Abstract
AbstractNoy-Meir's simple but insightful model of grazing-system dynamics was used to draw broader inferences from empirical data generated by a 17-year field trial with beef cattle grazing a Mediterranean grassland in northern Israel. After calibration of its parameters against the field results, the model predictions were tested against an independent set of data obtained from the study site; they were within acceptable deviations from the inherently noisy field data. The calibrated model was used to analyse the effects of changes to two key grazing-management factors – stocking density and early-season grazing deferment – on biomass dynamics and forage consumption. The simulated results were used to calculate forage deficits and supplementary feed requirements for optimum herd performance during the growth (‘green’) season and throughout the year. The results revealed a critical stocking density of 0.7 Animal Units (AU)/ha, above which early-season deferment reduced the amount of supplementary feed required to maintain the optimum production of the herd. Optimum stocking is higher when the grassland is used mainly in the highly nutritious green season. Responses of the strongly seasonal Mediterranean grassland to the interaction between stocking density and early-season grazing deferment were expressed by a calibrated model, in terms that determine the efficiency of forage supplementation of the herd during the green season and throughout the year.
Highlights
Since about the mid-20th century, models of grazed grassland ecosystems have been constructed that range in complexity from simple general functions (Noy-Meir, 1975; Lebon et al, 2014) to highly detailed simulations based on the physical and biological components of the ecosystem (Vickery and Hedges, 1972; Innis, 1978; Breymeyer and Van Dyne, 1980; Stuth and Lyons, 1993; Teague et al, 2008; Petz et al, 2014)
In a seminal and widely cited paper, Noy-Meir (1975) applied predator-prey theory to formulate a simple model of the dynamics and stability of continuously grazed systems
At the beginning of the green season most annual herbaceous species had germinated by 17 November, on average, and the herbaceous vegetation was usually well established 14 days after the first effective rainfall (Svoray et al, 2008)
Summary
Since about the mid-20th century, models of grazed grassland ecosystems have been constructed that range in complexity from simple general functions (Noy-Meir, 1975; Lebon et al, 2014) to highly detailed simulations based on the physical and biological components of the ecosystem (Vickery and Hedges, 1972; Innis, 1978; Breymeyer and Van Dyne, 1980; Stuth and Lyons, 1993; Teague et al, 2008; Petz et al, 2014). The simple models are more effective at providing insights into general system properties than they are at analysing management options; in contrast, detailed models can analyse management options but leave the broader picture obscure These limitations do not preclude the possibility posited in the current paper that simple models, which capture the fundamental functional relations of the system at the hierarchical level relevant to management, can be parameterized and used to explore the response space meaningfully. Noy-Meir’s model provided deep insights into the stability properties that emerge from the interplay of the two basic functions; it showed that at the ecosystem level of grazed, seasonal grassland, a few relatively stable, dominant properties determine the responses of sward growth and forage consumption by the herd to management manipulation, despite wide year-to-year variations in climatic conditions and system responses
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