity’ of the reserve over 20 years. The actual animal numbers in the system dropped by approximately 4 000 kg km−2after a drought in the early period of the study into the bounds as determined by a model incorporating rainfall and animal biomass. An approach to determine stocking density using animal type, biomass, rainfall and vegetation parameters is suggested. The development of this as a coarse‐scale (regional) and ranch‐specific model to cover a range of scales and heterogeneity in key resources is advocated.

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647953

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647954

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:1022-0119    

DOI:10.1080/10220119.1998.9647946

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor