Extensive pastoral land use systems are globally widespread and diverse, but seasonal migrations of herders and their livestock ("transhumance") provide common solutions to the constraints and opportunities provided by temporal-spatial patterns of accessibility, forage biomass and nutritional value in different parts of a larger pastoral zone. In many areas of West Africa, for example, pastoralists move into the drier Sahel region during the wet season and early dry season, not only because that is the time when herbaceous biomass is available, but crucially also because that is when surface water is available (water constraints are lifted). Here we use GPS collar information on distance travelled and time spent grazing, with data on available forage biomass and nitrogen content, and a simplified animal energetics and growth model, to evaluate the animal energetic tradeoffs inherent in transhumance as a pastoral land-use strategy, relative to sedentary options. The animal energy balance model estimates energy expenditures in basal metabolic and foraging activity, relative to energy gained during grazing.
Results/Conclusions
We use annual GPS datasets for transhumant herds in West Africa, including small ruminant and cattle with distinctly different migration pathways. Independent data on the spatial and temporal variability in biomass and nutritional value of forage along the transhumance route are used to estimate forage intake and digestibility. The differences in daily energy intake and expenditure are used to estimate animal growth and weight loss dynamics. For animals grazing on open range, daily net energy expenditures while foraging are similar to daily energy expenditures during migration, but forage intake is lower during migration, resulting in a net energetic cost. Simulations for sedentary animals, however, quantify the larger tradeoffs, in terms of forage availability and nutritional quality, behind the emergence of transhumance land use strategies.