N.F.G. Rethman

Review article: Vegetative growth, reproduction, browse production and response to tree clearing of woody plants in African savanna

Abstract This review deals with issues including the determinants of vegetative growth of woody plants (soil water availability, water stress, soil nutrient availability, carbohydrate reserves, plant hormones, atmospheric CO2 concentration, tree age, competition, defoliation and shoot pruning, fire, pathogens, soil and climatic conditions), reproduction (the ability of mature trees to flower and produce viable seeds, the ability of seeds to disperse and germinate, seedling survivorship), and browse production. Emphasis is placed on how these may be influenced by tree clearing, either through exploitation or through deliberate actions aimed at reducing the suppressive effect of an increase in woody plant density on the production of the herbaceous layer. A diverse range of aspects, related to the dynamics of the woody component of savannas, had received attention in the literature. It has contributed to some understanding of the nature and dynamics of the vegetative growth, reproduction and browse producti...

Inter‐related floristic changes associated with different long‐term grazing treatments in Sourish Mixed Bushveld 1

Eight camps, grazed annually during different seasons by steers, were surveyed. Herbaceous species were grouped into Decreaser and Increaser categories according to their reaction to grazing. The Increaser IIa (light overgrazing) and IIb (moderate overgrazing) herbaceous species were the most abundant and were significantly negatively correlated. Woody plants and aloes (Aloe transvaalensis) were positively correlated (P≤0.01) with Increaser IIb herbaceous species. The latter were negatively correlated (P≤0.01) with grass production. The production of Panicum maximum was negatively correlated (P≤0.01) with tree canopy cover, owing to its intolerance to severe grazing which contributed to an increased canopy cover. While representing the same amount of animal unit grazing days ha−1 (stocking density × period of occupation during growing season), a relatively high stocking density and occupation for part of the growing season had a less severe long‐term effect on the floristic composition than a relatively l...

The effect of form and level of nitrogen fertlization on the yield of Digitaria eriantha Steud

Abstract On dryland Digitaria eriantha ammonium sulphate and limestone ammonium nitrate were superior to potassium nitrate and urea. With reference to the level of nitrogen fertilization it was found that levels in excess of 300 kg N/ha, and up to 400 kg N/ha, could be economically justified, even on a pasture with a high basal level of fertility.

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 value of subtropical grass pastures for use as foggage on the eastern Transvaal Highveld

The utilization of frosted herbage of Pennisetwn clandestinwn, Paspalum notation, Digitaria eriantha, Cynodon sp. and Chloris gayana by Merino wethers during the winter confirmed the potential of this practice as an alternative winter feeding strategy under Highveld conditions. P. clandestinwn and P. notatum provided good quality foggage with average daily gains of 60–80 g sheep−1 being recorded. D. eriantha also provided good quality foggage when stem production was minimized. C. gayana gave good livestock performance, but did not persist under Highveld conditions. Although Cynodon sp. proved to be a relatively poor foggage species, it provided for the maintenance of livemass. Longer rest periods in the growing season increased the available pasture, grazing capacity and livestock production ha−1, but increased the probability of poorer livestock performance. This is probably linked with a lower protein content and a higher proportion of stem material in herbage rested for a longer period.

Influence of moisture stress on growth, dry matter yield and allocation, water use and water-use efficiency of four Indigofera species

The effects of moisture stress were evaluated in four Indigofera species (I. amorphoides, I. arrecta, I. coerulea and I. vicioides) using a pot experiment under glasshouse conditions. The aim was to examine the influence of moisture-deficit stress on physiological attributes and performance of the four Indigofera species. Based on plant available water (PAW) levels, three moisture levels were applied (no stress or 70–100% PAW, 40–70% PAW and 10–40% PAW) as experimental treatments and imposed on each species in a randomised complete block design with four replications. Moisture stress significantly reduced (p < 0.05) the total leaf area of I. amorphoides at moderate levels and that of I. vicioides at the most severe moisture stress level. The relative growth rate was significantly reduced (p < 0.05) in I. amorphoides but not in I. arrecta, I. coerulea or I. vicioides plants subjected to moisturedeficit stress. The reduction in total dry matter yield was due to the effect of moisture stress in reducing both leaf area and leaf area ratio. Drought stress-tolerance indices were not different (p > 0.05) for I. arrecta or I. coerulea grown under no stress and moisture-deficit stress conditions. The root mass fractions of I. amorphoides and I. arrecta were substantially increased (p < 0.05) due to moisture stress. Water-use efficiency was low in I. amorphoides under water deficit conditions, while that of I. vicioides was higher under severe stress conditions compared to non-stress conditions. Transpiration efficiency was, however, higher at moderate moisture-deficit stress levels than under non-stressed or severely stressed plants. Generally, the species investigated exhibited significant variation in terms of physiological attributes and performance response to moisture-deficit stress. Indigofera amorphoides was relatively sensitive while I. vicioides was able to maintain unabated growth under water stress conditions. This is highly relevant to programmes aimed at identifying suitable species as a source of fodder for livestock exposed to long dry seasons and frequent drought phenomena.

Dry matter yield and water use efficiency of five perennial subtropical grasses at four levels of water availability

The effect of four levels of water availability on the yield and water use efficiency (WUE) of five subtropical perennial grasses was evaluated in a small plot trial under a rain shelter. The soil profiles were brought to 100% (W4 = the control), 75% (W3), 50% (W2) and 25% (W1) of field capacity on a weekly basis. Under W4 (control) and W3 conditions, yields did not differ significantly from each other. Where water became less available (W2 and W1), yields were significantly reduced, in comparison with the control (W4). Buffel grass, a drought-tolerant species, produced yields (11.7–20.0t ha−1) under moderate to severe water restriction (W1, W2 and W3), which were comparable with yields obtained with adequate water (W4) for traditionally-irrigated grasses such as couch grass (12.0–15.8t ha−1) and kikuyu (5.6–11.8t ha−1). All five grass species tended to use water more efficiently under moderate to severe water-limiting conditions (W1 and W2) than under control conditions (W4). These results also indicate that traditionally drought-tolerant grass species should not be overlooked when identifying species for use under irrigation.

The utilization of eragrostis curvula and pennisetum clandestinum pastures for growing out and finishing young beef steers

Abstract Results obtained from three observation trials to assess the feasibility of utilizing Eragrostis and kikuyu with young beef steers are presented. Although both grasses were characterized by a high grazing capacity the performance of livestock, on an individual basis, was disappointing in the light of the high rate of fertilization and supposedly high feeding value of the grass. This was especially so with Eragrostis. The need for a comprehensive investigation of the nutritional value of these grasses in the grazing situation as a basis for supplementation is emphasized.

Livestock utilisation of grass species at different distances from water on both traditional cattle post and ranch management systems in Botswana

Individual forage species were appraised in varying distances along transects radiating from the water points of the ranch and traditional cattle post management systems. Measurements of plant height were used to assess the degree of forage species utilisation by livestock around water points. Livestock use forage plants more heavily near water points and the degree of use is more pronounced in the traditional cattle post areas, compared to ranch grazing. Utilisation of annual plants occurring at the 0m zone from water was greater than that of the same plant species growing at further distances. Periodic closure of water points aimed at reducing grazing pressure has been indicated as a method to promote production of forage around water points. With the exception of Panicum maximum, the abundance of which was low or non-existent in both management systems, Digitaria eriantha was the most utilised grass by livestock, followed by Schmidtia pappophoroides and Eragrostis lehmanniana. Stipagrostis uniplumis, one of the dominant grasses in the Kalahari Sandveld, exhibited a weak tendency to decline as distance from water increased. It was observed that high forage utilisation rates coincide with drought periods, implying that correct stocking rate adjustments should always match the available forage. Forage utilisation from water points in the traditional cattle post areas extended beyond 4 000m, especially during the dry periods.

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.