Browsing by Author "Richardson, F D"

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    A mathematical model of browse and herbage production in communal grazing lands of semi-arid regions
    (1999) Nhlengetfwa, Melusi; Richardson, F D; Hahn, B D
    The main purpose of this work is to extend an existing model of growing cattle and grass production in a semi-arid rangeland.The existing model which is basically Dye's (1983) model in differential equation form handles: i) the growth and performance of cattle measured in terms of weight, ii) the initiation of grass growth in early rainy season and its utilisation by the cattle . This model is being extended to simulate woody plants in addition to the grass and to simulate browsing by goats. The densities of vegetation and the stocking rates of both types of animals are being considered. Our model (SAVANNAS) will predict animal productivity in relation to rainfall and density of woody plants (or vegetation condition). A rainfall data file is being used to generate rain which divides into infiltration and run-off. Athough generally dry, semi-arid regions are agriculturally productive, more especially in terms of animal products. An understanding of the climatic conditions by the farmers is all what it takes. It is unfortunate that in these regions, rainfall, being the main driving force behind animal productivity, is unreliable in that it varies both within and over the years. It is in this regard, therefore, that models be built to simulate semi-arid environments. Such models, when run for several (semi-arid) representative rainfall years could be used by farmers. For instance, a model like SAVANNAS will be run for three rainfall years namely 1980/81, 1981/82 and 1982/83, which, respectively represent: very high, about average and very low rainfall (by semi-arid standards). SAVANNAS simulates processes that operate on widely different time scales. The growth and consumption of herbage and leaves and twigs of woody plants are modelled on a daily basis, while the numbers and ages of woody plants are updated every 120 days. The year is divided into four seasons, with the rainy season beginning in September and initiating herbage re-growth. SAVANNAS simulates herbage biomass, which means it allows the re-establishment of the previous year's grass plants . It divides woody plants into age cohorts with the first cohort being seedlings mainly, and the last cohort being adult trees which are usually out of the browsing range of herbivores. It is a model that has a focus on the effects of vegetation (woody plants and grass) on each other and the effects of the animals on vegetation and viceversa. Without overlooking their effects on vegetation production, fires are not considered in SAVANNAS. This is because in communal lands heavy grazing does not allow the accumulation of sufficient dry herbage for fuel.
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    Open Access
    Modelling the complex dynamics of vegetation livestock and rainfall in a semiarid rangeland in South Africa
    (2010) Richardson, F D; Hoffman, Michael Timm; Gillson, Lindsey
    Predicting the effect of different management strategies on range condition is a challenge for farmers in highly variable environments. A model that explains how the relations between rainfall, livestock and vegetation composition vary over time and interact is needed. Rangeland ecosystems have a hierarchical structure that can be described in terms of vegetation composition, stocking rate and rainfall at the ecosystem level, and the performance of individual animals and plants at the lower level. In this paper, we present mathematical models that incorporate ideas from complex systems theory to integrate several strands of rangeland theory in a hierarchical framework. Compared with observed data from South Africa, the model successfully predicted the relationship between rainfall, vegetation composition and animal numbers over 30 years. Extending model runs over 100 years suggested that initial starting conditions can have a major effect on rangeland dynamics (divergence), and that hysteresis is more likely during a series of low rainfall years. Our model suggests that applying an upper threshold to animal numbers may help to conserve the biodiversity and resilience of grazing systems, whilst maintaining farmers’ ability to respond to changing environmental conditions, a management option here termed controlled disequilibrium.
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    Modelling the long-term consequences of undernutrition of cows grazing semi-arid range for the growth of their progeny
    (2009) Richardson, F D
    Cattle grazing semi-arid range may be subjected to low intakes of metabolisable energy and protein as a result of drought, high stocking rates, or during the dry season low digestibility and low protein content of the forage (Butterworth, 1984). The growth and milk yield of cattle are reduced during such periods of undernutrition. Growing animals respond in different ways to nutritional restriction because of differences in age at the start of undernutrition, the severity and duration of undernutrition and the availability and composition of food during rehabilitation (O'Donovan, 1984). Severe chronic undernutrition of cattle in early life reduces growth and leads to smaller animals at any age (Morgan, 1972; Greenwood & Café, 2007). Undernutrition of breeding cows during the last trimester of gestation leads to a reduction in birth weight (Richardson et al., 1979) and in the cow's milk yield (Richardson et al., 1977) as a result of the effect of undernutrition on udder development (Mellor & Murray, 1985) so that pre-weaning growth is also reduced. On the other hand, compensatory growth may occur in rangeland cattle subjected to undernutrition after weaning. For example, Ainslie (1958) reported that when steers were subjected to different planes of nutrition between weaning at 8 - 9 months of age and 12 months the difference of 23 kg between group means was eliminated when all animals grazed together without being given any supplementary food. In this paper a mechanistic model of a rangeland production system has been used to explain the different responses to retardation of growth that have been recorded in the literature.