The'perfect'grazing management: how to choose best suited number and species of herbivore? (original) (raw)

2008

Abstract

Grazing is a commonly used management technique, e.g. to stop the expansion of competitive plant species, to enlarge plant species richness or to create spatial heterogeneity, favouring biodiversity in general. In order to accomplish an effective and efficient grazing management both herbivore and manager needs have to be accounted for. Several ungulate species are commonly used in nature. Several intra- and interspecific dissimilarities in animal morphology and physiology appear, and important differences in animal nutritional requirements and habitat selection are found. Therefore, not all herbivore species will be equally suited to fulfil management needs. Because of the considerable variation in biological (e.g. floristic composition), physical (e.g. scrub density, forage yield, water availability, forage accessibility) and chemical characteristics (forage quality), not all terrains are equally fit for all herbivores. This reveals the necessity to determine the carrying capacity of the system. Therefore a grazing capacity model (GCM) is being developed. This dynamic model considers crucial variables on both the terrain and the grazer level, such as forage yield, forage quality, palatability of plant species, accessibility of the area, soil erosion vulnerability, animal nutritive requirements, animal behaviour and general habitat health. It predicts the optimal grazer species and density, taking the seasonal variation in animal needs and terrain characteristics into account. In this short paper the outline of the model is presented together with the results of two case studies in coastal and estuarine special protection zones within the Natura 2000 network in Flanders (Nature Reserves Westhoek and IJzermonding). The model clearly reveals that grazing capacity is a dynamic feature, with seasonal, inter-annual, interspecific as well as intraspecific variation, differentiating GC in space, time and between herbivores. Additionally the GC outcome varies according to the used forage variable; lowest feasible Animal Unit numbers are generally found when using digestible energy as forage need variable, while phytomass generally results in significantly higher GC outcomes. Since all three forage need variables are relevant to the herbivore under consideration, it can be concluded that the manager should consistently follow the model at the lower predicted number of AU’s per season. For the studied coastal areas, the optimal herd densities per season show large differences. Model estimates of number of AU’s in winter (and spring) are far lower then in summer and autumn, suggesting that year round grazing is feasible only when a seasonally differentiated herd density is applied. The best grazer choice depends on food availability and quality as well as on specific management needs.

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