Presidential address

In intact plants C¹⁴‐assimilates were translocated from the treated leaf on the primary tiller to young leaves on that tiller, to secondary tillers, and also to the roots. Plants defoliated after labelling revealed movement of materials from the storage organs to all the new leaves. Here these materials were used as a respiratory substrate and for the production of structural materials. Although initially after defoliation new leaves retained all their assimilates, presumably for their own growth, assimilates were soon translocated from these leaves to the roots.     

The regrowth and utilisation of carbohydrate reserves of eragrostis cur vula after different frequencies of defoliation

Eragrostis curvula in pots was cut every 2, 3, 4, 6, or 12 weeks, for 12 weeks. Regrowth and the recovery of total available carbohydrates (TAC) in the next 7 weeks was measured weekly.

The regrowth (dry‐matter yield) of tops was progressively less with increasing frequency of defoliation. After the defoliation period the root weights decreased in the more lenient treatments, The crowns did not show this marked initial decrease, but increased steadily in weight. Immediately after the last defoliation, the TAC content of the roots and crowns decreased significantly, and it was not restored to the level observed at the last cut until the third week after defoliation.

A comparison of average relative growth rates of tops revealed that during the first three weeks after the last defoliation the highest values were obtained from plants defoliated least frequently. For this period there was a significant correlation between average grwoth rate and reserve status. The results indicate that the plants drew on their carbohydrate reserves for at least a short period during regrowth.     

The response of Acacia karroo plants to defoliation by hand compared to defoliation by goats

Abstract

The leaf and shoot growth of Acacia karroo plants defoliated by goats was compared to that of plants in which leaf only or leaf plus shoot‐tips were removed by hand, to simulate goat browsing.

There were no differences in leaf or shoot production between plants whose leaf only was removed compared to those that had both leaf and shoot‐tips removed. Leaf growth on the plants defoliated by goats at moderate levels was approximately three‐fold that on plants defoliated moderately by hand. The plants defoliated by hand produced approximately twice the amount of leaf that the undefoliated plants produced. Heavy defoliation by both hand and goat, produced approximately half the leaf growth that moderately defoliated plants produced, following equivalent defoliation. Shoot production of all plants defoliated by hand was no different to that of the undefoliated control plants and was considerably less than that of the plants defoliated by goats.     

Partitioning,storage and remobilization of 14C assimilated by Themeda triandra Forssk

The second youngest fully‐expanded leaf on the main tiller of vegetative Themeda triandra plants was exposed to ¹⁴CO₂. Thereafter, quantitative analysis of partitioning, storage and remobilization after defoliation of ¹⁴C‐labclled assimilate was conducted in serially‐harvested plants over a 27‐day period. In undefoliatcd plants, 50% of assimilated ¹⁴C was ultimately lost through respiration, while that remaining in the plant reached its final destination within two days of assimilation. A small, but significant, portion of assimilated ¹⁴C was invested into long‐term storage in roots and stem bases. These reserves were remobilized after defoliation, and a portion of the remobilized ¹⁴C was incorporated into new growth. Partially‐defoliated plants regrew more rapidly than totally‐defoliated plants, but they both made equal demands on ¹⁴C reserves. Reserve depletion took place from both roots and stem bases.     

A comparison of highveld fodders and fodder flows for growing out long yearling steers

Abstract

A fodder flow/animal experiment 40,8 ha in extent (7,5 ha irrigated), with 19 paddocks, was carried out over a two and a half year period.

The aims were: firstly, to compare fodder crops within seasons, and fodder flows over three seasons (winter‐spring‐summer), by means of pasture and animal parameters; secondly, to make economic comparisons between different fodder flows, for growing out beef steers.

The conclusions were that: Digitaria eriantha ssp. eriantha is superior to Eragrostis curvula cv. Ermelo for animal production; maize residue gives economically favourable results due to low costs and compensatory growth of cattle; an irrigated pasture providing fodder in more than one season of a flow, for example, Festuca foggage — Festuca pasture — Digitaria pasture, gives a very profitable fodder flow, because the costs of production are divided between two seasons; dryland fodder flows with the sequence maize residue — silage or hay — Digitaria or, with silage or hay replacing the maize residue, were also very profitable; and, the fodder flow giving the highest livemass gain per steer, is not necessarily the most economic. Different kinds and classes of livestock require different fodder flows.     

Effect of intensity of defoliation by goats at different phenophases on leaf and shoot growth of acacia karroo hayne

Abstract

Defoliation by goats (leaves plus shoots) during the growing phenophases resulted in a considerable stimulation of leaf and shoot growth relative to non‐defoliated plants. The response differed considerably depending on the intensity and phenophase of defoliation. Plants were most susceptible to defoliation and young shoot removal during the spring flush when carbohydrate levels were at their lowest. During the rest of the growing season carbohydrate levels were high. At these times moderate to heavy (50 % to 75 % leaf removal) defoliations resulted in the greatest leaf and shoot growth.

In contrast, the initial and continuing impact of avian and insect consumption of leaves only reduced leaf and shoot production. The stimulatory effect of defoliation in one season carried through the dormant season to the following growing season. Defoliations during the dormant season did not have any positive or negative impact on plants relative to non‐defoliated plants. Leaf and shoot material removed when the plants were growing, was replaced within weeks of defoliation.     

The production potential of pure and mixed stands of Eragrostis curvula and Medicago sativa on the eastern transvaal highveld

Abstract

Over a five year period pure stands of Medicago sativa and mixtures of M. sativa and Eragrostis curvula have done as well and better than E. curvula receiving 300 kg N/ha, and considerably better than treatments receiving 200 and 100 kg N/ha, indicating that evaluation using livestock and incorporation into production systems should now receive priority.     

The response of Acacia karroo plants to defoliation of the upper or lower canopy

The response of Acacia karroo trees to defoliation of either the upper or lower canopy only, was compared experimentally with that of plants whose whole canopies had been defoliated at a range of defoliation levels. These plants were very sensitive to defoliation of the upper canopy. A 100% defoliation of the upper canopy only, resulted in the same amount of growth as 100% defoliation of the whole canopy. This was considerably less than the growth of plants defoliated overall, at 25% and 50% leaf removal. In contrast, defoliating the bottom half of the canopy only, stimulated growth in the whole canopy to the same degree as defoliation of the whole canopy at 25–50%. The increases of growth were due largely to increased growth in the top half of the canopy. Plants were very sensitive to defoliation in the early‐flush phenophase. This probably masked the positive effects of the partial defoliations applied at this phenophase.     

Effect of the intensity and phenophase of defoliation and water stress on the rate of photosynthesis and the recovery of carbohydrate reserves in acacia karroo hayne

Abstract

A field study was conducted with Acacia karroo plants to determine changes in relative photosynthetic rates, the extent of carbohydrate reserve depletion and the rate reserves take to recover, following defoliation by goats at different intensities and phenophases, at a “wet” and a “dry” site.

The rate of photosynthesis of fully expanded leaves increased markedly following defoliation. Light defoliation increased photosynthetic rate the most. Total non‐structural carbohydrate levels dropped significantly after defoliation. The magnitude of decrease was directly related to the intensity of defoliation.

Following the heavy defoliations, recovery of carbohydrate levels was much faster than after the light defoliations. Rates of recovery were also faster following defoliation in the second half of the growing season, than in the first half. However, the plants that had been heavily defoliated in the second half of the growing season had not fully recovered carbohydrate levels before leaf fall in late autumn.

Moisture stress had very little effect on carbohydrate levels in comparison with the defoliation treatments.     

Effect of intensity and frequency of defoliation on aerial growth and carbohydrate reserve levels in Acacia karroo plants

Acacia karroo trees were defoliated by goats at two intensities and four frequencies; 2, 4, 8 and 12‐weekly. Leaf accumulation and carbohydrate reserve levels were compared to a non‐defoliated control, and to plants (defoliation controls) which were defoliated for the first time that season each time a frequency treatment was defoliated. These plants are activated by defoliation in such a manner that successive defoliations can result in this activation being additive. There is clearly a defoliation level below which they are not activated. Activation appears to be negated to a degree by defoliations at 2 and 4‐weekly frequencies, relative to the 8‐weekly defoliation frequency. The 12‐weekly frequency at heavy defoliation produced less than the same defoliation at 8‐weekly frequency. The 2‐weekly frequency treatments produced as much leaf as the 4 and 12‐weekly defoliations at the same defoliation intensity. The more frequently plants were defoliated, the more carbohydrate reserves dropped. However, plants adjusted to cope with very frequent defoliations. There was no connection between leaf accumulation and carbohydrate reserve levels following the different frequencies and intensities of defoliation.     

A comparison of manuring,mulching, and cultivation of Eragrostis curvula hay pasture

Abstract

Whereas cultivation had a detrimental effect on the production of Eragrostis curvula established in broadcast stands, both manuring and mulching had beneficial effects. This effect may be ascribed largely to better moisture conditions achieved with mulching although, at high rates of manuring, additional benefits could also be ascribed to the circulation of nutrients. The cost effectiveness of manuring is, however, doubtful. Although manuring improves the response to inorganic fertilization it will not reduce the need for such fertilization. The application of manure at 20 t ha^‐1 on large areas would be preferable to 40 t ha^‐1 on smaller areas, but this can only be recommended where costs are debited to feedlot waste product disposal.     

Note on the Taxonomy of Genetta

The recruitment patterns of six teleosts were studied at King’s Beach, Algoa Bay. February to May marked the major recruitment period for Diplodus sargus, Lithognathus mormyrus, Rhabdosargus globiceps and Trachurus capensis. The major recruitment of Sarpa salpa fry took place between August and September. Pomadasys olivaceum fry of < 5 cm total length were caught throughout the year indicating an extended breeding period. The estimated growth rates of new recruits between February and May (autumn/winter) was 0,60 cm month ^-1 for L. mormyrus, 0,75 cm month^-1 for ft globiceps and 0,40 cm month^-1 for T. capensis. The empirical growth curve derived for S. salpa indicated a growth rate of 0,75 cm month^-1. No growth estimates could be made for P. olivaceum or D. sargus.     

The influence of different forms and concentrations of nitrogen on total nonstructural carbohydrate allocation and growth in Digitaria eriantha

Abstract

Digitaria eriantha plants were grown under controlled conditions for a period of three months, during which time they were supplied with the following forms and concentrations of nitrogen: 50 mg/1 NO₃ ^?‐N, 200 mg/1 NO₃ ^?‐N, 200 mg/1 NH₄ ⁺‐N and a combined form containing both NO₃ ^?‐N and NH₄ ⁺‐N with a nitrogen concentration of 200 mg/1. Control plants received no nitrogen.

At monthly intervals both above and below ground components were assayed for soluble and insoluble carbohydrate contents. Dry mass and leaf areas were also measured.

The nitrogen treatments were found to influence the allocation of photosynthates into either soluble or insoluble carbohydrates. In above ground tissue, the soluble carbohydrate component decreased with time, while the insoluble component increased with time. In root tissue the reverse trends were observed. Both dry mass and leaf area development were enhanced in plants supplied with nitrogen in any form. It was suggested that growth of D. eriantha was influenced by carbohydrate fluctuations.     

Defoliation Effects on Herbage Production and Root Growth of Wet Meadow Forage Species

Root growth is important to the competitive ability of plants, and understanding how herbage defoliation affects root growth has implications for development of management strategies. Objectives were to determine the effects of defoliation intensity and frequency on root characteristics and herbage production of slender wheatgrass (Elymus trachycaulus [Link.] Shinners), Nebraska sedge (Carex nebrascensis C. Dewey), and “Steadfast” birdsfoot trefoil (Lotus corniculatus L.). Plants of each species were transplanted into containers that had been placed in the ground at wet meadow field sites the prior year. There were eight replications of a control and five defoliation treatments, which were combinations of different frequencies (two or five times) and intensities (light or heavy) and haying. Treatments were applied for a single growing season, and aboveground biomass was collected. Containers were extracted in October, and plant crowns, rhizomes, and roots were separated from the soil. Defoliation treatment did not affect total root weight, length, and surface area of Nebraska sedge or birdsfoot trefoil (P>0.10). Slender wheatgrass total root weight was less when defoliated five times (4.46 g·container^?1) than when defoliated twice (6.62 g·container^?1) during the growing season. More frequent defoliation of slender wheatgrass also reduced length (20%) and surface area (21%) compared to less frequent defoliation. However, defoliation frequency did not affect aboveground biomass. Defoliation intensity did not affect aboveground production or root characteristics of the three species. Abundant soil moisture in meadows likely buffers negative effects of defoliation. For all species, two defoliation events (e.g., haying followed by grazing) does not appear to negatively affect root growth and herbage production.     

Necrotising fasciitis in a bull due to infection with Arcanobacterium haemolyticum

Abstract

CASE HISTORY: A 2-year-old Hereford bull was lame for one week before becoming recumbent.

CLINICAL AND PATHOLOGICAL FINDINGS: The scrotum and ventral perineal region were cold and blackened caudally. The semimembranosus and semitendinosus muscles were firm on palpation. The bull was subject to euthanasia, and necropsy revealed that the skin and S/C tissues of the caudal half of the scrotum were grey and necrotic. The caudal and distal aspects of the semimembranosus and semitendinosus muscles were grey and necrotic to a depth of approximately 15 cm, and these changes appeared to track along fascial planes. The tissue had an offensive smell, and large amounts of flocculent, watery, brown fluid and some gas were present. Histology of affected muscle and S/C tissues revealed coagulative necrosis, with oedema and large numbers of bacteria that were predominantly Gram-positive rods. Adjacent blood vessels contained thrombi while the epidermis overlying the affected areas appeared diffusely necrotic, suggesting infarction. Culture of the fluid yielded a pure growth of Arcanobacterium spp., which was identified as Arcanobacterium haemolyticum, using an API Coryne biochemical test strip.

DIAGNOSIS: Necrotising fasciitis and myositis due to Arcanobacterium haemolyticum.

CLINICAL RELEVANCE: Arcanobacterium haemolyticum has not previously been reported as a cause of necrotising fasciitis in any species. Necrotising fasciitis is probably an under-reported condition in cattle due to its clinical similarity to clostridial disease.     

The influence of nitrogen fertilization,spring burning and height of stubble on hay and seed production of eragrostis curvula 1

Abstract

This investigation, conducted over a three‐year period, emphasized the strong effect of nitrogen fertilization on both hay and seed production. The optimum level of nitrogen, for conditions on the Eastern Highveld being 100–150 kg N/ha applied in spring, followed by subsequent dressings dependent on rainfall. Although the influence of spring burning and stubble height on hay and seed production was not as consistent, both had a strong influence under certain conditions and with more research into their action could become important considerations in management strategies for Eragrostis curvula.     

Grass defoliation affecting survival and growth of seedlings of Acacia karroo,an encroaching species in southwestern Zimbabwe

Abstract

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 of Acacia karroo seedlings. 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 and A. karroo biomass 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 more A. karroo seedlings 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.     

Growth patterns and annual growth cycle of Acacia karroo Hayne in relation to water stress I. Leaf and shoot growth

Abstract

A field study was conducted to describe the growth patterns of A. karroo at different levels of water stress in different edaphic situations.

Shoots are heterophyllous and are formed by free growth. The degree of development of a shoot, relative to others in the canopy, is governed by branch and position in the canopy. At least six phenological phases were identified in the annual growth cycle.

The pattern of growth and the phenological cycle are not changed by water stress. Initiation, emergence and development of shoots and leaves are governed by how favourable environmental conditions are. If there is little shoot growth early in the season, these plants can partially compensate by producing more leaf per unit of shoot if environmental conditions improve.

Leaf and shoot growth at the beginning of the season took place only if there was sufficient moisture available, and if the minimum temperature had risen above a threshold amount. Where there was insufficient soil moisture, no growth was observed before rains had fallen. The growth strategy of A. karroo differs markedly from that of broadleaved African savanna tree species. Growth in A. karroo is dominated by current growing conditions, rather than those of the previous season. They are able to make opportunistic growth at any time.

Soil depth had a marked influence on plant growth, presumably due to a larger available nutrient and moisture pool.     

Forage selection by blesbok (damaliscus dorcas phillipsi) as determined by point methods

The grazing behaviour and forage preferences of blesbok on the Van Riebeek Nature Reserve near Pretoria are described. Species composition of the grass component of this grassland vegetation as well as its utilization by blesbok were determined by means of point surveys. Blesbok feed exclusively on grasses and show preferences for certain species. Their preferences on burned range differ from those on unburned range. Judging by preference ratings, Themeda triandra and Eragrostis curvula are the most important food plants of blesbok in this Reserve. Rotational burning appears to be essential in the management of blesbok herds kept under such conditions.     

Die evaluering van vier Eragrostis curvula ekotipes met weidende skape

The evaluation of four Eragrostis curvula ecotypes with grazing sheep. Four ecotypes of Eragrostis curvula, namely, Cape Province 5, Ermelo, Pearston 204 and Makwassie 150 were evaluated in a grazing trial with sheep, at three stocking rates, during two growing seasons. Under conditions of very lenient utilization, where the animals could select freely, the animals on Cape Province 5 had significantly higher mass gains than on any one of the other ecotypes. The sheep on Pearston 204, on the other hand, showed higher live mass gains, although these were not significant, under conditions of more intensive utilization. There were no significant differences in the dry matter production and chemical composition of the clipped samples of the ecotypes.